450 backfiring popping

Vetracer231
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Vetracer231
I have a 2003 crf 450r it has a 2007 engine in it was running good for a while than got hard to start. I replaced intake valves which were bad and ran for about 2 or 3 races than started backfiring popping didn't change anything. It starts easily but just keeps backfiring when running and pull hot start it tries to run better but backfires and pops like crazy. I checked fuel drained fresh fuel valves are in spec pipe at header is tight and joint for muffler is tight don't see any cracking around intake anyone ever have this problem? Starts always with 1 or 2 kicks just backfires and pops bad
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holeshot100
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holeshot100 4/14/2017 7:44am
4/14/2017 7:44am
Pull your pilot jet and make sure it is not partially clogged.
Paw Paw 271
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Paw Paw 271 4/14/2017 8:06am
4/14/2017 8:06am
holeshot100 wrote:
Pull your pilot jet and make sure it is not partially clogged.
X2, but also check the stator.

Paw Paw
rrdecals
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rrdecals 4/14/2017 8:15am
4/14/2017 8:15am
leaky head pipe or o ring at header.
endurox
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endurox 4/14/2017 8:42am
4/14/2017 8:42am Edited Date/Time 4/14/2017 8:42am
Maybe you got a new valve that did not get hardened properly and has mushroomed
, thus no valve clearance.

The Shop

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Vetracer231
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Vetracer231 4/14/2017 9:11am
4/14/2017 9:11am
Clearances are good just checked . I'll check pilot jet and exhaust want to ride ran good 2 days ago. But did sit all winter
500guy
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500guy 4/14/2017 9:46am
4/14/2017 9:46am
Vetracer231 wrote:
Clearances are good just checked . I'll check pilot jet and exhaust want to ride ran good 2 days ago. But did sit all winter
Check all your air boots to the carb and make sure you don't have any leaks, also if it has a hot start lever make sure it's not partially engaged.

most of the time that popping is a lean condition created by something like this or stuff already mentioned.
peelout
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peelout 4/14/2017 9:49am
4/14/2017 9:49am Edited Date/Time 4/14/2017 9:51am
i just went through this same issue on my 525.

my slide seal looked like burnt bacon and i think the rest of my carb could use a re-build. however, i threw a new slide seal in and it helped. i figured out that my pilot (42) was way too lean. went up one jet and it cleaned it up but still a slight backfire on de-cel. i went up one more (48) and it basically went away.

my fix is going to come from rebuilding all the seals in the carb which i assume went bad from old gas, and then going back down on my pilot because it ran great last year.

like said earlier, your pilot could have some clogs. head pipe seal is less likely, but could be causing it too.

one more thing, check the torque on them sprocket bolts ffs
Vetracer231
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Vetracer231 4/14/2017 6:31pm
4/14/2017 6:31pm
You put a 48 pilot jet and slide seal is that a 525 ktm
peelout
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peelout 4/14/2017 10:18pm
4/14/2017 10:18pm
Vetracer231 wrote:
You put a 48 pilot jet and slide seal is that a 525 ktm
Yep, KTM 525.

I wasn't giving advice on what size jet to run, just that you are probably lean on your pilot. Also bad gas ruins seals and causes air leaks which also cause the popping
Beeby
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Beeby 4/14/2017 11:05pm
4/14/2017 11:05pm
I had the same issue with a 2007 (probably coincidence). It was the header pipe. It seemed a tight fit but nothing solved the issue. We looked at the hot start being open, valves and everything. Eventually my father suggested we went a bit old school and we packed the header with exhaust putty and the problem went away so we at least knew it was the header and could get a replacement that sat tighter. Might be worth a try for you to rule that out...

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