On replacing complex parts

Lots of motorcycle problems require us to replace the affected parts. Worn out tires, chains and sprockets, crash-damaged levers, leaking fork seals, etc. But there are two items people seem inclined to replace that rarely actually need it: the carburetors and the wiring harness.

Both of these assemblies are pretty complicated, at least in comparison to a tire, a chain, brake pads, or what-not. And that, I think, is the reason people decide to replace them — the foundation to understanding 12V electrics or carburetor theory takes time to build, and without that foundation, the only comprehensible solution is simply to replace the entire system. Don’t do it!

Real example: A customer brought us an early Honda CB750 on which the bike would usually go dead if he tried to turn on the headlight. He couldn’t find the root problem, so he assumed it must be the wiring harness and replaced that. (Not a trivial task by any means, but he did do it correctly, from what we could see.) When he was done, he had a lot of clean, new wiring, and when he tried to turn on the headlight… the bike would usually go dead. The actual problem was that a battery bolt had fallen into the back of the fuse box, causing a short between the headlight fuse and the frame. We ended up replacing the fuse box, since the problem had caused some of the fuse panel to melt away. (It took a while to chase this problem down, but working methodically with the correct wiring diagram will almost always lead you to a solution.)

Carburetor example: A customer had brought us his Yamaha XS650 cafe racer project for us to get the electrics all working. He lives a ways out of town, so when he ran into a problem getting the fuel system working properly, he took it to “some guy” closer to home. The guy, we’ll call him Fred, talked our customer into discarding the original carburetors and replacing them with Mikuni VM34’s. (This is a popular thing to do, but if the engine is stock, it’s almost never really necessary.) Fred then proceeded to “tune” the VM34’s beyond all hope of proper running. We spent two hours and a bit of internet and catalog research to repair all the problems and install the appropriate jets. (Idle speed screws were damaged and driven in tight enough to strip the heads, slides were in backwards, main jets were for another brand of carburetor… Poor things never had a chance!) It rode well enough afterwards that I extended the initial road test by five miles, enjoying the day and giving us a better read on the spark plugs. (Tester’s note: an XS650 with clip-on handlebars and a seat made of half-inch thick foam rubber will shake a body numb in under eight miles. Hopefully the next cafe is closer than that!)

We do sometimes replace carburetors, by the way. We replaced one a couple years ago on a Honda Twinstar CM200T, not because the carburetor itself had a problem, but because the original jet needle was badly worn. The only replacement jet needle we could find was $30 needle shipped from the UK, so instead we got a $25 Chinese-made carburetor from Florida. That Twinstar still had chromed curtain rods for mufflers, but at least it left running properly.

by Andrew | November 27, 2014 at 11:01 pm | Mechanic's Notes | No comments