Jet Types and Locations
Primary & Secondary Main Jets
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Located at the bottom of the float bowl
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These control fuel delivery during cruise and higher load
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Both must be considered when tuning—don’t assume only one is doing the work
Slow (Idle) Jet
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Located on the top of the float bowl
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Visible once the carburetor top is removed
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Controls idle and off-idle fuel delivery
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A clogged or incorrectly sized slow jet will cause lean stumble, poor idle, or hesitation even if the mains are correct
Changing Jet Size on an Aisan Carburetor (Reality Check)
Availability
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Replacement Aisan jets are essentially unavailable
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There’s no off-the-shelf jet assortment like you’d get for Holley, Weber, or Rochester
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That means modification is the only option
Richening the Mixture (More Fuel)
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Carefully drill the jet orifice larger
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Use precision numbered drill bits
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Go one step at a time
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A change as small as 0.001" is significant
Leaning the Mixture (Less Fuel)
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Solder the jet closed
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Then redrill to the desired size
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This is crude but effective if done carefully
Important Warnings (Don’t Ignore These)
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This method is not perfect
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Large jumps in jet size are a fast way to:
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Wash down cylinder walls
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Cause detonation
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Overheat or damage the engine
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If you’re tuning blind without AFR, plug reading, or vacuum feedback—you’re gambling
Bottom Line
Aisan carburetors can be tuned, but they are not beginner-friendly when it comes to jetting. Since proper replacement jets aren’t available, any jet change is permanent and unforgiving. Move slowly, measure everything, and respect how much difference a thousandth of an inch really makes.
