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.
An 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
Removing 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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).
The 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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
The 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.
Owners who shy away from rebuilding their own carburetors might be surprised to find it’s often a fairly straightforward proposal. The carburetors on most of our old bikes are quite simple, and experience says that success hinges on two key issues: cleanliness and thoroughness.
It takes more than a simple strip-down and cleaning with a can of carburetor spray to get good results. To do it right, you have to thoroughly soak and clean the carburetor body so that any trace of dirt is removed. That’s sometimes easier said than done, as modern carb cleaners are much milder than their forebearers and less effective at dissolving dirt and other deposits.
Fortunately, ultrasonic cleaners keep getting more affordable, with 6-liter units (the smallest we’d suggest) running around $120. Briefly, an ultrasonic cleaner uses high-frequency sound to create pressure waves. The pressure waves “tear” the liquid, creating millions of microscopic cavitation bubbles, and as the bubbles collapse they release enormous energy, agitating the liquid. Thus agitated, the liquid scrubs whatever surface it contacts, removing dirt and scale. Ultrasonic cleaners are remarkably effective, and they don’t depend on toxic chemicals: The cleaning solution is typically just tap water with an ounce or so of dish detergent to act as a surfactant.
Our project 1970 Honda CB350’s Keihin constant velocity (CV) carbs were particularly nasty, so we took a two-step approach, first soaking the carbs overnight in a can of Gunk carburetor cleaner, followed by a trip through our ultrasonic cleaner (see photo above). This worked well; the Gunk dissolved the major dirt and the ultrasonic cleaner finished the job, leaving the bodies looking clean and ensuring the passageways are clean. If you don’t presoak, spray the carbs with cleaner to remove the worst dirt before they go in the ultrasonic bath.
As removed: Years of sitting had not been kind.
Stripping these Honda Keihin constant velocity carbs isn’t particularly challenging. The important thing is to pay attention to which jets — there are three — go where and, if you’re replacing the CV diaphragms, properly aligning them on the slide during installation.
We got our master overhaul kit from Sirius Consolidated. The kit, which retails for $202.50, includes new floats, new chrome caps for the CV diaphragm, new diaphragms, new float bowl and cap screws, float bowl gaskets, new needle and seats, plus every jet and rubber plug for both carbs. All of the components are made in Japan. At first glance, $202.50 sounds like a lot of money for a carb kit, but the Sirius master kit is remarkably comprehensive and seems to us money well spent.
One note: While this how-to focuses on the major points of CB350 carb overhaul, space restraints always mean skimming over some details. Further, as we were working on a mostly bare bike we didn’t have to remove much to get to the carbs, so you’ll want to consult your manual.
As always, we suggest having a good shop manual on hand for parts identification and to aid in removing and reinstalling the carbs on your engine.
1. The first step is removing the carburetors. Our beater project bike was already stripped of most of its hard parts, so for us, this step was simple. If you’re unsure how to proceed, consult your manual. Note the connecting choke strap; disconnect it from the left carb before removing the carbs.
2. Next, remove the diaphragm cap and spring. The diaphragm is attached to the carburetor slide. Our slides were stuck. To free them, we lifted the diaphragm rubber up and sprayed a little brake cleaner around the slide and let it soak. We came back to it after we removed the float and jets.
3. Remove the float bowl and turn the carburetor upside down. Using a small punch and hammer, gently drive the float hinge pin out and remove the float. If the pin is stuck, clean the area with brake or carb cleaner; it should only take a few light taps to free it.
4. With a pair of pliers, twist the slow speed and needle jets. If they’re stubborn, spray them with brake cleaner. Once they’re loose, pull them out along with the retaining spring clip. Remove the rubber plug covering the idle jet next to the slow speed and needle jets.
5. Next, remove the retaining screw and bracket securing the float needle and seat assembly. Using a pair of pliers, twist and release the seat assembly. If it’s stubborn, spray the area with brake cleaner. It should pull out.
6. With the jets out we removed the slide, spraying it once more to break it free from the carb body. You can see the fuel residue that was holding it tight.
7. With the slide removed, look down into the carburetor body. The two brass points are the needle jet and slow speed jet emulsion tubes. Using a small punch, drive them out through the jet chambers in the float bowl. Again, a little brake cleaner can help break them free.
8. Here you can see the emulsion tubes, knocked loose and ready to fall out. The only jet that screws in is the idle jet, which is under the rubber plug we removed in step 4. Clean the bore with brake cleaner. Unscrew the jet with a small screwdriver; it should drop out.
9. Next, we replaced the rubber diaphragms and the needle. Using a pair of needle-nose pliers, remove the retaining clip inside the slide, followed by the plastic needle retainer and, finally, the needle. Note the orientation of the plastic retainer; it goes in flat-side down.
