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MC How-To
Step by step: From general maintenance to complete restorations, we share tips and tricks for working on classic bikes.

Rebuild Ducati Mikuni BDST Flat Slide Carburetors

 

This How-To might seem a little out of sync with our mission, focusing as it does on a 1992 Ducati 750SS. The name of the magazine is Motorcycle Classics, we hear you say, so isn’t 1992 too new?

Believe it or not, it’s been 26 years since Ducati introduced the 750SS for the 1991 year, putting it just inside our general policy of only featuring bikes 25 years old or older. Hard as it is to accept, 1991 was, well, a long time ago. In the motorcycle marketplace, our project 1992 Ducati is just old. It’s at that awkward age where it’s neither fish nor fowl; somewhat ignored by lovers of more vintage machinery yet too outdated to be interesting to riders looking for modern hardware. It is in fact a potential bargain Ducati, perfect for riders who appreciate the brand and its heritage.

An updated version of the 750 Sport of 1988-1990, the 750SS featured improvements over the outgoing 750 Sport including a wet clutch — the first Ducati so equipped since the Cagiva Alazzura and Elefant — and a pair of 38mm flat slide Mikuni constant velocity carburetors in place of the 750 Sport’s troublesome single Weber. The Mikuni swap yielded improved performance, with none of the flat spots or hesitation experienced on the Weber-equipped 750 Sport. But with the youngest Mikuni-equipped bikes now 20 years old (fuel injection came in 1998), the likelihood of one of these bikes needing a carb rebuild is high. BikeMaster

Fortunately, Mikuni BDST flat slide carbs are easy to rebuild — even on a Ducati — and parts are readily available, if you know where to look. Surprisingly, the same sources that don’t list kits for the 750SS do list O-ring and float needle kits for 1992-1993 Yamaha TDM 850s, which used the same 38mm Mikuni as the Ducati 750SS. Going to eBay and typing in the search term “1992 Ducati 750SS carb kit,” we found a few vendors offering kits that included not just the O-rings and float needle, but all new jets, the slide needle and the atomizer tube. Those kits ran in the $52-$56 range (per carb). In the end, we opted for O-ring and float needle kits from Power Barn for $32 each.

We should note that this was not a comprehensive rebuild. Rather, it was sort of quick and dirty, to see if we could get this 750SS running again. Which we did, and quite nicely, thank you. Although we did not soak the carb bodies, our subject carbs cleaned up well using spray carb cleaner followed by compressed air to chase the orifices. If you do soak them, make sure not to soak any plastic parts, cleaning them individually instead.

This is a fairly straightforward project and well within the reach of the average weekend warrior. If you don’t rush it and give yourself the weekend, you shouldn’t have any trouble pulling the carbs on Saturday and reinstalling them on Sunday. Outside of a synchronizer, this job doesn’t require any special tools, and if you have the experience it’s easy to sync the carbs by ear and feel. Twins are nice that way, readily betraying when one cylinder is firing smoother than another.

As always, we recommend having a good shop manual on hand for parts identification and proper torque specs.

 

1. Remove the gas tank and air filter, the throttle and choke cables, then loosen the carburetor to manifold clamps and remove the carbs as a unit. Next, remove the fours screws securing the float bowl and cable bracket. This photo also shows the mixture screw and spring.

 

2. The plastic float assembly hinges on a plastic needle housing. The housing is a press fit into the carb body. Grasp the housing firmly and pull it straight up and out, complete with the float. It should release easily.

 

3. Remove the pin securing the float to the needle housing and separate the two, exposing the float needle. Thoroughly clean but do not soak the housing and float.

 

4. The jet block is secured to the atomizer tube. Remove the screw and collar securing the jet block, then remove the jet block. This also releases the flat slide body, as we’ll see soon.

 

5. As its name implies, the jet block contains most of the carb’s jets. Left to right above the block is the block retaining screw and collar, the main jet, the pilot jet retainer and the pilot jet.

 

6. The starter jet screws into the jet block. Removing the main jet first releases the pilot jet retainer. Remove the pilot jet, then unscrew the starter jet.

 

7. As noted in Step 4, removing the jet block releases the flat slide body. Turn the carb over and remove the two screws securing the CV cover. Remove the cover, followed by the slide spring. Push the flat slide up from inside the carb throat, then remove the flat slide and diaphragm. Next, pull the plastic flat slide body straight up and out of the carb body, as shown. The slide is at upper right.

 

8. Repeat this process on the other carb body. Clean the bodies and passages thoroughly, then blow them dry with compressed air, making sure to blow air through all the orifices to ensure they’re clean. Do not soak any of the plastic parts. Next, start the reassembly process by installing the new O-ring on the float needle body.

 

9. Next, install the new O-ring on the slide body at the base of the atomizer tube. This photo shows the new O-ring installed, with the old O-ring held for reference.

 

10. Next, lightly coat the O-ring with grease and reinstall the flat slide body in the carb body, making sure it’s pushed fully home. Gently turn the carb over to prepare to install the jet block.

 

11. After cleaning the jet block, blow it dry with compressed air, making sure all the jet passages are clean. Install the new O-ring from the kit, as shown.

