Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

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Keith Fellenstein

Synchronizing Carburetors

Q: I have a question regarding “syncing” carburetors. Some gurus can synchronize them by ear, and I’ve met mechanics who are just incredible and use all their senses to accomplish the extraordinary. How does syncing contribute to performance? Usually, I measure the slides with the carbs on the bench with drill bits, selecting a size that just fits between the horn and the slide, and average the two (or three or four). Are there unique factors with individual cylinders that require a “power on” vacuum test, or am I good to go? I suggest to your readers to start a scrapbook, as I have with your articles. I look through the table of contents when I’m stumped and lo and behold, you’ve already done the work for me. — Mike Peterson/Chetek, Wisconsin

A: In a multicarburetor setup, synchronizing ensures that each carburetor is providing the same amount of fuel and volume of air at the same rpm. That in turn keeps the cylinders working in unison, with no cylinder working harder than the other. This, along with engine timing, contributes to an engine that runs smoothly and accelerates at its best. What you’re describing is generally referred to as “bench syncing” and it is a necessary first step whenever you have disassembled a multicarb setup. Depending on whether you’re dealing with CV carbs or direct slide carbs, the technique is a little different. On direct slide carbs such as the Amals on British Triumphs and Nortons, you can use the wire-gauge method you outline, using the idle stop screw to get the initial opening the same. Assuming you have the carbs off the bike, you can try this. For CV carbs, I keep a few 1/4-inch ball bearings on hand. I place them in the throat of the carburetor and open the butterflies. If the balls drop simultaneously, the butterfly valves are synced. Once that’s done, you need to make sure the cables are pulling identically for all the carbs. You can check at wide open throttle to see if the slides are all the way up to the same degree, but you can’t really tell if the intermediate range is matched. This is the range where you’ll spend most of your riding time. The easiest way to do that is with vacuum gauges. MC


Keith Fellenstein

Triumph cylinder

Q: I just started a total restoration of a Triumph Trident. The engine took on some water at some point, and I’m not sure if it is going to be worth the investment to rebuild it. My first question is, what size and type of wrench do I need to take off the cylinder? These are odd looking bolts. Also, the piston rings are fused to the cylinder walls. I have them soaking in penetrating oil, but so far I have not had any luck in freeing them up. — Jim T./via email

A: Those odd-looking nuts have 12 points and you need a box wrench to loosen them. A 13mm wrench works, but a 1/2-inch wrench fits better. Sometimes you have to grind down the thickness of the wrench to fit in the narrow space between the bottom fin and the nut. You may also have to grind down the circumference of the wrench for clearance between it and the cylinder base. For your seized engine, a 50/50 mix of ATF and acetone is reported to work well as a penetrating solvent. Applying heat cyclically can also help break things loose. If you’ve recently used the ATF/acetone mix, use a heat gun rather than a torch; a torch would be OK after the acetone evaporates. Don’t use MAP gas; it gets hot enough to melt aluminum. Propane should be safe if you keep the torch moving. Aluminum gives no warning; one second it looks fine and the next it’s melting. Alternating between hot and cold should create enough expansion differential that the rings will eventually break free of the cylinder walls. Applying a little torque to the crankshaft nut helps break it free, too. Remove the primary cover to access the crankshaft nut. Patience is your friend. MC


Keith Fellenstein

Honda carbs

Q: I have a 1981 Honda GL500 with 61,225 miles. The bike starts and runs up to about 4,500rpm, then breaks up and runs really rough. In terms of problem solving, I’ve done all of the typical tune-up items including replacing the air and fuel filters, and the spark plugs. I have also cleaned the carburetors. None of this solved the problem. After research on a CX500/GL500 forum, a member suggested replacing the spark plug wires and the coils, which I changed. This didn’t solve my problem either. I then located your article on CDI units and their function, and it seemed like the logical solution since I’m losing high-end. After changing both to a set ostensibly from a low-mileage bike, it still didn’t solve the problem. Do you have any suggestions? — Phil Quattrone/via email

A: Running problems with the CX/GL 500/650 bikes can be difficult to pinpoint, as there are several troublesome systems that can cause your symptoms. One of the toughest problems to solve is carburetion. The CX/GL Keihin carbs can be difficult to clean completely. See our article in the March/April issue about the CX500. The owner of that bike has written a comprehensive book on thoroughly cleaning and disassembling CX/GL Keihin carburetors. MC


Keith Fellenstein

Leaking Fork

Q: I just completed a 5-year restoration of my 1971 Triumph Trophy 500. My problem is a leak on the left front fork. It appears to be leaking from the small bolt that holds the bracket for the wheel. I’ve tried some black RTV on the threads and also teflon tape with no success. What can I do? — Al Rieske/Reno, Nevada

