Posted by Lars Olbimmerguy on April 25, 2003 at 22:24:51 from 206.163.168.9
In Reply to: Distributor posted by Dan Lafley on April 25, 2003 at 21:33:34:
I don't think that you should expect to see any difference. The vacuum comes into play as the engine RPM's go up. Now, if you are talking about at 3000 RPM, that's a different story. The first thing to check for would be actual vacuum at the hose. I don't know the spec, but you need to get a reading on the hose at a certain spec... ie xlbs.@xRPM. If that's happening, then the next thing to check would be whether the diaphragm in the vacuum fitting on the distributor is working. Again, it's a spec thing. If you take the distributor cap off and apply the right vacuum at the fitting, the internals that hold the points should move around the shaft, changing the timing as the vacuum increases.
That's a very simplistic overview of what's happening, but it happens at increased RPM's as the vacuum goes up in the line. Two things to look for... the right vacuum in the line at certain RPMs and movement of the points drawn around by the diaphragm in the vacuum fitting.
The purpose here is to change the timing so firing happens in a different relationship with TDC (top dead center) of the pistons as the engine RPM increases.