 

12. Slip the jet block over the atomizer tube and onto the carb body, followed by the sleeve and retaining screw. Secure the jet block to the carb body. This also locks the slide body.

 

13. With the jet block installed, install the new O-ring on the pilot jet. Lightly coat the O-ring with grease and push the pilot jet into its bore in the position shown.

 

14. Next, after ensuring it’s thoroughly clean, screw the starter jet into its bore next to the pilot jet.

 

15. Place the retainer into position around the pilot jet, then screw the main jet into place, securing the retainer and pilot jet. Note that the retainer partially blocks the starter jet.

 

16. Slip the new float needle into place on the float tang. Slide the float seat over the needle, then reinstall the float pin. Make sure the pin is installed from the outside in as the float bowl acts to retain it. Lightly coat the float seat O-ring with grease and reinstall the float assembly. Put the new float cover gasket in place and reinstall the cover and the throttle cable bracket.

 

17. Turn the carb over and reinstall the slide and diaphragm. The slide will only go in one way. Install the new O-ring for the CV airway, visible here at approximately the 1 o’clock position. Install the slide spring and the slide cover.

 

18. Install the spring followed by the washer and new O-ring on the fuel mixture screw. Lightly seat the screw, then turn back out 1.5 turns for an initial set. Reinstall the carburetor set, accelerator and choke cables, airbox and gas tank, then start the engine and make final adjustments to the running engine.

Rebuild BMW R75/5 Bing CV Carburetors

 

Bing constant velocity carburetors as used on BMW R75/5s and up are wonderful devices, generally trouble free and long wearing. Yet overhauls are still occasionally necessary, as we found with our subject 1973 BMW R75/5. Suffering from deteriorated jet and throttle shaft O-rings, the carbs were impossible to properly set and the only solution was a full overhaul.

For parts, we turned to the Bing Agency International. After describing our carburetors’ condition, Fay Laughridge suggested their #6 32mm CV rebuild kit, which comes complete with every gasket and O-ring needed, new throttle slide diaphragms, new slide needles and jets, new floats, float pins and float needles, new float needle seats, and new throttle plate screws.

A comprehensive Bing rebuild isn’t cheap. Our kit with optional needle seats came in at $261, and while you can source some of the bits cheaper the ease factor of picking one part number and knowing you’re getting everything you’ll need is huge. Further, as near as we can find, the Bing Agency is the only source for replacement float needle seats. The Bing Agency offers troubleshooting help and can also supply Bing manuals. BikeMaster

The Bing Agency also offers a float and bowl kit with alcohol-proof floats. The stock floats aren’t alcohol proof, but they seem to hold up reasonably well over time. Whether you want the upgrade — we went stock — is a judgment call you’ll have to make.

While this is a somewhat fiddly rebuild, it’s certainly within reach of the average mechanic. The trick, as always, is to take your time. When something doesn’t want to move, find out why before you force it. The only special tool required might be a carb synchronizer for tuning, but a good ear can do well on a twin. That said, for this project it’s essential to have a good manual on hand to help with parts identification and location, and final tuning.

 MC How-To

1. The #6 kit from the Bing Agency for 32mm constant velocity Bing carburetors includes gaskets, floats, float needles, CV diaphragms, slide needles and jets, throttle plate screws and every O-ring for both carburetors. Float needle seats are optional.

 

2. The first step is to remove the carburetors. We removed the gas tank, not strictly necessary as the carbs are so easy to access. Loosen the clamps at the intake elbow and the engine, then gently twist the carburetor free.

 

3. Once the carb is free, remove the throttle cable from the throttle plate and release the choke cable from the choke plate by loosening the holding nut.

 

4. Release the float bowl retaining bail and remove the bowl. Put the carburetor on its side and gently tap out the float retaining pin. Note: The pin has serrations on one end to lock it in place. Drive the pin out from the opposite end.

 

5. Remove the main jet with washer, needle jet assembly, idle jet and idle mixture screw, noting their locations. Also note the order of the needle jet assembly, which has three pieces: the venturi, the jet and the pre-atomizer.

 

6. Turn the carb over. Remove the four screws securing the throttle slide cover. Remove the cover and then the throttle slide complete with diaphragm. It will drop free easily.

 

7. Remove the four screws holding the choke cover and remove the cover. Remove the throttle spring and idle screw. Remove the nut securing the throttle cable lever to the throttle shaft and remove the lever. Remove the two screws holding the throttle shaft retaining plate and remove the plate.

 

8. To remove the throttle shaft you must first remove the throttle plate. The two screws securing it to the shaft are peened at the factory to prevent them loosening. The exposed threaded ends must be filed off before you try to remove the screws.

 

9. Using a tight-fitting screwdriver, loosen the screws. If they fight you, turn them back in and file the ends again. Repeat until the screws will come out, then pull the throttle plate free.

 

10. Remove the throttle shaft. Note the orientation. The screw head recess is to the outside, as shown. The throttle plate has a small dimple at 12 o’clock that also faces to the outside.

