Vacuum testers...

[shawntmartin]

Can someone give the short version of how a vacuum tester tests for optimal timing? Do I just adjust my distributor until I get the highest vacuum reading?

[Joekool]

A vacuum guage is used to find optimal idle air/fuel mixture, you turn the idle A/F mixture screws out one at a time till you reach the highest vacuum reading then turn the screw back in a quarter turn, adjust one screw at a time and adjust each screw multiple times and you will be all set.

[SecGenLvr]

I was only taught to use the vacuum tester as a means to check your car after a full tune up. I adjust my idle screws to RPM on my car then set the timing and plug the advance back in, then manifold vacuum should be like 29 in. merc. or whatever...
I always thought they just worked bacause when your car was working properly, which includes the iming being correct, then the presure in your manifold side of vacuum should be 29.... and I think when your timing in advanced you lose presure bacause the car is combusting early and causeing malfunctions in the flow of everything, and when it is late it is doing the same...
Whenever I saw that I had the wrong advance from the vac guage rarely trying to adjust my ignition fixed the problem...
John

[Joekool]

Originally Posted by SecGenLvr

I was only taught to use the vacuum tester as a means to check your car after a full tune up. I adjust my idle screws to RPM on my car then set the timing and plug the advance back in, then manifold vacuum should be like 29 in. merc. or whatever...
I always thought they just worked bacause when your car was working properly, which includes the iming being correct, then the presure in your manifold side of vacuum should be 29.... and I think when your timing in advanced you lose presure bacause the car is combusting early and causeing malfunctions in the flow of everything, and when it is late it is doing the same...
Whenever I saw that I had the wrong advance from the vac guage rarely trying to adjust my ignition fixed the problem...
John

Mabye 19"/hg but 29"/hg is impossible to pull with an internal combustion engine, 22"/hg of vacuum is about the max Ive ever seen an engine pull at idle. For a stock engine 17" to 20" of vacuum means the engine is in good health, add a big cam, intake or heads and that number will be lower. Have a wornout or damaged engine and that number will be lower and not steady. A/F mixture has the greatest effect on engine vacuum though taken to extremes yes timing will cause low vacuum readings, but a vacuum guage itself is generally not used to find optimal timing, mainly because an engine will show good vacuum with timing anywhere from 2* BTDC to 30*+ BTDC but neither end of the spectrum is anywhere near optimum.