Vintage Dirt Bike Q & A

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 Post subject: Exhaust Backfire
PostPosted: Sun Nov 12, 2006 3:38 pm 
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Joined: Sun Nov 12, 2006 3:15 pm
Posts: 3
PLease help...

I have a 78 Yamaha YZ 400 that will not start and backfires through the exhaust after a half dozen kicks or so. I have tried 2 different CDI boxes and checked that the magneto sheer pin was not broke.

Does anyone have any ideas?

BTW the spark at the plug looks good.

Thanks


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 Post subject:
PostPosted: Tue Nov 14, 2006 12:00 pm 
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Joined: Tue Mar 04, 2003 8:46 am
Posts: 215
Location: Alpine, UT
I'm fishing on this, but have you checked the intake between the carb and the cylinder for an air leak? A lot of times the old rubber cracks and lets air in. I've had that problem before.


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 Post subject:
PostPosted: Wed Nov 15, 2006 8:52 am 
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Joined: Sun Nov 12, 2006 3:15 pm
Posts: 3
Yes, the rubber appears to be in fair shape.


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 Post subject:
PostPosted: Wed Nov 15, 2006 9:01 am 
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Joined: Sat Jul 23, 2005 9:06 pm
Posts: 29
Location: Red Bud, MI
How are the reeds? Have you checked your compression? Certainly could be an air leak as well.


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 Post subject:
PostPosted: Wed Nov 15, 2006 9:48 am 
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Joined: Sun Nov 12, 2006 3:15 pm
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Compression is great... Reeds I have not checked


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 Post subject:
PostPosted: Fri Nov 17, 2006 12:29 pm 
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Joined: Thu Oct 20, 2005 8:14 pm
Posts: 66
Was this bike a runner that became sick or a rebuild that will not run??

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