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Dead Parrot · 03-10-2018, 08:50 PM

The behavior with ported could be due to your setup being on the very edge of starting to pull ported vacuum at idle. When you pop the throttle, the vacuum advance comes in, speeds up the idle enough to put enough vacuum on the ported port to hold the advance in place. Try it on manifold and readjust your idle.

Vacuum advance is a part throttle efficiency device. If it is working properly, it is out of the picture at or near WOT.
So at WOT: timing = initial + mechanical, in your case a max of 38*.
At part throttle: timing = initial + mechanical(varies with RPM) + vacuum. Under light high RPM throttle, you can have close to 50* advance.

03-10-2018, 08:50 PM	#4
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,509	Re: Vacuum timing The behavior with ported could be due to your setup being on the very edge of starting to pull ported vacuum at idle. When you pop the throttle, the vacuum advance comes in, speeds up the idle enough to put enough vacuum on the ported port to hold the advance in place. Try it on manifold and readjust your idle. Vacuum advance is a part throttle efficiency device. If it is working properly, it is out of the picture at or near WOT. So at WOT: timing = initial + mechanical, in your case a max of 38. At part throttle: timing = initial + mechanical(varies with RPM) + vacuum. Under light high RPM throttle, you can have close to 50 advance.