CRF Wont Stop Backfiring

[SincYZ450f]

Gotta '08 CRF450R standard bike with DEP S7 pipe that wont stop backfiring on decel. It runs as it should when on the throttle but as soon as you back it off it pops and bangs.

Problem came just after i changed the piston

Any ideas what can cause this??

so far have checked for exhaust leaks (fitted new copper ring)

larger main jet to richen mixture (no difference, jetting is good)

new spark plug

timing is good

checked for dust/filings in stator its spotless engine only done 30hrs

im lost, any help

[oakhillbiker]

O ring on the fuel screw leaking? how many turns out is it ? do you know how to use a tachometer/hour meter to set the fuel screw?

[SincYZ450f]

fuel screw 1 1/2 turns out

it ran fine before piston change and never touched settings on the carb.

When i took plug out it was really white like it was running lean but when i went out and gave it full throttle and hit the kill switch the plug came out a good brown colour.

runing out of things to check what could i have done during the piston change??

[rotordawg]

Did you take the carb apart for any reason? I did when I did my top end and put the carb slide plate back in upside down. (It has a name, but I can't remember). Very easy to do and it fits both ways.

[SincYZ450f]

no never took it apart until after it started backfiring to clean out the jets

[Drfletcherdc]

Maybe the pilot jet is too lean?

[SincYZ450f]

bike ran fine before piston change, its got me

[robg]

Pilot too lean or it's got some crap in it. How does it start?

[t_bois]

If you're sure the carb is not the problem, you may have a leak in the rubber insulator that mates the carb to the intake of the cylinder, or where the rubber insulator mates to the cylinder or carb.

[SincYZ450f]

its starts normal but doesnt idle as good as it used to

[XC-Kx250]

Have you checked your valve clearances yet?

[SincYZ450f]

valve clearances are well within tolerances

[kskyles]

have you tuned your pilot circuit? which pilot jet is in there?

have you double checked your timing?

[oakhillbiker]

let see pictures of the timing marks and what is your elevation and temp that you ride?

[SincYZ450f]

jetting was fine before piston change it ran perfect. what else can cause it backfiring. i did have the timing off a tooth when i fitted the piston and it wouldnt even kick over because the decompressor doesnt work when off the timing. so think ive got the timing spot on, had a m8 check it too.

[ridleyredraider]

Piston installed correctly? Arrow pointing forward? Not sure if that is an issue with 4 strokes, but might as well throw it out there.

The carb part being turned upside down that can stump you I think is called the vacuum plate?

[Luke315]

HOT START... It is very common for your hot start plunger on the carb to not go all the way back in. Did you check to see if the hot start plunger is moving freely.. Its very common.. Good luck

[SincYZ450f]

piston in the right way the inlet valves are slightly bigger than exhaust same as piston. not sure about vacuum plate??? any idea where it is or what it does?

[SincYZ450f]

as for hot start thats what i was thinking because the old hot start cable was siezed so its not been used in a while ive never needed it, so got a new cable on at the same time as the piston was done. spent today cleaning out plunger and that. is there anyway to bypass the hotstart just to test if its the problem.

[BigRichard]

Back your fuel screw out a quarter turn to see if it changes anything. I've had decel popping without any changes to the bike or weather.