B12 jetting problem, BST40 - Page 2 - Custom Fighters - Custom Streetfighter Motorcycle Forum

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Old Streetfighter Motorcycle Forum Post 08-08-2017, 11:30 AM   #11
scottyp
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Default Re: B12 jetting problem, BST40

Part of the issue is you're trying to tune 4 carburetors off of 1 AFR meter and rpm range.

What percentage is your throttle at from 2k-4k? What are your stock jet sizes?

From the symptoms you're describing, It sounds like your issue is more with your butterflies and emulsion tubes. throw it up on the stand, change the e-tubes and use the color tune to check your work on all 4 cylinders in that throttle range (I'm guessing between 15-30% up to 60% when you blip it for a wheelie)and adjust your needles from there. if it's still leaning out on you, Its the butterflies. I run my pilots a little rich just to match my mains.

Ideally, you're jetting should look like this, and you may have to switch to this after changing the tubes for a good baseline:
Pilot: +2 sizes over stock, so 45
Main: +6 over stock
Main Air Jet(not pilot air jet): Stock, but it doesn't hurt to have +1 and -1 sized on hand

Emulsion tubes and needles, I would also get 1 of each richer and leaner. Keep in mind your inner carbs may have leaner emulsion tubes and needles and that could be the entirety of your problem.
All of this is based off of elevation below 6000 feet. you'll need leaner jets if over 6000
Also it should be noted that you're gonna have to play around with configurations all over again once you change your tubes or needles. Remember to bench synch your carbs everytime you make any jet change.

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Last edited by scottyp; 08-08-2017 at 11:33 AM. Reason: forgot something
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Old Streetfighter Motorcycle Forum Post 08-08-2017, 11:35 AM   #12
scottyp
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Default Re: B12 jetting problem, BST40

Here's another really good tuning guide, complete with pictures for guys like me who don't read good.

http://www.dansmc.com/carbs2.htm

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