westtexus wrote:
I have an Edelbrock 750 carb. It has two vacuum ports on the front, one is very strong at idle, one is very weak at idle. Both provide timing advance when the engine is sped up, the timing stays at 8 degrees BTC at idle. Which port should I use?
Their should be little to no vacuum to the advance port during idle unless you have the idle set to high which exposes the ported vacuum slot in the bore by the butterfly notice if you reduce the idle speed the ported vacuum will drop also.
Here is another quote someone else asked it is very much like your question maybe this explaination is clearer:
On the carb, one is timed vacuum, one is ported vacuum or manifold vacuum. You are not going to get "more" vacuum from the manifold than the ported vacuum, it defies physics. Now the timed vacuum port only gives vacuum after a valve opens, butterfly that is. The timed vacuum port is above the butterfly, so at idle, that port sees little if any vacuum because it is "open" to the atmosphere (above the vacuum source, the manifold) You may get a quicker response at the distributor from the manifold because the port is about 10 times the size as the one in the carb. Some Phords have the distributor vacuum line coming right off the intake.
