Vacuum fluctuations

[Joe R.]

Was doing a routine check of my manifold vacuum on my 54 Blue Flame, and noticed that it fluctuates wildly between 15 and 22 in. HG. I have the gauge "teed in" and can dampen it by pinching the hose, and then I get around 18" HG.

I checked the valve adjustment, and they are fine, at 0.010 intake and 0.020 exhaust (hot).

Does anyone have any ideas about what can cause this? Is it normal? The engine seems to run fine.

Joe

[Jim T.]

Re: Vacuum fluctuations

Vacuum line going to wiper motor leaking?

[Joe R.]

Re: Vacuum fluctuations

No, that line is good. Wiper motor is good also.

[Jay G.]

Re: Vacuum fluctuations

Does the indicated vacuum reading cycle regularly (like once every other revolution)? If so then it's a bad valve in the engine.

Jay

[Joe R.]

Re: Vacuum fluctuations

Ouch! That was one of my fears, Jay. It's a fresh motor, only about 1,800 miles on it. I'll do a check on it tonight. Thanks for the suggestion.

[Jay G.]

Re: Vacuum fluctuations

Joe,

This link has some great information about how various different engine problems show up on a vacuum gauge.

http://www.centuryperformance.com/tuning-with-a-vacuum-gauge-spg-148.html

Regards,

Jay

[Duke W.]

Re: Vacuum fluctuations

The fewer the cylinders/lower the idle speed, the more rapid fluctuation you get on the vacuum gage. Pinching off the line as you did damps out the variance, but if the vacuum signal pickup is somewhere near the carb base, it should be fairly steady on a six cylinder engine. Usually the vacuum advance signal line is a good source on engines with full time vacuum advance. V8 engines with dual plane manifolds usually have a small vacuum signal passage from each set of four cylinders to the vacuum advance port, so the idle vacuum readings are fairly steady.

You could do the same thing as pinching the line by placing about a 1/32" restrictor in the test tubing. The danger in pinching the line is that if you pinch it off completely, you will just get the last reading, which may be false.

Any valve leakage should be easy to spot with a compression or leak down test, but if you get a steady 18" by pinching the line to restrict the size of the passage, I doubt if a valve is leaking.

Duke

[Joe R.]

Re: Vacuum fluctuations

Duke, I did a standard compression test (hot enine, all plugs out) and got 120-125 in all 6. Might I ASSume that if an intake valve were leaking, a cylinder could not get to that pressure?

I was taking the vacuum readings from the manifold, tee'd at the fitting that goes to the vacuum modulator valve in the Powerglide. I'll try at the distributor vac advance line coming from the ceter carb.

Thanks Duke and all.