armyofda12mnkeys Posted September 14, 2010 Share Posted September 14, 2010 I asked on dotheton, but just thought I'd ask here too since Yamaha folks have always been great. My friends '74 cb450 had a variety of issues. I had to clean his carbs as his bike started to run bad, and he let it sit for too long which gunked up carbs. We then put on uni-filters since his stock air filters were breaking up, and we put a higher main jet for that air filter. Bike starts after 5-10 kicks, but stumbles/splutters. Idle mixture doesn't seem to help at all. Bikes symptons are: 1. Backfires occasionally out of right exhaust. Sounds like a gunshot hehe. (read that lean or rich conditions can cause it, but idle mixture seems to be fine). 2. Has a noise coming out of right air filter. We took off air filter and see some air is pushed back to air filter (splats a lil' gas droplets back if you put your hand there). 3. While engine is weak at startup, if you give it some throttle it may not cut off... if so, after riding for 5m, it may have high idle. Bike rides decent in 3rd gear giving throttle, but at the lights, it stays at 3k rpms. I am guessing timing? like maybe intake opens at wrong time and forces air into carb?, and exhaust opens at wrong time, causing un-ignited gas to hit the hot exhaust and ignited there? Exhaust is hot, so gas is ignited most of the time it seems though. Used a Colortune sparkplug to confirm that gas is igniting on that side (didnt pay attention though if it was missing ignition during those exhaust-gunshot or carb-intake sounds). Link to comment Share on other sites More sharing options...
chris66 Posted September 14, 2010 Share Posted September 14, 2010 have you changed the idle jets as the main jet will have very little effect on low down revs. the spitting back is not necessarily timing or valves it could be the carbs haven't been cleaned properly. chris Link to comment Share on other sites More sharing options...
armyofda12mnkeys Posted September 15, 2010 Author Share Posted September 15, 2010 have you changed the idle jets as the main jet will have very little effect on low down revs. the spitting back is not necessarily timing or valves it could be the carbs haven't been cleaned properly. chris >have you changed the idle jets didnt change the idle jet, didnt think the air filter was that crazy of a mod that it can't at least run 80% of its former glory and idle fine. I did see DimeCityCycles has 2 sizes for these cb idle jets. I could get one i suppose but i doubt thats issue. it was kinda running the same way before upgraded air filters/cleaned carbs >it could be the carbs haven't been cleaned properly. hmmm i cleaned the carbs i thought well, maybe i'll take another look. what specifically would cause that pocket of air to be forced backwards like that? Link to comment Share on other sites More sharing options...
chris66 Posted September 15, 2010 Share Posted September 15, 2010 Anything that stops the cylinder firing properly will cause a little splash back , And if you are using single filters its more noticeable because it has very little distance to travel to wet the filter Same thing was happening to my yzf 750 and it was a bloked idle circuit, The fuel wasn't getting through on one cylinder just splashing back. The other three were carrying it hence the bad running and every so often it would catch and backfire . As I said there is many reasons for a cylinder to missfire this is just the cheapest, Plugs, leads, coils, all should be tested before expecting the timing ,values etc. I have written this with my phone so sorry if its bad grammar.Chris Link to comment Share on other sites More sharing options...
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