 

11. Remove the four screws and plate securing the diaphragm to the slide and remove the diaphragm. The slide needle is held by an internal spring clip and has four stops. Remove it by twisting it 90 degrees then pulling it out. If it clicks and stops, twist another 90 degrees and continue. Repeat until it pulls free. Note the number of stops to remove; you’ll want to reinstall the new needle the same number of stops. 

 

12. Soak the carb body and all parts in solvent. Rinse and, if available, give them a final clean in an ultrasonic washer, rinsing when done. Blow out all ports with compressed air. To replace the float needle seat, thread a 7mm x 0.75mm pitch tap into the seat until it stops.

 

13. Next, clamp the end of the tap firmly in a vice. With an assistant holding the carb body, place a block of wood over the jet ports and hit the wood with a hammer until the body separates from the seat. We heated the body with a heat gun, making removal easier.

 

14. Make sure the seat bore is clean. Coat the outside of the new seat with anti-seize compound. Using a wooden dowel, drive the new seat into place until it bottoms out.

 

15. Put a new O-ring on the throttle shaft. Coat the O-ring with silicone lubricant and install the shaft into the body, noting its orientation. Install the throttle shaft retainer and screws.

 

16. Install the throttle plate in the shaft making sure the dimple on the plate is facing up (toward the top of the carb) and out. Coat the retaining screws with permanent-type thread-locking compound. Screw firmly in place.

 

17. Install the throttle cable plate on the throttle shaft. Reinstall the idle screw and spring. Place a new O-ring on the choke valve shaft and coat with silicone lube. Install the choke valve with index hole on choke plate aligned with oblong intake port on choke valve body. Using a new gasket, reinstall the choke assembly to the carb body.

 

18. Install new O-rings on the idle jet, main jet, needle valve venturi and idle mixture screw. The central needle valve jet has three pieces: Install the pre-atomizer followed by the jet and finally the jet venturi. Install the idle and main jets and the idle mixture screw.

 

19. Place new float needle into seat. Install new float and pivot pin. Seat the serrated end of the pin by lightly squeezing it into the float bowl post. Set the float so it’s parallel to the carb body when it touches the needle. Install new float gasket followed by the float bowl.

 

20. Install new diaphragm on slide. The diaphragm is “clocked” to the slide, with a molded, raised rubber edge that locks into a corresponding slot in the top of the slide. The outer edge of the diaphragm (just visible above thumb at left) has a similar molded edge that slots into the carb body when the slide is dropped in place. Replace slide cover and four screws.

 

21. Adjust the idle screw until it just touches its stop, then screw it in one turn. Lightly seat idle mixture screw, then turn out 1-1/2 turn. Attach throttle and choke cables and install carbs. Finally, start the engine and fine-tune the carbs.

BMW /5 Charging System Upgrade

 

As good as BMW’s classic /5 series is, there’s always room for improvement. One thing old /5s — and just about every airhead from 1970-1994 — can really benefit from is a charging system upgrade. While the stock system basically works fine, it can come up short in voltage delivery, especially in low-rpm urban situations where the engine doesn’t spin at high enough revolutions to maintain the necessary voltage to keep the battery up. 

Euro MotoElectrics in Denver, Colorado, specializes in electric system upgrades for BMW, Moto Guzzi and Ducati. Over the years, they’ve developed numerous starting and charging kits for vintage and contemporary BMWs, including the EnDuraLast III kit for 1970-1977 airheads. 

The comprehensive kit includes a new 4-wire stator plus a new rotor, diode board, diode board mounts, alternator brushes, an adjustable voltage regulator and wiring. The major benefits of the system are increased output (240 watts versus 180 watts stock), with charging voltage coming on at significantly lower rpms than the stock system.

For our BikeMasterinstall on a 1973 R75/5 we also had Euro MotoElectrics send us a new front crankshaft oil seal (easy to do while you’re there) along with a rotor removal bolt and an optional resistor lead for the charging light. The charging light excites the system, but if the light fails the system won’t charge. This simple modification is just a safeguard in the unlikely event the charge light does fail.

This is a high-quality, well-thought-out upgrade and everything goes together exactly as it should. That said, we suggest making this a weekend project to give yourself plenty of time, as it is fairly involved for the average weekend warrior. Required tools include a good soldering iron and solder/flux, a torque wrench, and, if you replace the crankshaft seal, a seal puller.

 

Depending on the exact year of your bike, you may or may not have to install an insulated Y-post connection for the upgraded 4-wire stator. The extra wire — added starting in 1974 — increases the efficiency of the AC to DC conversion. We forgot to order the available kit and came up with our own Y-post, a 4mm bolt with mylar for insulation. Simple enough, but the optional Y-bolt kit from Euro is only $2.75 and worth getting. That brings us to cost, which we consider very reasonable given the improvements gained (more on that in a second). The complete kit goes for $350, with the rotor tool adding $8.25, the seal $7.95 and the charging light circuit mod another $20, for an all-in of $388.95 if you also get the Y-post kit. By comparison, a replacement BMW rotor and stator alone will set you back almost $500.

As for performance, our system went from barely producing 13 volts at 4,000rpm to kicking out 14.25 volts as low as 1,500rpm. That’s an impressive improvement, ensuring full charging even if all you do is lope around town. As ever, we suggest having a good manual on hand for parts identification and proper torque specs.