A: There are three potential leak points here. Two of them are likely and one unlikely. The first is the drain screw and sealing washer. You can get fiber or copper washers for these; I prefer to use copper. Make sure the sealing surface on the fork leg is clean and flat. Often, if a fork slider has been powder coated, the coating will cover the sealing surface, making it difficult for the sealing washer to seat. The second likely source is the bolt at the bottom of the fork securing the cone-shaped restrictor inside the fork slider. The restrictor provides a hydraulic lock at the extreme end of fork travel, stopping the forks from bottoming out metal-to-metal. There is tremendous oil pressure in that space at the extreme end of fork travel. If the aluminum sealing washer is worn out, you will often find a leak here. You may be able to retorque the bolt holding the restrictor, but a permanent solution will be a new crush washer. The final and least likely leak source would be if the studs holding the axle cap had been over-tightened and either cracked the stanchion or pushed through into the oil chamber at the bottom of the stanchion. NOTE: A follow-up email from Al indicated the leak was indeed from the bolt securing the restrictor. Retorquing the bolt fixed the leak. MC


Keith Fellenstein

BMW Wiring

Q: Can you please help me with the correct connection of the wires on the ignition coils of my BMW K100RS? — Corrie/via email

A: The brown wire is common to both coils, as is the green/yellow wire. The blue/black wire goes to the front coil while the black/red wire goes to the rear coil. The front coil is for cylinders 1 and 4. Looking at it from the bottom, the blue/black wire is at the top, brown in the middle and green/yellow at the bottom. On the rear coil (cylinders 2 and 3) the top is black/red, the middle brown, and the bottom green/yellow. MC


Keith Fellenstein

BSA diagnosis

Q: I recently purchased a 1968 BSA Firebird Scrambler. It took many tries to finally get it started. When it finally did start, there was a loud backfire through the mufflers. The next kick got it started for good, but it wouldn’t idle down very far. Thinking it was probably stale gasoline, I rode it for a few blocks to check out the clutch, brakes and transmission. All seemed fine, except for a tendency to die when the throttle was released. Long story short, it died right after I returned.

The next day, I tried starting it again. It wouldn’t fire. Thinking it was the battery, which was very weak, I installed a new battery with a full charge. Still, there was no spark at either plug when cranking it with the ignition switch on. I removed the points cover to see if there was anything obvious that would cause this. Much to my surprise, it had an electronic ignition. I don’t know the brand, but I’m guessing it’s a Boyer. Now I’m really stumped. I know it must have been properly installed as the engine did start and run. Where would you suggest I start in diagnosing the problem? For what it’s worth, the white wire to the Zener diode is disconnected. The diode is mounted to the frame under the fuel tank, but the wiring schematic shows the white wire is not used on the 1968 Firebird Scrambler model. The polarity is correct, with a positive ground, unlike many that have been wired incorrectly. Any help you might be able to give will be greatly appreciated. There aren’t any BSA mechanics in my area! — John Botts/Ponca City, Oklahoma

A: You can easily test for spark with a Boyer. First, pull a spark plug and lay it on the cylinder head to ground it. Next, disconnect the trigger wires from the unit under the points plate. With the ignition on, touch the ends together. Every time you touch the trigger wires together you should get a spark. As for the wire to the Zener being disconnected, look around under the tank or seat to see if someone installed a different regulator/rectifier like a Podtronics. If you have one of those installed you disconnect the Zener diode, as the Podtronics device does the work of both the rectifier and the voltage regulator. I’d also check to make sure the rotor under the pickup plate is tight on the camshaft end. If it is loose your timing becomes erratic at best and your bike becomes unstartable at worst. MC


Keith Fellenstein

Hard starter

Q: Why is my 2002 Triumph Bonneville so hard to start? If it has not been started for a few weeks, it is a gorilla to start. The carbs have been re-jetted to a 130 main, a 42 pilot jet, with the needles using two shims. I have replaced the spark plugs and charged the battery. What is the problem? — Rick Romanesque/via email

A:  Your 2002 is different from the old Triumphs I’m used to working on, but since I recently had one the same age through my shop for a similar issue, I’ll take a shot at it. The one I worked on had sat for three years without being run, so the carbs were completely gummed up. But worst of all, and not found the first time, the petcock filters inside the tank had totally disintegrated, allowing all kinds of junk to come back in and clog the just-cleaned carburetors. Once the petcock was replaced and the carburetors cleaned (again), the bike started and ran as it was supposed to. So check your fuel flow through the petcock and the quality of the fuel flowing through the petcock, then make sure that all the carburetor passages are clean, including the fuel enricher/choke. MC

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