10. The diaphragm is clocked to the slide, with the diaphragm locating tab as the reference. Roll the diaphragm at the tab over the slide and make a mark on the slide as shown. The tab on the new diaphragm must be clocked to this same location when installed.
11. Next, using a thin metal blade, slowly work your way around the diaphragm’s plastic retainer, loosening it lightly with every pass. After a few passes, the retainer will be loose enough that you can pull the diaphragm out of its seat and off the slide.
12. Slip the new diaphragm over the slide and align its locating tab with the mark you made on the slide. Using your thumbnail, push the diaphragm into its seat. Work around the slide until it’s fully seated. Push the retainer tight.
13. Install the new slow speed jet and needle jet emulsion tubes. This photo shows the slow speed jet emulsion tube being installed. They both drop into place. Screw in the idle jet and cap it with the new rubber plug.
14. Next, give the needle and slow speed jet O-rings a very thin smear of grease. Locate the jets in the slots in the retainer spring and push them home. They are different sized and will fit only one way. Install the new needle and seat and its retainer.
15. Install the new float and check the float height. Our manual specified 26mm. To check, hold the carburetor nearly vertical so the float is touching but not pushing the needle. To adjust, gently bend the float tang where it touches the needle up or down. Install the float bowl with its new gasket and screws.
16. Drop the new needle into the slide, followed by the plastic retainer (flat side down) and the retaining clip. Drop the slide into its bore, aligning the diaphragm locating tab with its corresponding notch in the carburetor body. Install the spring and cap and secure with the new screws.
17. Install the idle/air screw. A rough preset is to screw it in until it lightly seats, then back out one full turn.
18. Reinstall the carbs, making sure the choke connecting strap is properly located and secured. For initial set, loosen each idle screw until it no longer contacts the throttle. Screw it in until it just touches the throttle, then turn it one more full turn. Refer to the manual for final adjustment with the engine running.
The clutch: Always used, often abused, it doesn’t get the respect it deserves. That’s somewhat ironic, given the clutch’s role in ensuring you get from A to B. Yet it’s somewhat understandable, as clutches are typically undercover and out of sight — and out of mind.
The typical motorcycle multi-plate wet clutch is made up of two sets of alternating discs; a fiber-faced set driven by an outer clutch drum and a set of metal discs keyed to an inner clutch hub. A spring-loaded pressure plate bolted to the inner clutch hub clamps the discs together, coupling the clutch drum and clutch hub together to transfer power from the engine to the transmission, and then the rear wheel. When you pull on the clutch lever, linkage pulls or pushes the pressure plate, overcoming spring pressure and uncoupling the clutches so you can shift gears.
This constant coupling and uncoupling causes wear. If you’re nice to your clutch, bringing engine speed up lightly for launch and letting the clutch engage quickly and cleanly, the plates can last indefinitely. Stoplight drag races, on the other hand, are clutch killers. The high revs induced for a high-speed launch translate to longer and harder engagement. The clutch plates slip and heat up, resulting in wear of the fiber plates and overheating and warping of the metal plates.
The previous owner of our 34,000-mile 1981 Suzuki GS1100EX had installed “heavy duty” clutch plates. Clutch feel was poor, however, and there was no adjustment left in the cable, which was new. A tear-down showed overheated and blued metal discs and wearing fiber discs. More worrying, it also revealed a missing piano wire retainer for the innermost metal disc and an incorrect pressure plate bolt. That missing piano wire added about an hour to the job; replacement requires removing the inner clutch hub. Our advice? If the innermost metal plate is good (no signs of bluing) and the wire is in place, leave it be.
New metal disc (left) and overheated original.
For parts, we turned to clutch specialists Barnett Clutches & Cables, picking up a complete set of plates — fiber and metal — plus new springs. All in, our Barnett clutch discs and springs cost $218. That may seem like a fair piece of change, but given Barnett’s reputation, quality is not going to be an issue. We sourced the missing clutch plate piano wire retainer, outer cover gasket and pressure plate bolt/washer from bikebandit.com for $19. Importantly, much of the process outlined here applies to other air-cooled Suzuki inline fours, including the GS650, GS850, GS1000, GS1100 and GS1150 from 1978 through 1986, as the basic design was used across the range. The GS750 models used a different setup.
This job is completely within reach of the average weekend warrior. Replacing the piano wire retainer complicates the process, but disassembly is otherwise straightforward. Draining the oil is optional (we doubled up for an oil change) — the clutch is on the right, the sidestand on the left; positioned on the sidestand, no oil will drain out when you remove the outer clutch cover. As always, we suggest having a good shop manual at hand. Happy motoring!
