Hi folks,
Does anyone have a good jetting suggestion for a Weber 34 DMSA on a healthy stock 1438cc twin cam? Just bolted on a rebuilt carb with the following:
Primary/Secondary
Idle 50/60
Main 135/145
Air 180/160
Emulsion F7/F7
I'm getting some stumble as I open the throttle off idle, and Guy Croft's site has a post suggesting 55 or 60 idle jet for the primary would help that.
Also do these air correctors seem big given the main jet size?
Opinions welcome, thank you!
Jetting recs for 34 DMSA on 1438 motor
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Davin
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18Fiatsandcounting
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Re: Jetting recs for 34 DMSA on 1438 motor
Sorry for the late reply; I missed this post.
Those jet sizes look like a good place to start. Yes, a 50 idle jet on the primary may be a bit small, so you could try a 55 and see if the stumble improves. You could also just swap the primary and secondary idle jets and see what that gets you.
The secondary air corrector seems a bit small to me, and I would have expected something around 180.
A lot of the jetting will depend on the venturi diameters (not the barrel size), and I believe for a 34DMSA those are 25mm primary and 27mm secondary although variations are possible. These values are stamped inside circles on the carburetor body, on the side facing the driver when the carb is mounted on the engine.
Focus on the main jets and idle jets, and you're 90% of the way there. Air correctors and emulsion tubes are for smoothing out the mixture across the rpm band, correcting for altitude, different cams, compression, fuels, etc. Unless you have access to a dyno or a race track, it's almost impossible to sense much difference in air correctors or emulsion tubes unless you are way off. Which you aren't.
-Bryan
PS: A 34DMSA is pretty hefty for a stock 1438 engine. Not saying it can't work, but it's a big carb for that engine, which may be part of the stumble issue. Just not enough airflow from 1438ccs to get enough vacuum in the venturis to start drawing the fuel in, hence the stumble.
Those jet sizes look like a good place to start. Yes, a 50 idle jet on the primary may be a bit small, so you could try a 55 and see if the stumble improves. You could also just swap the primary and secondary idle jets and see what that gets you.
The secondary air corrector seems a bit small to me, and I would have expected something around 180.
A lot of the jetting will depend on the venturi diameters (not the barrel size), and I believe for a 34DMSA those are 25mm primary and 27mm secondary although variations are possible. These values are stamped inside circles on the carburetor body, on the side facing the driver when the carb is mounted on the engine.
Focus on the main jets and idle jets, and you're 90% of the way there. Air correctors and emulsion tubes are for smoothing out the mixture across the rpm band, correcting for altitude, different cams, compression, fuels, etc. Unless you have access to a dyno or a race track, it's almost impossible to sense much difference in air correctors or emulsion tubes unless you are way off. Which you aren't.
-Bryan
PS: A 34DMSA is pretty hefty for a stock 1438 engine. Not saying it can't work, but it's a big carb for that engine, which may be part of the stumble issue. Just not enough airflow from 1438ccs to get enough vacuum in the venturis to start drawing the fuel in, hence the stumble.
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Nut124
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Re: Jetting recs for 34 DMSA on 1438 motor
I have no experience with this carb but I would always first check float level adjustment. This level impacts the transition from idle to main jets in all weber carbs.