QuestionI'm wondering if you can give me any tips. I know it isn't a honda but I've had this problem for a while. The bike backfires when I let off of the throttle. I do have aftermarket exhaust on but I had the problem before that. The new exhaust is louder than factory pipes and they said that rejetting wasn't required. Is it running to rich? It seems like it uses more fuel than it should. Any help you can give me would be great. Thanks
AnswerKerry, I can tell you that backfiring from the exhaust is related to air leaking into the exhaust system (exhaust leaks at the header pipe/cylinder head or at any exhaust pipe joint/junction. Bikebandit.com has some online illustrations of parts, so I went there to take a quick look. I see a "mixture control" valve in the air filter section, so will assume that there is some kind of vacuum operated control to reduce backfiring on the decelleration phases.
Manufactures have to go to great lengths to design an exhaust system that will be quiet, allow the engine to make decent power and to meet EPA regulations as far as emissions and noise limitations. If you lop off the stock mufflers and change the length and/or the backpressure in the system, then the factory calibrations are pretty much out the window.
If the mixture control valve is part of a pulse air system or a anti-backfire control system, then those components should be checked for proper function. After 25 years, all kinds of deterioration occurs in any motor vehicle system, so there could be problems with the factory installed parts, if you still have them in place.
The exhaust system does more than just gather up a bunch of loud explosions and point them towards the back of the bike. There are serious high frequency pressure waves generated in the exhaust pipes that reflect back up the pipes during various phases of the exhaust cycle. Some pressure waves, arriving at the exhaust port can create a vacuum on the cylinder and inlet valve, sucking fuel right through the engine w/o burning it. When the raw fuel mixture hits the hot exhaust gases and the pressure waves that are surging back and forth inside, the mixture explodes into what is heard as backfiring.
Find some kind of manual that explains the factory installed parts and functions (Yamaha shop manual or aftermarket versions) and then test the present systems for functionality.
Backfiring can come from being too lean or too rich. Too lean mixtures won't fully ignite, so the remaining charge often gets burned in the exhaust pipes. Too rich mixtures will also burn in the pipes, disrupting the normal process of exhaust flow.
That is about all MrHonda has to say on the subject of Yamaha exhaust and fuel control systems.
Bill Silver
