What should manifold vacuum read on a 1975 6/300?

 

16-22" is what I have always heard, the higher the better.

 

Thanks. I'm trying to figure out why my C4 is not shifting and thought I'd start by making sure the mod valve has enough vacuum.

 

In the manuals a V8 manifold vacuum is supposed to be a little higher overall than a 6. 18" to 20" for V8. And of course it is altitude dependent. Subtract 1" vacuum for every 1000' above sea level for a corrected figure.

17" would be a good number for a 6, at factory idle RPM. A lot of people like to goose the throttle up and try and get a higher number, but that's not how it works. Connect to a source of manifold (constant) vacuum and measure vacuum at a typical factory idle spec. Note whether it is steady.

Compression can be checked by disabling ignition and blocking throttle and choke open and cranking engine over for 5 to 10 seconds. Should get the same 17" on the gauge. Slow or late ignition timing would cause low manifold vacuum and would contribute to shift problems.

 

Just replying to see what Tedster9 has to say

 

With factory cam, engine in good shape and well tuned should be 18'ish (as posted). Ignition timing, carb settings/condition and health of the engine can change that number quite a bit.

 

The highest vacuum I can get is 17-18, but the needle
won't steady and hovers around those two numbers. Also if I accelerate the needle drops. Do you think I have a vacuum leak?

 

On older engines a little needle shaking is fine, as long as it's not doing the Fandango all around the face.

Upon acceleration, or any "goosing", engine vacuum drops, then regains equilibrium.... The gauge needle should drop to near zero then back up to 30" - 35" Hg, then back to equilibrium. In this case, 17"-18" HG.

 

Agree vacuum sounds OK and behaves normally.

If you measure while running under load with throttle fully open vacuum should drop to near zero.

 

Sounds pretty good! 17" to 18" is respectable for a six, and means the engine is basically sound.

A slightly wavering hand or needle could just mean the carb needs fine tuning. An actual vacuum leak would mean a low figure like 12" or 13". Improper or late ignition timing will show up as low manifold vacuum. Vacuum gauge reading takes experience, but it's easy.. If you dig around online and search, there are a few different vacuum gauge charts and illustrations on what to look for and tests to perform, these are REALLY handy. They are all generally the same but there are always a couple different tests or indication in each chart. I don't try to memorize them. The stuff these things can do in the hands of an experienced mechanic is pretty amazing. Every toolbox should have one.

 

That's good to know. Thanks

 

Yes Tedster9 I agree. I just bought the vacuum gauge and for what it costs it's a great tool to have.

 

They are great for carburetor and engine tuning, power valve selection, all kinds of stuff. A lot of savvy folks keep one tapped into the manifold permanently to keep an eye on it inside the cab in the gauge cluster. They are part of what helps make the difference between an engine that runs pretty good and REALLY good.