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Post Info TOPIC: Backfire @ idle, through carb


Newbie

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Backfire @ idle, through carb
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Hi, new here, can anyone help? I've got a problem with my 2001 ttr250!

I hadn't used it for nearly a year, but started & ran it regularly though..... I realised over the last few times that it ran perfectly on the choke but when I switched off the choke it had started searching for an idle, popping & when the revs really troughed, it would backfire to a greater or lesser extent & eventually backfire in a big way & stop. When I say backfire..... loudly, through the carb & air box leaving a smoky haze around the carb area. Bringing up the revs, upping the idle screw, working the throttle would keep it going but as soon as it was left alone it would start searching again & eventually trough, backfire & stop!

Riding it is fine, the only change is that it has become hesitant at low revs, not wanting to pick up. There now appears to be a bit of a black, sooty residue in the carb intake boot & snorkel!

New fuel, no change.

Obvious next step was carb rebuild...... no change.

Fuel lines & filters clear.

Checked carb intake boot, it is perfect!

Run out of ideas, now....... any ideas??

 



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Super Guru

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Welcome to the forum!
Always suspect the carb first! The pilot circuit is very fine (not just the jet, but the ducts as well) and easily plugged by dirt or fuel residue. Sounds a bit like lean running on the pilot circuit to me because you say it's ok with the choke out - ie getting the extra fuel it needs.
Alternatively, timing chain jumped a tooth?


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Moo


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Just watch out it doesn't set your air filter on fire and then the whole bike on fire! Seen it happen before.

Is the timing out? Shouldnt the inlet valves be closed when firing?

Is there carbon deposits stopping the inlet valves from closing properly?

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Super Guru

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Hi there and welcome to the forum!

Moo and mossproof make good points.

Out of interest, did you clean or replace the pilot jet? If you cleaned it, did you ream it with a bit of copper wire as we usually suggest?

Brian



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Newbie

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Thanks all, I'm thinking a check on the valve clearances next & then back to the carb, if not resolved!

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