LOUD backfire!!! Hawk 250
 

bike has a very loud backfire on deceleration..

- Mods -
stock pz30 carb
0.5mm overstock pilot jet
115 main jet
needle raised +2
mixture screw about 1 1/4 out
POD filter
spark arrestor out
baffle drilled out (about 8 holes)
cat removed

any ideas????

did you check valve clearances?

Usually "Popping/Backfire on deceleration" means : Pilot jet too small or Mixture Screw is off (usually a bit lean)... might need to check valve clearances too

Also... according to "Mikuni"... "some heavily modified bikes (free flowing exhaust) will always have some kind of backfire/popping on deceleration"...

You might try 1 1/2 to 1 3/4 turns or more on the mixture screw... make sure you go no more than 3 turns out... go to a bigger pilot jet if you go 3+ turns out

yes, just did them last week

air leak in exhaust most lightly..

air is getting in and igniting the unburn fuel
that gets in on a closed throttle....

Fuel / Heat / Air = BANG

It may have always done it... now yer gutted the muffler
& removed the CAT you can now hear it...

My XT660 pops/bangs like a bastard on a trailing throttle.
But it has a AIS (Air Injecton System) to let air in to burn the unburnt fuel....
Clean air bullshit and all that..
never head it till I put the Metal Mule pipe on...




....

I agree with Pete. Sounds like an exhaust leak.

If the bike has PAIR (exhaust air injection) blocking it off may cure it. Assuming it's tuned right and there are no defects this will likely stop the pop. If it doesn't have PAIR then you might have an exhaust leak or it might just be going lean on decel, not uncommon.

Please explain what you mean.

i drilled out the pilot jet to 0.5mm

Thanks for the clarification. If I'm correct in my understanding, that equates to a #50 slow jet. Since the Keihin style uses much larger slow jets than Mikuni, I presume that size to be in the ballpark.

Do you have an air injection fitting or has it been blocked off?

whats that? lol
but seriously, I'm not sure what that is.

i not sure where it would be leaking? i removed the header pipe last night and re installed it to make sure everything was proper, and it looked fine..

Did you replace the gasket?

no sir.
to the head?

Yes the one at the head