vacuum 327/350 68' vette
 

I had been running ported vacuum to my distributor. Since my Q jet does not have a connection for manifold vacuum I used a t fitting to connect directly from my manifold to the distributor vacuum canister. Once I did this I started hearing a rattling sound. I checked around and it seemed to be coming from the distributor or the canister. Pinching off the vacuum to the canister and the sound stopped. I have checked the canister applying a vacuum to it and it does move and holds until the vacuum is released. I left it connected and the car seemed to run okay. Any ideas on whats causing this and if something needs replacement? I switched back to ported for now.

Greg,

Go read this, it'll help you understand timing and vacuum http://repairs.willcoxcorvette.com/c...m-advance-101/

Willcox

It shares the lines with accessory wipers and headlights using the T.

Hi Greg, If your carb is the original 68 I don't think there is a ported vacuum. There are 2 vacuum ports, one on the right side by the divorced choke and another on the lower front left. Both are manifold vacuum sources. The side tube was used for the smog pump and the front one was for distributor advance but both are manifold vacuum. My 69 is the same as they didn't start using ported vacuum until later.

thanks for the info. I do have a pretty good grasp on ported vs manifold vacuum pros cons and history but the detailed info is a refresher. I had been running manifold vacuum with a holley. Recently swapped back to the original Q Jet. The original 68 Q jet has no ports on the carb for manifold vacuum like the holly did. I tried the T fitting to return to manifold vacuum but got that funny rattle from the distributor/vacuum canister. The car runs fine on the ported just a little less smooth off the line and car doesn't seem to run any hotter than it did when I used to run manifold vacuum. Since the rattle was a concern I just returned back to the ported set up.

All smog stuff is off this car. The only vacuum port available is on the rear of the manifold close to the distributor. One hose exits this port for the accessories :headlights and wipers.

I wonder if maybe the rod on the advance canister is rattling against the stop with full vacuum as you said it was coming from the advance can area. When you first said a rattle I thought it might be the check ball in the pcv valve but that's not near the advance can. You could try one of the two vacuum sources on the carb and see if you still have the rattle as those are both manifold vacuum with a 68.

The port on the lower front left is the ported attachment on my carb no vacuum is produced from this at idle. the other right side port has a hose is attached to the choke not sure if this is manifold vacuum.