1. We did an oil change at the same time, so we started by draining the oil with the engine hot, then removing the oil filter for replacement.
2. To begin, remove the 10mm bolt securing the rear brake lever. Before removing the lever, note the small alignment punch mark on the lever shaft; it aligns with the pinch slot in the lever.
3. Next, remove the pin securing the clutch cable to the clutch release arm. Remove the 10mm bolt securing the clutch release arm, then lift the release arm up off the release shaft.
4. Using the proper JIS Phillips driver, remove the clutch cover bolts, working slowly in a cross pattern. They can be stubborn, making an impact driver highly recommended.
5. The clutch cover screws vary in length. To ensure proper location, sketch out the cover on a piece of cardboard, mark and punch out the approximate screw locations and then transfer the screws onto the sketch.
6. With the screws removed, the cover should break free easily. If it doesn’t, reinstall the release arm and pull it toward you to “push” the cover off.
7. With the cover removed, loosen then remove the six clutch pressure plate retaining bolts and springs, working slowly in a cross pattern until the bolts with springs and limiter barrels come loose. Remove the cover and set aside.
8. Next, remove the clutch plates, starting with the fiber outer and ending at the final metal inner. The plates alternate from fiber to metal. Use a pick and magnet as necessary to aid removal. If the piano wire is in place retaining the innermost metal plate and the plate isn’t blued from overheating, you can leave the innermost plate in place.
9. The piano wire was missing on our clutch. The wire holds the innermost plate in tension against a cone washer and the clutch hub. The hub must be removed to install a new piano wire. To remove the hub, fold back the locking tab washer, then remove the 34mm nut securing the hub.
10. To install the piano wire, install the cone washer and a metal clutch plate on the hub. Lightly clamp the plate to the hub. Feed the wire around the hub in its slot, then push the spring ends through the keeper hole in the hub.
11. Make sure the wire is properly seated around the hub. Reinstall the hub, the tab washer and retaining nut. Put the transmission in gear. Have an assistant apply the brake and torque the retaining nut to 50.5ft/lb.
12. Before installing the rest of the plates, soak the new friction plates in clean engine oil. Barnett suggests a 1-3 minute soak. After they’ve soaked, hang them from a hook to drain off excess oil.
13. Install the plates, alternating from fiber to metal and so on, ending with a fiber plate. This is how it should look before the clutch pressure plate is reinstalled.
14. Although not strictly necessary, we opted to replace the clutch pressure plate springs as we think it’s good practice. The full set only added $22.16 to the project. We also replaced the incorrect retaining bolt and washer.
15. Reinstall the pressure plate using the new springs. You can see our replacement bolt at the 11 o’clock position. Working slowly in a cross pattern, tighten the bolts until they seat, then torque to 9.5ft/lb. Rotate the center release rack so its teeth face rearward.
16. Install a new gasket on the cover face. The cover has two locating dowels that will hold the gasket in place. Sealant is not necessary, just make sure the sealing faces are clean and dry. Gently tap the cover home if necessary.
17. Next, transfer the cover screws from the paper or cardboard locating template. Using the correct JIS Phillips driver, work in a cross pattern until they all seat evenly and securely. Reinstall the clutch release arm, aligned as shown, and connect the clutch cable.
18. To finish, install the new oil filter then fill the engine with fresh oil. Start the engine and check for leaks, then let the engine rest and top off the oil as necessary. Assuming there are no leaks, you should be ready to ride.
If you’ve replaced carburetor throttle cables on a vintage 4-stroke engine, you know that the basic process is relatively simple. The complicating factor for anything other than a single-cylinder engine is making sure the carburetor slides are synchronized to each other so they pull off idle evenly. If your carbs are adjusted properly before you replace the cables, getting the slides synchronized isn’t that difficult, as we describe.
Vintage Yamaha 2-strokes equipped with Yamaha Autolube oil injection have one more component. These bikes have a cable working in conjunction with the throttle cables, operating an oil injection pump and increasing oil pump flow as throttle is applied. As a result, a 2-cylinder Yamaha 2-stroke such as our subject 1973 RD350 has four cables — two carburetor slide cables, one Autolube oil pump cable and one throttle cable at the handlebar twist grip. Additionally, the multicable setup has a cable junction that links the top or twist grip cable to the carburetor and oil pump cables. The cable junction can fail, falling apart from years of use. On our bike, a previous owner had clamped and zip-tied the junction to keep it in one piece.
