Vacuum gauge - where to hook up on Carter AFB?

[Oliver S.]

Hello,

I'd like to know where to hook up a vacuum gauge on a Carter AFB for exact idle mixture setting and what should the gauge read.

Oliver

[Duke W.]

Idle mixture/speed procedure

Use a tee to adapt the vacuum gage to the vac. adv. signal line. Idle vacuum depends on engine configuration, particularly valve overlap and idle speed - higher idle speed increases vacuum. Total idle spark advance timing is also part of the idle manifold vacuum equation.

The detailed procedure to set idle speed/mixture is in the service manual. Do you have one for your car?

When asking engine technical questions, you should always state the model year, engine option, and whether it is OE or modified, and what you know of the modifications, if any.

Duke

[Oliver S.]

Re: Idle mixture/speed procedure

Hello,

@Duke, thanks. My Corvette is a '65 with 300 hp manual transmission, power brakes and an original carburetor -> Carter 3721SB. I've got the shop manual and a book about Carter cabs ... but I didn't find information regarding the hook up of the gauge.

Oliver

[Duke W.]

Re: Idle mixture/speed procedure

The basic shop manual for your car is the 1963 Corvette Shop manual, and it has the idle speed/mixture procedure for the AFB-equipped 300 HP engine. There is also a 1965 supplement for your car, but the above procedure remains unchanged - just some minor changes to the carb. calibration that has no effect on the procedure.

Duke

[Christopher R.]

Re: Vacuum gauge - where to hook up on Carter AFB?

The 3721 has a vacuum fitting on the base on the front (Or, is it on the back?) of the carb where the distributor vacuum advance hose goes. Hook up the vacuum gauge hose there.

Look at the manual re whether the distributor vacuum advance hose should be connected or disconnected during the procedure. If disconnected, then it's easy. Just swap hoses. If the manual says the distributor vacuum advance hose should be connected, then buy a "tee" and length of hose at your FLAPS, and splice in the gauge so that both of them are connected at that same port.

[Duke W.]

Re: Vacuum gauge - where to hook up on Carter AFB?

Idle speed and mixture MUST be adjusted with the engine in the normal operating configuation, and the test equipment MUST NOT alter the configuration, which means the vacuum gage must be connected in parallel.

The easiest way to temporarily attach a vacuum gage in parallel is with a 1/8" plastic tee with a short piece of 1/8" vacuum tubing and splice it into the rubber hose on the on the vacuum advance control.

Let's not complicate it by disconnecting hard lines attached with B-nuts.

Prior to performing the idle speed/mixture adjustment the ignition system should be checked - dwell angle, initial timing, and total idle timing, which is a functional check of the VAC.

Duke