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Last Post: sungod
Posted On: 1 Day Ago
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1 Week Ago
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Know It All
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Join Date: Sep 2015
Posts: 162 Sportster/Buell Model: sportster Sportster/Buell Year: 1968
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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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1 Week Ago
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XL FORUM TEAM MEMBER
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Join Date: Feb 2009
Location: ny
Posts: 723 Sportster/Buell Model: sportster stroker xlch Sportster/Buell Year: 1970 Sportster/Buell Model #2: sportster xlh Sportster/Buell Year #2: 1973 Other Motorcycle Model: 99 inch sportster Other Motorcycle Year: 2003
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Quote:
Originally Posted by 01rk
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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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?
Last edited by sungod; 1 Week Ago at 22:01..
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1 Week Ago
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Biker
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Join Date: Dec 2022
Location: Piedmont North Carolina
Posts: 42 Sportster/Buell Model: XL 1200 Sportster/Buell Year: 2000 Sportster/Buell Model #2: XL 1200 Sportster/Buell Year #2: 1993 Other Motorcycle Model: H-D FLH Other Motorcycle Year: 1980
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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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1 Week Ago
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XL FORUM TEAM MEMBER
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Join Date: Jul 2008
Posts: 11,217 Sportster/Buell Model: 1200s Sportster/Buell Year: 2001 Sportster/Buell Model #2: xlch Sportster/Buell Year #2: 1974
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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 |
1 Week Ago
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Senior Chief Master Mechanic
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Join Date: Oct 2019
Posts: 972 Sportster/Buell Model: 883 Evo Sportster/Buell Year: 1989
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Have a nice day
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1 Week Ago
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Flat Track Champion
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Join Date: Feb 2015
Posts: 685 Sportster/Buell Model: custom Sportster/Buell Year: na
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Quote:
Originally Posted by 01rk
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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shulda left the S & S on it, IMO |
1 Week Ago
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XL FORUM TEAM MEMBER
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Join Date: Jun 2009
Location: Sunshine Coast
Posts: 8,562 Sportster/Buell Model: XLB, XLCH, Sporton Sportster/Buell Year: 1962 Sportster/Buell Model #2: XLCH Sportster/Buell Year #2: 1966 Other Motorcycle Model: XLCH (Another one) Other Motorcycle Year: 1966
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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)
__________________
"I know only too well the evil that I propose. But my inclinations get the better of me."
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1 Week Ago
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Know It All
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Join Date: Sep 2015
Posts: 162 Sportster/Buell Model: sportster Sportster/Buell Year: 1968
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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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1 Week Ago
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XL FORUM TEAM MEMBER
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Join Date: Feb 2009
Location: ny
Posts: 723 Sportster/Buell Model: sportster stroker xlch Sportster/Buell Year: 1970 Sportster/Buell Model #2: sportster xlh Sportster/Buell Year #2: 1973 Other Motorcycle Model: 99 inch sportster Other Motorcycle Year: 2003
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Quote:
Originally Posted by 01rk
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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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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1 Week Ago
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XL FORUM TEAM MEMBER
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Join Date: Jun 2009
Location: Sunshine Coast
Posts: 8,562 Sportster/Buell Model: XLB, XLCH, Sporton Sportster/Buell Year: 1962 Sportster/Buell Model #2: XLCH Sportster/Buell Year #2: 1966 Other Motorcycle Model: XLCH (Another one) Other Motorcycle Year: 1966
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Quote:
Originally Posted by 01rk
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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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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