Low Vacuum, where to look?

ID89GT · Feb 8, 2011

I have a 1980 Coupe with a 351w that is rebuilt. The car did sit for ten years and since then I have had the carb rebuilt, and the intake manifold gasket replaced. The car has also had a recent valve adjust which is what im thinking may be my problem..

The cam card called for .020 for the valve lash clearance. Well this was to much since the exhaust valves have these spacer type discs in them. I would take the car out for a drive and the spacer would slip out and I would have to pull over and tow it back home.. I ended up putting the clearance at .014 and it was fine after that.

The car only pulls 5 inches of vacuum at idle and it is steady. The car also runs really rich, even after downsizing the jets. The carb is a Holley 600 DP, has mechanical secondaries. No matter what I do with the metering screws the vacuum don't change..

Any help on where to look, or how to test if my valves are to tight would be awesome, thanks

PoppyMod · Feb 8, 2011

ID89GT said:
I have a 1980 Coupe with a 351w that is rebuilt. The car did sit for ten years and since then I have had the carb rebuilt, and the intake manifold gasket replaced. The car has also had a recent valve adjust which is what im thinking may be my problem..

The cam card called for .020 for the valve lash clearance. Well this was to much since the exhaust valves have these spacer type discs in them. I would take the car out for a drive and the spacer would slip out and I would have to pull over and tow it back home.. I ended up putting the clearance at .014 and it was fine after that.

The car only pulls 5 inches of vacuum at idle and it is steady. The car also runs really rich, even after downsizing the jets. The carb is a Holley 600 DP, has mechanical secondaries. No matter what I do with the metering screws the vacuum don't change..

Any help on where to look, or how to test if my valves are to tight would be awesome, thanks

I would start checking all of the ruber vac lines first. Pull the intake and re-seal using all of the best practices. If you have a CAM card, research for it's intended vac setting. The grinder should be able to provide the compression PSI, VAC reading etc. ...I would suspect all manner of ills, lf the PO used spacers. What a botch!

woodsnake · Feb 8, 2011

If you have had that much trouble with the way the engine runs, I would SERIOUSLY consider a few options.
1. a new cam and lifter set.
2. Totally reaccomplish the valve train set up on the motor.
Depending on what you mean by spacers, it sounds like nothing has been done correctly with your geometry. If you are losing shimswhile under a load, I would say that you just need to plan on a new cam and lifter set, as well as get prepared to have some work done on the heads.

ID89GT · Feb 8, 2011

There are no vac lines except for the one going to the dizzy and I've replaced it already. All the vac caps have been replaced as well.

As for when the shims would come out, ya it would only be while under load. I would be able to rev it up just fine sitting there in neutral and such, but when I took it out to drive is when it did it.

68RCodeConv · Feb 17, 2011

How big of a cam do you have? 5 inches of vacuum is not much - takes a pretty radical cam to go that low.

Bump the initial timing to about 12. The vacuum should improve and it should allow you to close the butterflies enough to get the carb on the idle circuits (in other words the idle screws should start having an effect). If it likes 12 then try 15. Warning - a lot of initial can cause hot start problems. A mini starter can solve this problem.

And there is absolutely no way the lash caps (the little discs) on the end of the exhaust valves should be coming off at .020. You have serious valve train geometry problems. I'll bet the pushrods are the wrong length. Rotate the engine and see where the rocker arm is touching the end of the valve. It should stay right in the middle.

jcode68 · Feb 17, 2011

You mention that it revs good while in neutral but not under a load while driving and also runs rich. Might want to double check your firing order to make sure it is correct. If you are off it will rev fine in neutral but under load will have no power. All it takes is just 1 plug wire to be crossed. Would also explain running rich since it wouldnt be burning the fuel going to 2 cylinders. Not sure this would have any effect on vacuum, but it's a simple thing to check and costs nothing to fix.

Low Vacuum, where to look?

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Low Vacuum, where to look?

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