Ok, here's one for you guys. We have a 300EX that is backfiring when you let off the throttle. Yeah - yeah, I know what most of you are thinking - 'richen the ole idle mixture screw up a bit' - but we've tried that. I took the screw all the way out with the stock pilot jet, then went up a size on the jet and backed the screw all the way out in stages until it died - but the dang thing still popped at every step of the way.

I have also raised the needle one notch and gone up several main jet sizes. I have checked for vacuum leaks and the engine does seem to be jetted correctly (if not overly rich) on almost every circuit. The quad has a White Brothers E series with the default number of discs and a UNI filter (oiled) but other that that, the engine is stock.

Any ideas out there?

Thanks,

Dennis

 

Decelleration backfiring is a tell tale sign of leanness. The obvious fix is to raise the needle and not touch the main unless there is a problem at full rpm's...but you did that. Try the white bros approach and tune it with the discs by subtracting a few at a time...it's a quick and easy way to tune without messing with the carb. The number of discs may have a bearing on the rebound pulses back to the carb during decelleration.

 

You have an exhaust leak.

 

well her's a couple of reason depanding on where you ride you might have to down jet it try that and an easy to tell if you need to up jet rase somone and if it starts cracking at high rpm you need to up get and try this on the hills in third gear and if that's not your problem check your head pipe for leaks at the head and onther thing is it is a straight pipe just keep that in mind ther's really nothing there to hold back

 

Are you running 93 octane? Try just regular ol'87 octane. I noticed that when super was run in the 300 4x4 Fourtrax, it backfired continuously. When switched to regular, the back firing ceased.

 

mine also backfires..but just in 2nd and 3rd wide open when i let off the throttle....im thinkin puttin in a 150-155 main jet....i already have a 145. also i have moved the needle and it doesnt help at all.

 

I think i know the problem. On my white bros. there was no graphite packing in the end of the silencer where it meets the headpipe. This is where your exhaust is leaking. Dig your graphite packing out of your old pipe but be carefull not to break the packing. you will have to bend silencer where it meets the headpipe for the packing to fit. This fixed my backfiring a little but didnt stop it.

 

Thanks for the suggestions guys. An exhaust leak sounds reasonable - although I did order and install a new graphite o-ring. Maybe it's leaking up where the headers mount to the engine. This is actually my old man's ride so I'll have to get him to check it out in the next day or two.

Thanks again,

Dennis

 

If its leaking by the headpipe this would be easy to diagnose. It would be louder and you could feel air coming out. When you put the silencer did you tighten it down before you tightened down the headpipe? I did this and it leaked really bad.

 

I would have to agree that it is leaking around the headpipe. The same thing happened with my wifes Wolverine.