vacuum gauge question

[Kagan]

Hi there,

I bought a vacuum gauge and connected it to the fitting on the side of Demon Carb. The manual describes it as "direct manifold vacuum for diagnostics, accessories or distributors that use manifold vacuum for timing advance". The other is "ported or timed vacuum for spark advance to occur with throttle opening". My Electronic distrubutor is connected to the second one.

The gauge moved very fast between 23-27 in/hg. When I played with the carb idle screws the values moved up to 25-29 in/hg and down to 21-25 in/hg. But the gauge moved really fast.

What do you think about these? Valve problem, idle mixture problem? Are these correct? or any other faults with the engine and/or carb?

thanks in advance..

 

[wild70stang]

leaky head gasket. Its not air tight so as the piston travels its sucking air in through area that is not sealed. A trusty shop manual helps, if not web sites for vacuum info work too such as this Browser Warning

 

[brianj5600]

Why do you think something is wrong with the car? Is it down on power? Smoking? Pushing water out the overflow? Running rough?

I you are worried about the vacuum gauge jumping, I would not. Does it have a tiny restrictor in the line? Vacuum changes every time an intake valve opens. There should be a tiny orifice in the line to keep the needle from jumping.

 

[ForceFed70]

Why do you think something is wrong with the car? Is it down on power? Smoking? Pushing water out the overflow? Running rough?

I you are worried about the vacuum gauge jumping, I would not. Does it have a tiny restrictor in the line? Vacuum changes every time an intake valve opens. There should be a tiny orifice in the line to keep the needle from jumping.

I agree.

Could be that your guage is accurately reading the manifold vacum. Does is still "jump around" at higher RPM?

 

[BarnStang]

Found a nice "color" gauge chart. You will need to gain some experiance with your gauge. It's not cut and dry. THere is a lot of grey area, simply meaing that some of the readings are open to interpertation. THe chart is a "guide" only.
http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/vacuum.pdf


THe needle should not be jumping around, but it depends on what the engine is doing...make sure you have a good properly functioning gauge first.

I like using a vacuum gauge, some don't. I can tune a carb by ear, but I always watch the gauge while I am setting things. I use it as a reference, not as a right or wrong.

You do have the gauge connected properly. Full manafold vacuum. If you are setting idle mixture on a Holley, set for maximum vacuum while reducing curb idle-the one on the throttle shaft. It will not be like setting an air pressure regulater, you are looking for a best reading. See how that goes.