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01rk 01rk is offline
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Default Backfire

I put a VM38 on the '74 XLCH I bought and now it backfires through carb when I get on the throttle, if I put the enrichner on a little it's not as bad, turning the air adjustment screw does not help. It starts and idles fine. Does not backfire with the S&S that was on it when it bought it.
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Originally Posted by 01rk View Post
I put a VM38 on the '74 XLCH I bought and now it backfires through carb when I get on the throttle, if I put the enrichner on a little it's not as bad, turning the air adjustment screw does not help. It starts and idles fine. Does not backfire with the S&S that was on it when it bought it.
Popping could indicate a possible lean pilot jet. Do you know what number pilot jet you currently have.How many turns on the adjustment screw?
Needle clip position? How about WOT ok there? Carb is clean?Float set?

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All of the above being good, and absent any vacuum leaks, raise the needle two notches (move the needle clip down on the needle). If that improves, but does not completely cure the problem, you may need a richer needle. A larger pilot fuel jet will help if the motor goes lean just off idle. Tuning, tuning tuning. And use fresh plugs during testing; a dirty plug may present bad driveability even if you get the jetting right. I usually recommend buying a box of plugs.

Jim
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download the tuning manual from american mikuni
the enrichment circuit is by-passed when the slide is open. sounds lean but where it pops will determine what the next step is.
since there is no accel pump, when you yank on the throttle it may not be pulling enough fuel through the main.

http://www.mikunipower.com/VM01.htm
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Have a nice day
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Quote:
Originally Posted by 01rk View Post
I put a VM38 on the '74 XLCH I bought and now it backfires through carb when I get on the throttle, if I put the enrichner on a little it's not as bad, turning the air adjustment screw does not help. It starts and idles fine. Does not backfire with the S&S that was on it when it bought it.
shulda left the S & S on it, IMO
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I wasn't willing to say that Jeffro, but %100 agree.

Mikuni MIGHT be useful if you have access to a dyno AND you race on tight twisty circuits. The ones with a lot of 2nd and 4rd gear work.
With a really healthy engine on long straights the 38 will not flow enough on the top end.

Could me 1/2 a second a lap under the right circumstances. (But probably not)
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What ever the stock jets are is what is in there, new vm38.
I am 80 years old I don't plan on racing anywhere just want it to run ok
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Quote:
Originally Posted by 01rk View Post
What ever the stock jets are is what is in there, new vm38.
I am 80 years old I don't plan on racing anywhere just want it to run ok
The carb circuits get dialed in based on throttle position so it is important to identify when the pop occurs and then adjust only the carb circuit for that throttle position. It's really easy once you understand the concept.
Take a look at the diagram:mikuni.jpg
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Quote:
Originally Posted by 01rk View Post
What ever the stock jets are is what is in there, new vm38.
I am 80 years old I don't plan on racing anywhere just want it to run ok
That's a very big reason the S&S is so popular.

Simple as a rock.

We already know required jet sizes for stock engines. Put them on, they just work.
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