 

1. First and most importantly, disconnect the battery by removing the ground cable. The alternator is under the front engine cover. Remove the three Allen-head bolts securing the front cover to the engine and remove the cover.

 

2. Next, disconnect the electrical connections at the alternator (five spades on our R75), then remove the three Allen-head bolts securing the alternator cover and stator to the engine.

 

3. With the Allen-head bolts removed the alternator cover and stator should remove easily as a unit to reveal the alternator rotor. The stator is pinned to the cover with a roll pin at the factory. The new stator is not.

 

4. Remove the Allen bolt securing the alternator rotor to the crankshaft. Install the long removal bolt. It threads into the rotor only, pushing against the inside of the crank to pull the rotor free.

 

5. Tighten the bolt, then tap the rotor with a brass or rubber mallet. It will usually pop off its taper. Be careful not to overtighten the bolt; it could break.

 

6. With the rotor off, we elected to replace the front crank seal. It wasn’t leaking, but it’s easy enough to do. After drilling a small pilot hole, we used a seal puller and it popped right out.

 

7. Installing the new seal is a simple matter of positioning it and pushing it evenly around its circumference with hand pressure to start, followed by seating with a flat driver such as a socket or small block of wood, working evenly around until it’s flush with the case.

 

8. Using a soldering iron, melt the solder securing the three wires from the stator to the junction block on the alternator cover and remove them from the block. The blue device is a “solder sucker” — a handy vacuum that sucks up the solder and gets it out of the way while you’re working.

 

9. With the three alternator wires pulled free from the junction block, remove the two screws securing the block to the cover and remove the block.

 how-to

10. Next, place the cover on the jaws of a vice with the stator clear and gently drive the stator out of the cover, working back and forth from one side to another. It usually comes out quite easily.

 

11. With the stator removed, flip the cover over and remove the two nuts securing the brush holder to the alternator cover, followed by the brush holder itself. Note the insulating collar on one of the locating posts.

 

12. With the brush holder removed, use a soldering iron to melt the solder at the brush lead connections and remove the brushes from the holder.

 

13. The kit comes with new brushes. The insulation on the leads from our old brushes was in good shape so we transferred it to the new brushes after first lightly soldering the wire ends to keep them closed.

 

14. Next, install and solder the new brushes to the holder, making sure that once installed the brush wires will be under the holder and the solder joint at the top for proper clearance to the cover. Reinstall the brush holder, noting the insulating collar on one locating post.

 

15. Our early alternator required adding a Y post for the new 4-wire stator. We drilled a 5mm hole in the position shown, then used a mylar insulating collar and mylar washers to isolate a fixed mounting post made from a 4mm bolt with one nut fixing it to the cover and another to fix the fourth wire and the spade terminal connection.

 

16. Next, install the new stator in the alternator cover, making sure it’s clocked so the stator wires freely pass through the cover as shown. This photo shows the new stator’s fourth wire secured to the Y post we installed.

 

17. Install the junction block for the stator wires and solder the wires in place. It doesn’t matter which goes where. If necessary, ream out the holes for the wires using a small drill bit. Clip the excess off the wires when done.

 

18. We next swapped out the original diode board, a simple matter of removing the four Allen screws securing it to the engine. Some engines had rubber mounts for the diode board. Ours did not. 

 

19. Before installing the new diode board we wired in a new power lead to the main terminal on the starter. This is to ensure a solid voltage path.

 

20. The original power lead from the wiring harness was redundant. To keep it available but isolated, we insulated the connecting spade terminal with shrink wrap tubing.

 

21. Connect the wiring to the D+ terminal on the back of the new board (the new wire from the starter is also shown connected). Note: Our early bike required shortening the original diode board mounting screws 0.25 inch each to mount the new diode board.

 

22. Next, we installed the new rotor, using the supplied new bolt and lock washer and torquing it to 14ft/lb.

 

23. Install the alternator cover and stator. To keep the brushes clear of the rotor slip rings, carefully pull the brush springs back, cocking them on the brush holder mounting posts. Snug the bolts down to roughly 6ft/lb. Do not overtighten. Reseat the brush springs.

 

24. With the alternator installed, connect all the wiring as shown. The blue wires are additional grounds.

 

25. Our final step (we could have done this at any time) was to replace the original voltage regulator and install the optional charging light energizing circuit. The fuel tank must first be removed to gain access to the voltage regulator.

 

26. Replacing the regulator is a simple matter of disconnecting the regulator harness before removing two bolts, transferring the harness to the new regulator and bolting it to the frame. The optional charging light circuit attaches to one of the regulator D+ wires using a Posi-Lock tap connector.

 

27. The other end of the circuit connects to the right ignition coil on the spare blade for the switched battery circuit to the coil, which is a green/blue wire. Finally, we reinstalled the front cover before reconnecting the battery. MC

Adjusting 1977 Suzuki GS750 Valves

How-To 

Suzuki’s 1977-1983 2-valve 4-cylinder GS series engine was one of the best. Whether in 550cc, 650cc, 750cc, 850cc or 1,000cc guise, these air-cooled twin-cam fours were praised when new, and over the years they’ve proven themselves to be remarkably robust, able to withstand gross negligence and abuse. That’s good news, because it means that with proper care, you can expect a GS to run, well, almost forever.

