Lean on Throttle in Carter 1-barrel Carbs


Lean at high speed (part to wide open throttle) in Carter W1, WA-1, WO, WDO 1-barrel carbs

Possible cause:

Low fuel pump pressure

Restriction in gas line connection to needle seat. This does not allow sufficent gasoline to enter bowl for high speed driving

Low float level

Opening in needle seat too small (the wrong needle and seat)

Restricted outside vent hole

Restriction between metering rod and jet

Improper metering rod setting or parts that operate metering rod worn. (bowl cover, pump arm, throttle rod, throttle shaft arm)

Wrong metering rod (too large in diameter) or wrong metering jet (hole too small)

Restriction in casting from metering rod jet to nozzle

Damaged nozzle

Screw nozzle fails to seat in slip nozzle (air bleed nozzle)

Two nozzle gaskets installed on nozzles that should have only one nozzle gasket. A nozzle that has a tapered seat where it seats in the casting does not require a nozzle gasket (early W-1 carburetors). If the nozzle has a flat seat, it takes a nozzle gasket

Air horn assembly loose on casting

Worn throttle shaft

The anti-percolator should be closed air tight when fuel is discharged out of nozzle. That’s why it is very important to set anti-percolator properly so that it is opened at idle speed or closed throttle and closed just before fuel is discharged from nozzle.


Lean on acceleration

Possible causes:

Weak plunger spring.

Worn or dried out plunger leather or weak pump spring.

Leather loose, not installed tight on plunger

Damaged or cracked plunger leather

Wrong plunger assembly

Intake or discharge check not seated in casting

Leaking or sticking intake or discharge check

Passages in casting for pump restricted

Pump jets restricted or not seated in casting

Pump disc check sticking or leaking

Pump not adjusted properly

Wrong parts

If linkage from throttle to accelerating pump is worn, the mixture will be lean. When throttle is moved open from closed position, the pump jet should start to discharge instantly. Therefore, there should be little play in these parts.

Updated on 06/23/2022

Was this article helpful?

Related Articles

Need Support?
Can't find the answer you're looking for?
Contact Support