my 73 vette is only registering 15 on the vacuum gauge when I hook it up to the quadrajet. Any ideas what could be causing such a low reading.

 

Sure!!! Everything that uses vacuum to operate it COULD be hiding the problem. But, that's not how you diagnose a vacuum problem.

First, you disconnect ALL vacuum hoses from the intake manifold and the carb [and cap them off]. Now hook up the vacuum gauge to one of the intake manifold fittings. Start the engine and read your engine's BASELINE vacuum amount. You may have to adjust timing by rotating the distributor to determine the absolute maximum vacuum the engine will make at idle.

If that reading is 13 in Hg, then that's all the engine can make...given the components in it and the age of them (ring, valve wear, etc).

But, I'm betting that you will get something in the range of 17-19" Hg on the gauge. Thus, the engine has plenty of vacuum POTENTIAL...but there are leaks/losses in the vacuum systems -- somewhere.

To find them, hook up one vacuum line at a time and see what vacuum level you get, then. If it drops, there is some cause for leakage in that line/system. Continue to chase it down by blocking off stuff or adding it back in to see when vacuum drops.

That's the general process. Hop to it!!!

 

thanks for the heads up, I'll start there.

 

because of the low vacuum the head lights will not pop up.

 

Or they could be the cause of the low vacuum.
Disconnect them from the manifold as 7t1vette said and see what happens.

 

With 15" of vacuum everything including your headlights should work normally. As 7T1vette said, something there is probably hiding the problem. Figure out how much vacuum your engine makes without everything connected and then check each system individually.

 

Curious: Is your engine stock? Heads? Cam? Intake?

My heavily modified engine runs about 7 inches at 900 RPM. Lots of overlap. I have absolutely zero vacuum lines connected to it, not even a vacuum advance or PCV.

 

I was pulling 11-12 and my headlights worked.