Tuning Problem!
Thread Starter
Drifting
Joined: May 2002
Posts: 1,546
Likes: 282
From: Back in Nebraska
Tuning Problem!
I’m having problems with acceleration. It is a freshly rebuilt 350 with a Qjet. I’m thinking that it might be the vacuum advance. On a 1970 350/300, was the vacuum drawn directly off of manifold vacuum or from a metered carb port. Any advice on the problem would be helpful.
Tech Contributor
Joined: Aug 1999
Posts: 14,371
Likes: 6,365
From: At my Bar drinking and wrenching in Lafayette Colorado
The vacuum advance unit is not a factor under acceleration - you're not pulling any manifold vacuum under acceleration, so the vacuum advance and its hookup is irrelevant since it's fully retarded.
For tuning, do the basics:
- Check total timing and set it up to 35-36 degrees. Make sure dwell is at 30-31.
- Verify centrifugal advance is fully deployed by 2500-3000 rpm
- Verify carb is correctly jetted for the carb number, and verify float level at 3/8"
In answer to your question about vacuum advance hookup: Your car came stock with the vacuum advance hooked up to ported vacuum. Most of these cars run better at idle with the advance run off manifold vacuum. During cruise and off-idle conditions ported and manifold vacuum will peform the same, so use whichever source produces best quality idle. Note that manifold vaacuum at idle will increase hydrocarbon emissions, so don't run manifold if you need to pass an emissions test.
For tuning, do the basics:
- Check total timing and set it up to 35-36 degrees. Make sure dwell is at 30-31.
- Verify centrifugal advance is fully deployed by 2500-3000 rpm
- Verify carb is correctly jetted for the carb number, and verify float level at 3/8"
In answer to your question about vacuum advance hookup: Your car came stock with the vacuum advance hooked up to ported vacuum. Most of these cars run better at idle with the advance run off manifold vacuum. During cruise and off-idle conditions ported and manifold vacuum will peform the same, so use whichever source produces best quality idle. Note that manifold vaacuum at idle will increase hydrocarbon emissions, so don't run manifold if you need to pass an emissions test.
