Carb Questions

[Billla]

Originally Posted by SchreinerM

I'm new to carb tuning as well. Is that typically to set timing with the vacuum advance plugged? Is this true of the vacuum on the carb as well?

Yes, static timing is ALWAYS set with the vacuum advance plugged. The service manual is your friend

I'm not sure what you mean by vacuum on the carb, but the distributor advance will be connected to either the carb or manifold - whichever it is connected to should be disconnected at the distributor and plugged at that end...otherwise we end up with a vacuum leak.

[SchreinerM]

Thanks for the info. I've read the manual, but must have missed that portion.

As for the vacuum on the carb, I was refering to the diaphragm on the passenger side that opens the butterfly on the front side of the carb.

Again, greatly appreciate your help.

[Billla]

Originally Posted by SchreinerM

As for the vacuum on the carb, I was refering to the diaphragm on the passenger side that opens the butterfly on the front side of the carb.

This is the vacuum secondary, you can leave this connected.

Go and time :-D

[60ratrod]

Originally Posted by SchreinerM

Thanks for the info. I've read the manual, but must have missed that portion.

As for the vacuum on the carb, I was refering to the diaphragm on the passenger side that opens the butterfly on the front side of the carb.

Again, greatly appreciate your help.

the diaphramed vacuum port on any carb is generally refered to as a gated port. i usually find it better to hook the tranny to that port and the carb to the fully open port.

[Billla]

Originally Posted by 60ratrod

the diaphramed vacuum port on any carb is generally refered to as a gated port. i usually find it better to hook the tranny to that port and the carb to the fully open port.

Not meaning to go off-topic, but a few thoughts.

Carb vacuum secondaries are internally connected to ported vacuum on all the carbs I'm familiar with, so usually there's no opportunity to connect them differently...and you wouldn't want to. If your experience is different, a picture of what you're describing would be helpful.

Ported vacuum is vacuum taken from above the throttle butterflies, and so has little or no vacuum at idle. Non-ported vacuum is taken from the manifold directly - either from a fitting on the manifold or from a carb fitting that is below the throttle butterflies.

Generally, transmission vacuum modulators should be connected directly to manifold vacuum, usually to a the fitting behind the carb on the intake manifold where power brakes (if present) are connected as well.

For distributor vacuum advance, there is active debate about where it should be connected, but a good rule of thumb is that stock or near-stock cammed engines should use ported vacuum, but a more performance-oriented cam should use manifold vacuum as the additional vacuum - and so, additional vacuum advance - at idle can smooth out a bumpy idle with the bigger cams. Note that this only affects idle and near-idle; once the butterflies start to open the two fittings will show nearly identical vacuum.