312 vacuum


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By YellowWing - 15 Years Ago
Wondering if anyone knows what the vacuum is for a 312 with the following.

Fresh rebuild

Mummert 265 cam 224 deg@.050

G heads milled .020

Pistons .015 below the deck

I am having a couple minor running issues ( full report to come as soon as I get them ironed out). In the process of doing engine checks I found My vacuum is 8 inches @ 800 rpm. This seems low to me. Checked for vac leaks and could find none.

By marvh - 15 Years Ago
What have you set your valves at?

If valves are too tight you will have a low vacuum.

marv
By charliemccraney - 15 Years Ago
It should be more than 7. I'd guess somewhere between 13 and 18 at least.

Ignition timing, and idle mixture are other things that can reduce vacuum.
By YellowWing - 15 Years Ago
Marv, I set the valves at .018 per the cam sheet but... Engine now has 150 miles on it so I'll check again.
By marvh - 15 Years Ago
YellowWing (10/25/2010)
I set the valves at .017 per the cam sheet but... Engine now has 150 miles on it so I'll check again.




I would open them up a little more say to .019 and see if there is an improvement.

I have usually found that works.



John M maybe on here and has an opinion on what to set the valves for that cam.



I have an Isky E4 cam in my car which calls for .015 clearance. I am running .019 with no noise and 18in vacuum.

marv
By YellowWing - 15 Years Ago
Marv, checked specs and they were set at .018. Checked some of them tonite after some break in miles and some were as low as .015. Also think I have a ower valve problem which could be causing me to have more throttle open for idle which would also lower the vacuum. Mike
By shakey pete - 15 Years Ago
are you running a hot cam, if you are thats probable why your vaccum is low a stock cam runs 17 to 20 inchs /vaccum if  it is set up right that includes your timing and carb adjustment and a cloudy day  just my thought ShakeyBigGrin
By YellowWing - 15 Years Ago
I am on the way to having the problem solved. First thing was a bad gauge (was a Mac tools gauge) it was reading low, actual vac was 12 inches. Readjusted valves which made no difference. Removed the carb and found a couple problems, secondaries were not closing completely, the stop was adjusted correctly but the small link that pushes them closed when the primaries are at idle had some slack in it. But the biggest problem is a bad power valve. It is leaking so throttle plates were open .050 inch at idle! Once I have a new power valve in it and can close up the throttle plates. I think the vacuum will then be normal.



Interestingly the engine was still running very well, just some idle inconsistencies and a slight surge at cruising speed. I had suspected a bad power valve and that is why I had done a vacuum check to determine what size to install for the replacement. Will let you know how it works out. Mike