Thread: new carb, runs rough in gear

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Are you running a big cam?

One with an advertised duration of 280-290 degrees will idle between 10-12" of vacuum. (Manifold vacuum, sourced from lower than the throttle blades anywhere on the intake manifold.)

Stock cams - and very mild ones on bigger engines such as yours - will idle at 18-19" of vacuum.

I believe late double pumpers come with idle screws at all four corners.

It sounds to an extent that the two bibs (outlets) you're checking have different sources.
One ported and one manifold.

If your idle is say 600 rpm and you're running a stock or mild cam - and the engine is in good shape - you should be getting 18-19" vacuum.
Use that source for all vacuum referenced idle and running adjustments.

Idling at 600 rpm and plugging a ported source - above the throttle blades (most times*) you should get zero and possibly 1-2" vacuum.
Spin the engine up to 1000-1200 rpm and you may show 5-10" of vacuum.

*Some Holleys have a manifold source bib higher than the throttle blades (right side).

Carter/Edelbrocks have two bibs at the front of the carb - near the base plate - and the driver side (in the USA) is manifold vacuum with the passenger side being ported vacuum.


I do have a question though; if you're running on the street why a double pumper?

Imho you'd be a lot better off with a vacuum secondary Holley or a Carter/Edelbrock with air flow demand secondaries.
Gas mileage is considerably improved over the double pumper as well.

I see guys pop double pumpers on moderately built street engines now and then, drive only on the street and in general make life tough for themselves.
By the same token, I see cars at the dragstrip that would be better off with vacuum secondary carbs....