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The TTR250 FAQ web pages -> TTR250 -> Repairs and maintenance -> Need help diagnosing backfire at the header that stalls the motor
Post Info TOPIC: Need help diagnosing backfire at the header that stalls the motor
Sprok


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Need help diagnosing backfire at the header that stalls the motor
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When lugging at low RPM I'm getting a weird backfire and motor stall that sounds like it's coming out of the header. Is this normal or maybe timing chain or something?

 

The carb is freshly rebuilt and otherwise runs great when not idling. Stock everything (US 1999 model). Unknown time on the motor. It was only ever used offroad and had less than 600 miles on the trip odometer thing (that the owner didn't even know you can reset).

 

I have a video. In the video it actually sounds more like a normal backfire but in-person it's clearly "chuffing" out of the header at the front of the motor. You can see in the video I almost fall over because when it backfires the motor stalls. There are also some "clicking" noises when running at low RPM. I haven't owned many singles and they all sound like a box of marbles to me so maybe it's normal. Or maybe my timing chain slapping around?



-- Edited by Sprok on Tuesday 21st of December 2021 05:56:31 AM

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JOHNYBOY


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sounds like you have a loose header pipe or gasket blown will cause it to run lean & backfire or possible tight exhaust valve but that would have low compression

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94Garry


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Mine was doing this, i notice the rubber inlet from carb to engine had split, i have replaced it and its fine now.

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Sprok


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Hmmm, that carburetor joint on mine is bad. I thought it wasn't leaking because it is just the outside rubber that pulled off of the metal retainer and I re-sealed it with RTV. It's not cracked or anything. Maybe it's leaking somehow that's only noticeable at low RPM. That's a difficult one to work with because those joints are extremely expensive and I don't want to replace it if I don't have to.

I'll check the header and head gasket as well. Valves should be good because a dealer recently serviced them but this is the same dealer that messed up all sorts of stuff on the bike. I hate to going tearing in to stuff if I don't have to because it runs really well except for the low RPM issue.



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Sprok


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I replaced the carb joint and that did seem to help when riding around my yard at least. However, I now have the bike on the road and I can tell there's definitely something wrong. When holding the throttle steady at any RPM, it surges forward and back. Not smooth at all. Rarely the motor still does the same thing in the video above where it essentially stalls then continues.

I keep thinking it's something to do with the timing, maybe the chain. I may have to crack it open and take a look.

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admbnm


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if you start the bike,carefully squirt a bit of wd40 or similar around the carb stub,airbox join,the engine will increase in revs IF their is a bad seal,all the best

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Sprok


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I found that if I pull out the choke a little bit that fixes the problem. Hmm...

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TTRfan


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Sounds like the pilot jet may still be partially blocked. 



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Sprok


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Maybe. I know it was perfect when I rebuilt the carb. Maybe something got in there.

Feels more like an air leak though. I didn't replace the o-ring on the choke because it looked really good but maybe it isn't. Same with the pilot screw. It looked fine but maybe it's not or maybe I need to adjust it a bit. I don't know if the pilot screw would even be in effect at say 40 MPH... maybe, because I'm at steady throttle.

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TTRfan


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The pilot screw wouldn't have any real effect at 40 mph.

It sounds as though you need to go through and methodically recheck the carb including the inlet stub and its O ring and that the jets and galleries are clear in the carb.

Ream the pilot with a bit of copper wire to make sure it is absolutely clear.

If the problem persists then it points to a valve or ignition problem such as the pickup on the stator.

Brian



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