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So i thought it might be bad gas, but it isn't getting better. It only backfires on decelleration, and it isn't like one snap, more like several bangs. valve lash is ok, i can't hear any exhaust leaks, i have a 172 main and the stock pilot, no screw adjustments, i run cam2, and performance wise there is no other problems (no deadspots, no studdering, has great throttle response). anyone have some suggestions? thanks.
 
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So i thought it might be bad gas, but it isn't getting better. It only backfires on decelleration, and it isn't like one snap, more like several bangs. valve lash is ok, i can't hear any exhaust leaks, i have a 172 main and the stock pilot, no screw adjustments, i run cam2, and performance wise there is no other problems (no deadspots, no studdering, has great throttle response). anyone have some suggestions? thanks.[/b]
So you have a cam and exhaust but havent changed the pilot circuit at all and are running a 172 on a 06, sounds like you need to put a pilot and adj air mix screw popping on decel is a lean cond in pilot circuit.
 

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backfiring upon decel 'CAN' be a lean pilot circuit....

or....it can be a bad header gasket.....

or several other conditions.... cam timing, valve stuck open, ign timing....etc..



however...in this case.... if it were me..... first thing I would do is check the header crush gasket...or simply replace it...

then....if that doesnt fix the issue.... I would back out my pilot screw about 1/2 turn.....if that helps...but didnt cure the issue.... maybe back it out another 1/2.... again...if it helps...but doesnt cure the issue... go to next size pilot... in your case... #45... and start over with jetting......

p.s......#172 is probably a bit too rich for your main..... try #162........ will probably pick up some rpm
 

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Thanks guys, i will try the easiest fix...the crush gasket, i found a couple old posts suggesting the same. i'm not sure i changed it after the exhaust install, since it had only ben ridden 10 hrs.

i had a 160 main but man, it ran hot, the pipe glowed after riding, the cooling fan was constantly on...and two brothers are the ones suggesting the 170 main. i'll pull the plug and see what color it is.

other than alligning the timing marks, how else can you check the timing?
 

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I myself wouldnt worry much about the timing at this point.....

neither valve timing OR ignition timing.....it would be more rare than common to have that be the issue here....

its probably a lean pilot circuit....but since you mentioned it runs very well and no hesitation...... I would not want you to begin messing with the jetting......and a bad crush gasket has the same effect (popping) due to allowing fresh air to enter into the CC during the intake stroke while decelerating......via the leaking gasket.....(which in turn causes the lean mix and popping)...but allows for perfect running engine when ON the throttle.....

p.s....let us know what you find out....
 
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