BikeMasterAn often neglected maintenance item on these engines is valve adjustment, although experience shows that gently ridden machines tend to hold to spec for tens of thousands of miles. Our subject 1977 Suzuki GS750 is a perfect example of this. An original condition survivor, it shows a low 12,500 miles on the odometer and looks like a bike that was only ridden when it was nice outside, and put in the garage when it wasn’t. Checking the valves confirmed this: All the valves were within specification and no adjustment was needed.

The valve clearance (or lash) on the 2-valve GS750 is very tight; 0.03mm-0.08mm, or 0.001in-0.003in. Loose is better than tight, and in our case the valves were all on the high end at 0.0025in-0.003in. The valves are shim over bucket, with adjustment shims carried in a recess in the actuating bucket. Removing shims requires the correct valve shim tool, readily available from Motion Pro for $16.50. Shims are available from various sources individually or in kits, with individual shims costing $12-$15 and kits around $75-$100. Local shops typically stock the shims, and once you know what you need you can usually purchase them from the local Suzuki dealer or independent shop.

You’ll need a good set of feeler gauges, making sure the set includes the thin sizes needed for the Suzuki. Besides that, all you’ll need are standard hand tools. The actual checking doesn’t take too long, but be prepared to either let the bike sit as you locate the necessary shims — or do the job twice if shims aren’t available locally and you need to keep your bike on the road while waiting for them. As always, have a good manual at hand to confirm proper specifications.

 How-To

1. The first step is to remove the gas tank. There are two lines to the petcock, a fuel line and a vacuum line that opens the petcock when the engine is started. They’re different sizes and only go on one way. Remove and plug the lines. Remove the single bolt at the rear of the gas tank.

 How-To

2. Remove the gas tank, then remove the horn from its mount. Remove all four spark plugs. This makes it much easier to rotate the engine as you index each camshaft lobe as you won’t be working against compression.

 How-To

3. Next, remove the four camshaft cover end caps. Use the proper JIS Phillips head driver. One end cap screw on our GS was damaged to the point we had to drill it out, using successively larger bits until the head of the screw broke off. From there it was easy to remove the remaining screw shaft.

 How-To

4. Working in a cross pattern, remove the valve cover retaining bolts and the valve cover breather cover; the valve cover won’t clear the frame for removal with the breather cover in place.

 How-To

5. Remove the right side ignition cover. Using a 19mm wrench, rotate the engine clockwise until a camshaft lobe is straight up. Here it’s the #4 exhaust.

 How-To

6. Check the lash, then rotate the engine as necessary to get each camshaft lobe in the position shown, checking each one. As it turned out, our engine needed no adjustment, each valve coming in at 0.0025in-0.003in clearance.

 How-To

7. If necessary, adjustment is done by changing the adjusting shim. To do this, insert the shim tool as shown, hooking it under the camshaft and positioned so its raised center ridge contacts the edge of the valve actuating bucket. Rotate the tool down and toward you and depress the bucket.

 How-To

8. With the bucket depressed, remove the shim using a magnet or tweezers. Any oil under the shim will tend to hold it in place. If necessary, use a dentist’s pick to lift it from its seat. It will then pull free.

 How-To

9. The 2-valve GS750 engine uses 27.5mm-diameter shims. The shims come in 0.05mm increments from 2.15mm-3.10mm. Choose the proper shim by subtracting the readings and shimming up or down as necessary. If the valve is loose, you’ll need a thicker shim. If tight, you’ll need a thinner shim.

 How-To

10. Once the valve lash is correct, clean the valve cover gasket surfaces. Install the half moon end plugs, two on each side. If the new valve cover gasket has strengthening ribs as shown, cut them off before installing the gasket and valve cover. Install the gasket and plugs dry.

 How-To

11. Install the valve cover followed by the valve cover breather cover. The valve cover bolts on our engine were mildly corroded. We wire brushed them clean, then coated the threads with engine oil before installing them.

 How-To

12. Tighten the cover bolts evenly, working in a cross pattern. Install the spark plugs, side covers and ignition cover. Install the horn. Install the gas tank and reconnect the fuel and vacuum lines. MC

Replace 1970 Honda CB350 Fork Seals

replace fork seals 

The forks shown here are off our 1970 Honda CB350 project bike, which, given the state of the rest of the bike, we knew needed new seals at the very least and more likely new fork tubes. Stripping them apart we learned two things: First, early series 1968-1971 CB350 forks are amongst the easiest we’ve ever worked on; and second, yes, we need new fork tubes.

To be fair, our original fork tubes are perfectly serviceable. They might not be pretty, but they’re straight and the critical wear surface area where the seal makes contact is fine; it’s just the top third, hidden under the upper fork tube covers, that’s rusty. Although we’ll be swapping in a new set of fork tubes from Forking by Frank, what you see here is pretty representative of what you’ll see on most old survivors; fork tubes that are a little battle scarred and ugly, but still perfectly usable.

