Technical Idle vacuum rapidly moving gauge

I’ve been working this problem for some time, finally solved it and wanted to share to maybe help another user:

Issue: at idle with Holley carb and dual plane manifold the vacuum needle bounces around.

All the gauge charts shown as bad valve guides, but after a pressure and leak down test and no issues found the hunt was on.

for me what it came down to was the vacuum port at the lower front of the carb was only picking up one bank, I’m using a dual plane manifold no air gap, and the correct dual plane gasket that separates the plenum, though the open gasket had the same symptoms.


I found the following, the milling on a Holley carb primary’s is very small so it tended to pull a signal from the closest bore, dropping in a 1/2” spacer cured the issue but took away a little from low end performance. After a lot of googling I found another user that used the power brake port off the carb for a steady vacuum, this in fact does work without the spacer, if you flip over the carb you’ll notice the milling between secondary ports is wide and a direct path to the connection.

On a side note, I moved my PCV to this rear vacuum connection and it cleaned up my idle as well and was able to lean out the mixture just a little more as the pcv valve was letting more air in on one bank than the other.

hope this helps, if your running a single plane this is not your issue, when I tested a single plane the vacuum was steady