Motor Stumbles when accelarating

DRBarnhart

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This thread is starting to confuse me so I thought I'd throw in my :zz2cents: hopefully to see if we're all on the same page...
When I went home at lunch I got on it and twisted the throttle twice and it backfired...
This really doesn't have any bearing on what you're trying to do. When you're talking about a "lean" or "rich" condition you're talking about what's happening inside the cylinder. So, when you twist the throttle then try to start the engine the gas/air mixture in the cylinder is "lean" so the fire ignites the gas it can find which is in the intake. About all this tells you is that you didn't need to twist the throttle twice to get the bike running!

Here's a couple of things to keep in mind while you're tuning... backfiring in the intake is indicative of a "lean" condition in the cylinder and backfiring (it's actually afterfiring) in the pipes indicates a "rich" condition. If you turn the idle mixture screw in it gets leaner and turning it out makes the mixture richer.

And, when you turn the accelerator pump screw in (clock-wise) to the point of making contact you've turned the pump off. So... the more counter-clockwise turns you make on that screw the more fuel that's fed to the intake. (I tune my carb with the screw all the way IN (clock-wise) to take the pump out of equation until I get the idle and mixture adjusted. Then, after the idle and mixture are right I slowly turn the pump adjusting screw out to find the sweet spot!)

I hope this helps simply things... :D

Dennis
 
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Bigbass

Member
Got rid of the intake backfire for the most part. The idle mixture screw is only turned out about 1 1/4 turns when it idles perfect. Now I am trying to get rid of the loud annoying backfire on deceleration.
 

MARV

Well-Known Member
Got rid of the intake backfire for the most part. The idle mixture screw is only turned out about 1 1/4 turns when it idles perfect. Now I am trying to get rid of the loud annoying backfire on deceleration.

see if you have black soot around the exhaust flanges.

you probably have an exhaust leak which can suck air and add to your backfire.

you can tighten up the flange bolts but might also need new exhaust gaskets
 

Bigbass

Member
There is no black around the gaskets and I can feel around the flanges for the most part when I first start it to check for leaks and feel none at all. I was wondering if I needed to back off my jet just a little. I went from a 31 to a 33 and that is when it started.

Not real good when it comes to jetting stuff. I can rebuild every part of this thing from the bottom up just have always had a problem with fule air mixtures on carburetors.
 

Bigbass

Member
You asked for me to keep you all posted so here it is. Thanks to all of you that got involved I have finally got the stumble gone and the backfire gone without spending an arm and a leg to do it. I put the .031 back in and made real fine adjustments and it runs like a top with no hesitation and no backfire. You guy's really are the greatest.
 

Moespeeds

Well-Known Member
You can check for an exhaust leak by holding a lit cigarette, cigar, or incense for you crunchy types under the exhaust flanges while the bike is idling. Watch to see if the smoke is disrupted as it flows around the flanges. Backfiring in the pipes can also indicate a lean condition, if there is too much air in the mixture it doesn't properly combust in the cylinder and goes out into the pipe, where it can ignite and backfire.
 
You asked for me to keep you all posted so here it is. Thanks to all of you that got involved I have finally got the stumble gone and the backfire gone without spending an arm and a leg to do it. I put the .031 back in and made real fine adjustments and it runs like a top with no hesitation and no backfire. You guy's really are the greatest.
:2thumbs:

:cheers:
 
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