250 sx fresh build back firing on cobra pipe

[Russman1986]

I got a fresh 250 SX build valve lash set perfectly but it's backfiring on a fresh vintage Cobra exhaust like we're talking Flames out of the tailpipe what do you think I put a lot of work on that motor and I would very much appreciate it if you weren't a dick in replying (dhoc bikes)

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[Rob Canadian]

Is it shooting flames when trying to start it OR runs then when you give it gas it backfires and flames.

[Russman1986]

When it runs and you let off the gas

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[350for350]

When It backfires as you let off the throttle, it's usually a lean condition. Did you rejet your carb after you installed the aftermarket exhaust?

[Russman1986]

No its stock

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[Russman1986]

What should the jet be can I get a link

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[Rob Canadian]

No its stock

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More info needed on the carb. Stock as in OEM(?) Aftermarket carb(?) Jet sizes and elevation.

Was the carb cleaned good?

[Russman1986]

Ya ran fine with stock pipe it was a all oem bike I don't know elevation I'm in Illinois

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[edizzle89]

yea probably need a re-jet. pretty much any motorcycle or dirt bike, atv, atc, etc. will need a re-jet after any change to the intake or exhaust that would cause the engine to see more air flow.

[yaegerb]

I got a fresh 250 SX build valve lash set perfectly but it's backfiring on a fresh vintage Cobra exhaust like we're talking Flames out of the tailpipe what do you think I put a lot of work on that motor and I would very much appreciate it if you weren't a dick in replying (dhoc bikes)

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Other than set valve lash, what did you rebuild? Carb, top-end? As others have mentioned a backfire (with flames) out of the pipe is either a timing issue, carb issue or a leak between the exhaust pipe and head or a hole in the exhaust. Either way...shooting flames isn't good.

Starting simple.....

Did you put the copper crush washer back in the exhaust port?
Is the pipe good and tight and sitting flush against the exhaust port?

[Russman1986]

I built that motor from scratch. Brand new piston and rings crank all barings and gaskets. It did have the crush ring in it no holes or leaks in the pipe. It's mounted on the head well. I just cleaned the carb with the build. I never had this problem with the old pipe, we ran it all week at teikefest. It just started when I put the new cobra pipe on.

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[Rob Canadian]

OK. Just to be clear. You rebuilt the engine and ran fine with old exhaust at Trikefest. Carb was clean and ran well. It sat for a bit I assume (?)

You put the Cobra exhaust on recently and this is how it is running? Old gas?

How does it run otherwise? Fires up good. Good power? Just the issue of backfire on decel?

[yaegerb]

start the bike, get it to flame and then pull your choke on and see if you notice a difference.

[Russman1986]

OK. Just to be clear. You rebuilt the engine and ran fine with old exhaust at Trikefest. Carb was clean and ran well. It sat for a bit I assume (?)

You put the Cobra exhaust on recently and this is how it is running? Old gas?

How does it run otherwise? Fires up good. Good power? Just the issue of backfire on decel?
The carb is clean it has new Gas it ran fine when we took the stock pipe off. Then just backfire on decel with the Cobra pipe I drove it around a little bit before I took the old pipe off no problems

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[Rob Canadian]

Try the test by yaegerb in post 13. This will determine if it is running lean and you can rejet for the new pipe.