BikeMasterRemoving the forks is straightforward. First, remove the fork drain screws and drain the old oil. With the bike suitably supported, remove the front wheel and the front fender. The upper end of each fork tube is choked down and fits hard up inside the upper triple clamp. Remove the chrome bolts (these are also the fork oil caps) securing the tubes to the upper triple clamp. Next, loosen the clamp bolts on the triple clamps, two on each side, and pull the fork legs out.

As always, we suggest having a shop manual on hand for parts identification and placement, and to confirm proper torque specs.

replace fork seals 

1. Early series CB350s used external fork springs. The springs aren’t noticeable installed as they’re covered by the upper fork tube sheet metal. They fall loose from the fork as soon as you remove the fork tube from the triple clamp assembly. Note the position of the plastic bumper inside the spring; it should be installed at the top or upper end of the spring. Once the spring is removed the chrome fork leg cover around the top of the fork leg will pull straight off.

replace fork seals 

2. Getting back to the task at hand, the next step is to remove the snap ring securing the oil seal and fork bushing — and hence the fork tube — to the fork leg. If the snap ring is rusty, spray it with penetrating solvent and let it soak for a few hours before trying to remove it.

replace fork seals 

3. With the snap ring removed, hold the fork leg, compress the fork tube then give the tube a sharp pull out. It may take several tries before the seal releases and the fork tube, complete with seal and fork leg bushing, pulls free from the fork leg as shown above.

replace fork seals 

4. With the fork tube separated inspect all parts. As noted in the text, our fork tubes were rusty at their upper ends but critical wear surfaces were still OK. This photo shows all the parts of the fork, with the exception of the orifice tube inside the fork leg, which we left in place during cleaning. It’s a very simple design.

replace fork seals 

5. During cleaning, carefully inspect the fork tube bushing for excess wear. Both bushings on our forks showed wear, but fortunately not enough to warrant replacement as new bushings are expensive and getting harder to source.

replace fork seals 

6. Thoroughly clean all parts in a parts washer or with a suitable solvent. Spray brake and electric parts cleaner works fine. Dry with compressed air. We took a few extra minutes to clean and polish the fork legs and the chrome fork leg covers. The fork tubes cleaned up well, although we’ll ultimately replace them. Here are our parts ready to go back together.

replace fork seals 

7. Loosely assemble the fork tube in the fork leg. Slide the fork bushing over the tube and push it down into the fork leg. Smear a light coat of grease or fork oil on the lip of the new seal and around the top of the fork tube and carefully push the seal over the tube and down to the fork leg as shown.

replace fork seals 

8. Next, making sure it’s straight, push the fork seal into the fork leg as far as you can using finger pressure. To drive the new seal home, take an old seal and slide it over the fork tube and on top of the new seal. You’ll use this as a driver.

replace fork seals 

9. Place a suitable piece of 1.75-inch PVC pipe on top of the old seal and drive the new seal home with several blows from a rubber hammer.

replace fork seals 

10. Confirm the seal is fully seated by inspecting the groove for the snap ring, which should be fully visible. Next, reinstall the securing snap ring. 

replace fork seals 

11. Exposed to the air, the fork springs can and do rust over time. Looking for a quick and easy way to remove the rust, we soaked our springs in a small bucket of Evapo-Rust rust remover. The spring at top is untreated while the lower one shows the results after a 24-hour soak. All we had to do was rinse it off and we were good to go.

replace fork seals 

12. With the seal installed, push the chrome fork leg cover back on, short side down. Next, slide the spring with plastic bumper over the fork tube. Install the forks in the triple clamps. Tighten the top chrome bolts, then the upper triple clamps. Remove the top bolts and fill each fork with 200cc of fork oil. Honda originally specified 10w30 engine oil but we prefer a straight-weight like Spectro 15w fork oil. Install the fender and wheel. Push the front end down several times, then tighten the lower triple clamps. MC

Change a Motorcycle Tire

1974 Yamaha DT125 Enduro

Changing tires isn’t a particularly fun or rewarding job, but like many chores, it’s a good one to know how to do when the occasion arises. Frankly, this issue’s How-To reminded us of how rusty we are at the job, rarely changing our own tires because A) it’s usually rolled into the cost of a new tire and B) you still need to get the wheel and tire balanced once you’re done, also usually rolled into the cost of a new tire.

If you want to change your own tires, there are a few tools we suggest having on hand. You won’t use them all at one time, but they’re worth having and they’re relatively inexpensive, to boot. Our lineup consists of a valve core tool for removing and installing valve cores ($4.95), a valve repair tool for cleaning threads on damaged valve stems ($5.95), a puller for fishing the valve stem on a tube through the rim hole ($11.95), a valve stem mounting tool for tubeless rims ($17.95), a set of tire irons ($25.95 each for heavy duty irons — extra leverage and strength is always nice — or $13.95 each for standard 11-inch irons — great for smaller tires and they’ll fit in a tank bag), and a set of rim protectors ($7.95 for the pair and especially appreciated with aluminum or cast rims). We picked up everything you see here from BikeMaster, including the new tube ($12.95).

