Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

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Keith Fellenstein

Frame powder coating

Q: I have a question that I thought you might be able to help me with, or at least steer me in the right direction. I’m restoring a 1984 Kawasaki ZX750E Turbo, and I’d like to have the frame powder coated. The problem is the factory applied federal standards stickers near the steering head. They would be destroyed in the powder coating process, but they would also be destroyed in trying to remove them. Do you know if these have to remain on an older, restored motorcycle, or can they be removed? I could take photos of them and have them reproduced and re-apply them, but I don’t know if that’s legal, or if legality needs to be considered in this case. Do you have any info on this in your experience? I appreciate any help you can give me, whether directly or from another source that you know. — Bill Adams/Pleasant Plain, Ohio

A: You may not like my answer, but here goes. I’m not a fan of powder coating frames for a number of reasons, some mechanical and some cosmetic. Let’s start with the mechanical. The thickness of the coating can interfere with refitting engine and frame components if you are not completely obsessive in masking off all the bolt bosses and threaded openings before having the frame done. More significantly, the flexibility and thickness of the coating can mask corrosion under the coating, and hide fractures in welds and other areas of the frame that you don’t want hidden. Those are my major concerns. A minor concern is that when, not if, the frame gets chipped, you can easily touch up paint, but powder coating, not so much. To your main concern, I don’t know of a way to protect the frame labels from the heat needed to fuse the powder coating. With many states increasing scrutiny of VIN numbers on older bikes, it becomes imperative to make the inspector’s job easier so you don’t spend time arguing with them over proper frame/engine numbering. My recommendation is to find a good paint shop and enamel the frame. Thanks for reading my column, and I’m sorry if this isn’t what you wanted to hear. MC


Keith Fellenstein

KTM 250MX Starting Troubles

Q: I have a 1986 KTM 250MX and am unable to start it using the kickstarter. I have installed a new spark plug, and the carburetor is clean. The timing is correct, and I’ve installed a new power reed. It will start if it’s pulled behind a four-wheeler. — Clair Raible/Black River, New York

A: Without seeing the bike, I can only guess, but for a 2-stroke, starting troubles that are overcome by pull starting usually point to bad crank seals. Once you have it running, will it idle reasonably, or does it die? That’s another pointer to crank seals. If you can do a pressure test, you should be able to pinpoint the problem easily. If it idles once you have it started, it may be poor compression. Again, a pressure test will give you the answer. See our How-To in the March/April 2015 issue or online. MC


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

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