Reply to "351C typical vacuum readings"

At one point I installed installed port plates on the intake side of the manifold and unknowingly blocked the exhaust crossover ports. The result was that the car took 20 minutes or more to warm up. Eventually I isolated the issue, fixed it, and the car returned to warming up very quickly. I have since added a phenolic spacer under the carb to mitigate any heat related issues.

I do not think this will have any affect on your manifold vacuum at idle when the engine is warmed up but it may help with your fine tuning efforts because the engine gets up to temperature faster.

My engine has a 292 solid lifter cam from Clay Smith Engineering and we have been able to get just shy of 13 hg vacuum at idle.

You may be able to get higher vacuum by increasing the initial distributor advance. Try gradually increasing it and you will notice higher RPM's. When you lower the idle to approximately 900 RPM's via the set screw on the carb then you may see higher vacuum. Keep doing this until you have achieved maximum vacuum. You may also notice that the engine sounds better at idle.

In turn this will affect your distributor advance curve. The above process increases initial advance and thus changes the advance curve. I am using the MSD 8477 distributor which has a readily configurable advance curve and total advance. Duraspark and other distributors may need tuning by a specialist.