BikeMasterThe biggest challenge can be getting an old tire off. Tires get stiffer with age, and tire beads have a tendency to weld themselves to the rim over time, making it hard to break the bead and pull the tire over the rim. It’s not unusual to have to cut off stuck and stiff decades-old tires to avoid damaging the wheel rim.

The front tire on our 1974 Yamaha DT125 Enduro didn’t put up much of a fight. With the valve core removed and the tire deflated, the bead broke with simple downward hand pressure. When that doesn’t work, push a tire iron between the bead and rim in one spot, then pry the iron down to push the bead down and off the rim, working around until it falls loose. Rim protectors are great if you’re worried about marring your rim. We used them for the dismount, but we didn’t bother with them during remount as our new tire went on easily. If there’s a colored balance dot on your new tire, line it up with the valve stem hole. And finally, give the bead a light coating of soapy water or tire mounting paste to help the new tire slip over the rim.

Tools needed for changing a motorcycle tire

1. Tools of the trade: You won’t need them all at once, but it’s nice to have a good selection of tire tools ready at hand. The tool below the inner tube is for installing valves in tubeless rims.

Changing a motorcycle tire

2. The first step is breaking the bead. With smaller tires you can usually do this by hand, pushing down on the deflated tire’s sidewall until it lets loose from the rim. If it won’t, break it loose with a tire iron placed between the rim and the bead, prying down on the iron.

Changing a motorcycle tire

3. If you use rim protectors, put them on the rim first. Next, slip the tire irons behind the tire bead, then pry back and pull the bead up over the rim. Pry the bead up with one iron, then hold the iron down, following with the second to pull the bead up over the rim.

Changing a motorcycle tire

4. Old tires can be a bear to stretch over the rim, but ours pulled over fairly easily. Once started, reposition the rim protectors as needed and work around the rim until the tire is free.

Changing a motorcycle tire

5. With one side off you can often push the other bead over the rim without using the tire irons. With the bead in the rim recess and the tire at its loosest, push it off as shown. It will usually roll off.

Changing a motorcycle tire

6. With the tire off, remove the rim strip (ours had completely deteriorated) and clean the inside of the rim. Our steel rim had a quite a bit of rust. If this was a daily rider we’d consider replacing it, but since this bike only sees occasional field use we felt comfortable just cleaning off the loose scale.

Changing a motorcycle tire

7. Although we didn’t bother, you can coat the inside of the rim with a rust treatment or use rust-resistant paint to help stave off future rust. Once the rim is cleaned stretch a new rim strip into place, making sure to center the hole for the valve stem.

Changing a motorcycle tire

8. Lubricate the bead, position the new tire and push the inner bead over the rim in one spot. Work around the rim evenly left to right, pushing the bead down by hand. Use a tire iron to stretch the last bit of the tire over the wheel rim.

Changing a motorcycle tire

9. With one bead over the rim, put the tube inside the tire, with the valve centered on the valve hole in the rim. Feed the tire valve through the rim and secure it loosely with its retaining nut. Make sure the tube is inside the tire and rim so it won’t get pinched when the bead is pushed down in the next step.

Changing a motorcycle tire

10. Starting at the valve, push the bead down into place by hand. Work around the tire evenly left to right, pushing down and finishing with a tire iron to stretch the last bit over the rim.

Changing a motorcycle tire

11. Tighten the valve stem retaining nut and install the valve core. Air up the tire, then remove the core and deflate it. Check that the bead is evenly seated on both sides. Reinstall the core, air the tire up to the appropriate pressure and install the valve cap.

Replace Pre-Unit Royal Enfield Primary Chain

Keith Fellenstein's 2002 Royal Enfield Bullet

Always turning and pulling a load, the primary chain is yet another oft-ignored motorcycle maintenance item. Fortunately, they’re usually quite robust, and outside of regular tension adjustment they typically require little attention.

Hidden from view as they are, it’s not surprising that many owners ignore them, yet they do need occasional replacement. True to its early-Fifties British roots, like many British motorcycles Royal Enfield’s familiar single-cylinder Bullet employs a separate engine and transmission with a chain-driven primary drive — as do even the newer “unit” engined Enfields; they may have fuel injection, but they still employ some old school technology.

BikeMasterThe primary chain plays an important role in your powertrain, transferring engine output from the crankshaft to the clutch, which then feeds the power to the transmission and finally the rear wheel. The basic design has stayed the same for decades, mostly because there’s really not much to change. Different self-adjusting schemes have been tried to help limit user maintenance, but the vast majority of chain-driven primaries rely on a simple manual chain tension adjuster.

That’s fine, but most adjusters are located inside the primary cover, so primary chain adjustment requires removing and refitting the primary cover. It’s not actually much of a chore, but it’s enough to keep some owners from ever checking for primary chain stretch. That’s a problem, because as the chain stretches it starts whipping up and down, aggravating wear to the chain and sprockets and presenting the real possibility of the chain hitting the inside of the primary case and damaging it.

For this How-To, we replaced the primary chain on Q & A man Keith Fellenstein’s 500cc 2002 Royal Enfield Bullet. As you’d expect from him, Keith’s given his old Enfield model care, including adjusting the chain regularly, so he was a little surprised when he found the primary chain stretched to its limit after 17,000 miles. Normally, we’d expect at least 20,000 miles, and we know Norton and Triumph owners who have done double that.