If any one of these cables breaks or is damaged, you’re likely looking at replacing the entire cable assembly, as individual cables are increasingly difficult to find. Yet replacing them as a set is a good idea, as cables are service parts that have to be replaced occasionally. Regular use takes a toll, leading to stretched cables and worn-out cable housing ends. Nonuse does, too,
resulting in sticking cables from years of sitting.
Original “repaired” cable junction was on its way out.
Fortunately, thanks to specialty parts providers like HVCcycle, complete cable assemblies for Yamaha 2-stroke twins are readily available.
Replacing the cables isn’t a particularly difficult job, and can be done by the average home mechanic in a few hours. You won’t need any special tools, but like so many of these projects, patience is the single most important ingredient to success.
Protocol is important in this project, as well: With the new cable assembly installed, it’s important to first synchronize the carburetor slide pull, followed by the oil pump pull, and finally by removing any excess cable play using the adjuster at the handgrip.
Doing this job revealed a few other tips. Tip 1: If your RD still has the original rubber carburetor air intake boot, replace it at the same time. It has to come off and it’s probably hard as a rock; getting old ones back on is a bear. Tip 2: As you install the new cable, make sure the cable ends fit into place properly. On our oil pump cable, a slight excess of solder on the cable end barrel kept it from properly slotting into place on the oil pump pulley. The fix was easy: Using a small hand file, we gently filed off the excess solder until the cable dropped smoothly into place. It didn’t take more than a few minutes to address.
A complete assembly from HVCcycle was $70.95, and the one we received was as good if not better in quality than the original. As always, having a good shop manual at hand will help you work through the process. Happy riding!
1. Disconnect the fuel line at the petcock and drain the gas tank into a container.
2. With the gas tank drained, disconnect the fuel crossover line and plug the line and the tank outlet. The crossover runs from the left to right side of the tank; you only need to disconnect one side to remove the tank.
3. With the gas tank off, unscrew each carburetor slide cover and remove the slide assembly. The accelerator cable slots into the bottom of the slide, held in tension by the slide spring. Compress the spring and release the cable.
4. With the cables removed from the slides, remove two screws securing the right control assembly and cable stay plate. Pull the top half away and unhook the cable from the twist grip.
5. With the accelerator cable disconnected from the twist grip, thread the upper cable down between the handlebar and instruments and pull it free. Note the cable routing guide on the left side of the steering head.
6. Next, remove the right side engine cover to expose the oil pump. Rotate the oil pump pulley counterclockwise and release the cable from the pulley. Unscrew the cable housing from the side cover and pull the cable free.
7. Next, thread the upper section of the new cable through the cable guide at the steering head and up between the handlebar and instruments.
8. Lightly grease the handlebar end and the cable barrel. The original cable has a rubber sleeve to hold the cable housing tight in the assembly. Reuse it if possible. Pull the cable through the lower half of the control assembly and secure it in the twist grip. Secure the two halves of the control assembly and the cable stay plate.
9. Next, route the oil pump cable through the side cover and screw the cable housing into the side cover.
10. With the cable housing screwed into place, rotate the oil pump pulley counterclockwise and secure the cable end barrel into the pulley.
11. Attach the accelerator cables to the carburetor slides. The easiest way is to first slot the cable into the slide, followed by the slide needle and retaining plate. The spring can then be wound around the cable, compressing it as it winds on, until it’s fully in place.
12. Install the slides in the carburetors, followed by the carburetor slide covers. Remove the right side panel, loosen the clamp securing the intake boot to the air filter housing and the clamps securing the boot to the carburetors and remove the boot.
13. Next, lift the seat and loosen the clamp securing the oil filler tube to the oil tank. Remove the two bolts securing the oil tank. Pull the oil tank far enough away from the frame to disconnect the oil breather hose. Next, remove and cap the oil supply outlet and tube and remove the tank.
14. To adjust the carburetors, first make sure both cables are slack and that the slides are resting on their idle stop screws. Using your fingers to feel the slides for movement, adjust the cable tension at each carburetor to take out most of the cable slack.
15. Using your forefinger and thumb, feel the carburetor slides for even lift as you pull the twist grip; they should rise simultaneously. If not, slowly adjust the cable tension on the “slower” slide until it lifts evenly with the other slide.
16. With the slides set, lock the cable adjuster at the carburetors by setting the lock nut on the housing.
17. Next, set the oil pump pull. Turn the throttle twist grip until the carburetor slides are just starting to lift, then adjust the oil pump cable housing until the index mark on the oil pump pulley lines up with the pin on the oil pump plunger.
18. To finish, reinstall the engine side cover, the air intake boot, the oil tank and right side panel, and the gas tank. Finally, remove any cable slack at the twist grip by adjusting the threaded cable barrel at the twist grip.