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  #1  
Old 18th April 2008
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Default backfire through carb....

all you resident guru's, what would cause a back fire through the carb? It only happens above 3000 rpms while cruising. I have an s&s super e with a 29.5 idle speed jet and a 68 main jet. any help is appreciated. Thanks
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Old 18th April 2008
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Default

after doing some research it seems that it is because of a lean condition, but it just seems odd that it is only above 3000 rpm's. I would think that it would be happening at all rpm's. When I pulled the plugs they looked black and sooty. Any one have any advice.
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Old 18th April 2008
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Default Above 3000 rpms you're well into the main jet, that may be...

...where you're a little lean.

Your intermediate jet selection seems a little rich at 29.5 for a 1000cc motor.
That's on the high end of the S&S specs, and they're generally on the rich side.

Check it out: http://www.sscycle.com/modules/instr...ds/51-1012.pdf
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Old 18th April 2008
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I had that problem and found out it had absolutely nothing to do with mixture.

It sometimes hiccuped/coughed even when it was hot and I found out it was the stock worn out diaphragm in the carb.
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Old 18th April 2008
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Default What diaphragm? He's running...

...an S&S Shorty E.
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Old 19th April 2008
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sorry I skim. =)
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Old 19th April 2008
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Might want to check for the old air leak. At 3000 or above your engine has more vacuum so you might have a leak that doesn't present itself until the higher vacuum sets in.
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Old 19th April 2008
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You may also want to check your timing advance mechanism...
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Old 20th April 2008
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Your main jet is to lean, 3000rpm is where the intermediate system changes to the main jet circuit. Go to a 70 main and try again. I agree with Ivan though 29.5 is a bit rich for the intermediate jet 26.5 is about right.
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Old 21st April 2008
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Quote:
Originally Posted by keeno View Post
Your main jet is to lean, 3000rpm is where the intermediate system changes to the main jet circuit. Go to a 70 main and try again. I agree with Ivan though 29.5 is a bit rich for the intermediate jet 26.5 is about right.
I started out with the 28 jet and had to back my mixture screw out 3 turns before she would idle good. According to s&s I am only supposed to be out 1 1/4 turns. Keep in mind that I am bored .050 over. I will swap out the main to a 70 and see if that helps. I will keep you guys posted. Thanks.
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