Our Enfield’s primary chain was so stretched it was about to start rubbing on the cover support pillar.

Fortunately, this is a pretty easy job and quite within reach of the average weekend warrior. Outside of a torque wrench and a block of wood (more on that later), it doesn’t require any special tools, and even if it’s your first time you can probably complete the job in a relaxed morning. That’s assuming, however, you don’t find any surprises once inside. Those surprises can include worn clutch plates (clearly visible when you remove them); worn engine and/or clutch sprockets (highly unlikely); and a worn chain tensioner shoe (ours was fine). If you find any of those issues, your best bet is to just stop, order up the necessary parts and finish the job after the parts arrive.

Back to that torque wrench and block of wood: The crankshaft and clutch hub nuts are only torqued to 40ft/lb; the alternator stator and clutch cover bolts are torqued to 7ft/lb. The block of wood is to lock the clutch drum, as you’ll see in the How-To. As always, we suggest having a good shop manual on hand for parts identification and placement, and to confirm proper torque specs.

1. Disconnect the battery as a general precaution. Remove the left footpeg, then remove the cotter pin securing the rear brake rod from the brake lever. Remove the brake light switch bracket followed by the brake rod and let the brake lever swing down out of the way.

2. There is no drain plug for the primary, so make sure to have a large drain pan. Remove the large nut in the middle of the primary cover and gently remove the primary cover, letting the oil drain. Clean out any excess oil.

3. Remove the three nuts and washers securing the alternator stator. Remove the stator and hang it from some wire, as shown. Remove the nut and lock washer securing the alternator rotor. We were able to break it free easily with a hammer blow to the socket wrench.

4. With a felt tip pen, mark the rotor face for orientation when reinstalling. Remove the alternator rotor. Locate the Woodruff key that keys the rotor to the crankshaft and remove it. Finally, remove the spacer that fits between the rotor and the primary drive gear.

5. Next, loosen the adjuster so there’s slack in the chain, then remove the lock nut and nut securing the adjuster to the primary case. Remove the adjuster.

6. Working in a cross pattern, remove the three bolts and washers securing the clutch pressure plate to the clutch hub. Note there are six springs total, three that fit on the outer securing plate and three on the pressure plate.

7. Remove the pressure plate followed by the clutch discs. The discs alternate between fiber and steel; keep them in order to ensure they’re reinstalled correctly. Next, remove the clutch pressure plate pushrod, as shown here.

8. The clutch hub nut is a locking-type nut and can require a little more effort to remove than the alternator rotor nut. It can be removed with a socket wrench by putting the transmission in first gear, then reinstalling the brake lever and locking the brake so the transmission won’t spin. Or, as we did, you can simply spin it off using an air wrench.

9. Remove the nut and spring washer. The clutch hub will often just wiggle loose, but ours was stuck tight. To remove it, we used a universal puller, using the three clutch cover bolts to secure it to the clutch hub then slowly tightening the center bolt against the transmission input shaft. Doing it this way made it come loose easily.

10. Finally, lift the front sprocket, clutch hub and chain away from the engine as an assembly. At this stage, check the teeth on the sprocket and hub carefully. Ours were fine.

11. Confirm the replacement chain is correct. Wrap the chain around the sprocket and clutch hub and reinstall the sprocket and hub to the engine in the same way you removed them. You may have to spin the transmission input shaft until the splines line up properly.

12. We didn’t reinstall the brake linkage to tighten the clutch hub nut. Instead, we cut a piece of hardwood to fit so it would lock the inner and outer hubs together as we tightened the hub nut against engine compression. Crude, yes, but a good trick and it works well.

13. After installing the lock washer and clutch hub lock nut and torquing to 40ft/lb, reinstall the clutch cover pushrod.

14. Reinstall the clutch plates in the same order as they were removed, followed by the pressure plate. Place three of the springs on the clutch cover locating studs and three on the cover securing plate. Using a cross pattern, bolt the securing plate down to 7ft/lb.

15. Install the rotor spacer and the Woodruff key. Install the alternator rotor, then the lock washer and nut and torque the nut to 40ft/lb. Install the alternator stator and tighten the securing nuts to 7ft/lb. Check the air gap between the rotor and stator. It should be 0.010-inch all around. If it’s not, loosen the nuts and adjust the stator as necessary.

16. Next, make sure you have proper chain tension. Our book said there should be 1/2-inch total deflection. Adjust by loosening the adjuster lock nut and screwing the adjuster up or down. It takes a minute to find the right spot. Finally, spin the engine over and check to make sure there are no tight spots.

17. Clean the mating surfaces and reinstall the outer primary cover. Remove the level screw on the side of the cover and fill until oil just starts to drain out. Keith uses ATF in his Enfield primary,  but not everyone agrees with that.

18. Finally, reinstall the rear brake rod to the brake lever, followed by the brake light bracket, washer and cotter pin. Next, reinstall the left footpeg. Finally, if you disconnected the battery, hook it back up. You should be ready to go.