427 manifold vaccum question

[Bill S.]

i have a 66 L72 427/425 hp. is 15 too high? that is what i read at idle. if so what causes it and how can i correct it? most importantly what are the issues caused if it is too high,bill

[William C.]

Re: 427 manifold vaccum question

At what RPM? To much manifold vacuum is seldom if ever a problem. Msot have too little

[Bill S.]

Re: 427 manifold vaccum question

at 850 rpm off the port in front of the dist. another member sugested it indicated a problem. it is a newley rebuilt engine to GM L72 specs.

[Duke W.]

Re: 427 manifold vaccum question

As a reminder to all, reporting idle vacuum without reporting idle speed doesn't mean much because manifold vacuum varies with selected idle speed for a given engine configuration.

With full time vacuum advance, idle speed vacuum increases over the OE ported vacuum advance at the same idle speed.

Most other owners who converted to L-72s to full time vacuum advance report about 14-15" Hg manifold vacuum at idle speeds ranging from 800-900.

If you lower your idle speed, manifold vacuum will decline. Set the idle speed at the lowest speed that is commensurate with acceptable idle quality. Because this criterion is subjective the final result varies among owners.

The vacuum advance control must also provide full vacuum advance at 12" or idle may be unstable. The OE L-72 VAC is 12", but it should always be checked for compliance with spec or replaced with a NAPA VC1765 or equivalent.

Duke

[Bill S.]

Re: 427 manifold vaccum question

i stated idle seed at 850 as per spec. i read 14 to 15 at this speed with constant vac. will teast w/o thanks,bill