Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

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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


Keith Fellenstein

Cool runnings

Q: I have a problem when checking the cooling level in my coolant tank. I suppose when the bike was new the tank was more or less clear and any color of coolant could be readily seen. Over the years, the plastic has degraded to the point where the coolant is nearly impossible to see. In order to check the coolant level, I have to get down on all fours to view the tank and shake the bike so the coolant can be seen moving. At 70 years of age, this is hard to do. I could simply install a new tank if one is available, but is there anti-freeze out there that is not yellow in color? Any suggestions? — George W. Miller, Jr./via email

A: I have this same problem with a number of plastic containers, from coolant tanks to batteries, and there is no easy fix that I can see. What I do that helps me to see the level is find a way to shine a strong light from behind or beneath the container and let that illuminate the fluid level enough to see. If there is no way to get a flashlight behind the tank, maybe one of those thin, flexible lights can be snaked up behind the tank. Good luck! MC


Keith Fellenstein

Norton luggage rack

Q: I’m trying to find a sissy bar and luggage rack for my 1975 Norton Roadster, to no avail. It could be either of the two or a combination sissy bar/rack. It doesn’t need to carry much weight, either. The items I’d carry are an extra helmet and rain gear. Any suggestions? — Mitch Zyman/Merrick, New York

A: There are a couple of options, one of which we did an article on a couple of years ago. Here’s the link to that one. Also, check out Andover Norton’s website accessories page. Scroll down a bit and you’ll find their luggage racks. MC

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