Pages: 1
Posted by: Bear4570
Backfires are generally caused by unburned fuel in the exhaust system. Either to rich, black spark plug tip or a mixture to lean to fire in the cylinder, white spark plug tip. That unburn fuel and air dump into the exhaust collects and burns there. When the motor does this try pulling the choke on and see if it gets better or worse. Gets better, you are to lean and vice-a-versa. Before changing jets however make sure float setting is correct and the mixture screws are set right. My float was so far off that the thing shouldn't even have run at high speed. Seems the guy liked to drain the carb before trailering and the float bouncing up and down and bent itself downward, lowering the level by almost a quarter of and inch.
Posted by: hondabuster
Its due to a lean idle circuit. Either you have a intake leak, exhaust leak, or the idle circuit is lean due to dirt or adjustment.
This is from the mikuni website
4: Backfires in Exhaust
Note:
It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.
The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.
Why This (normally) Happens:
1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.
if you pull the choke on, just before letting off the throttle, the backfire should stop. ( if the choke circuit is working).
Posted by: BLUEZL600EFI
I bought a 1997 Bearcat 454 used a few months back. I am having a problem when ridden hard after it warms up it backfires. I checked the plugs and it seems lean to me. I don't have much experience with 4-strokes. I was wondering if I just put in a jet size larger, or is therre something else I can try first.
Thanks.
Posted by: BLUEZL600EFI
I feel like an idiot I ran over 100 miles on it the other weekend and never thought to pull the choke! That's what happens when you ride and efi snowmobile I guess.
Posted by: BLUEZL600EFI
It only does it when I let off the gas, even if I let off easy. I try to never go from full throttle to no throttle on any of my toys. I already tried changing the needle position. I was going to try bigger mains next, especially with the colder weather coming.
Posted by: BLUEZL600EFI
I did clean it a few weeks ago. Also found crap stuck to the inlet in the tank that would cause a bog when it was 1/2 empty. Thought originally that's what caused the backfiring. But it's still there. I will try the fule screw first then up the main jet I guess.
Posted by: BLUEZL600EFI
Nothing I tried made it better. (did not try the choke). I have a k&n air filter on it. but it backfires with the orginal filter almost as much! I checked for air leaks but I am starting to think that may be what is wrong. I tried spraying starting fluid aroud the boots and it did not rev. This is really throwing me for a loop.
Posted by:
When is it backfiring? Deceleration? If so, turn the fuel screw out a 1/4 turn. If this makes it better but still does it, go another 1/4.
Check the needle position as well. Maybe someone tried to find some power by changing the jesus clip.
See what main jet you've got in it too. I believe that model came with a 142? Not positive though.
Posted by:
Might be wise to pull the carb off, dissassemble while looking for dirt & give it a good cleaning.
Posted by:
Fuel screw should be set from factory at 2 turns out. If so, go to 2 1/2 & try. Go to 3 turns out if it helps. Is motor & exhaust all stock? That's an awful lot of backfiring. Check the carb boot as well for cracks in the rubber or any other air leaks. Did a needle change make a difference? Has anything that you've tried?
