67 3x2 Vacuum Advance Enhancement

[Tom C.]

April '08 Corvette Enthusiast article by John Hinckley details method to modify tri-power distributor vacuum advance from "ported" to "full" thereby improving idle stability, low-speed engine cooling, throttle response, & fuel economy. All parts have been purchased, I'm anxious to move forward. Has anyone already performed this modification? Any additional insight? I recognize this modification is NOT factory correct.

[Duke W.]

Re: 67 3x2 Vacuum Advance Enhancement

I recommended this modification for L-72 and L-71 on this board years ago, and several have done it with with excellent results as stated. The VAC needs to be changed to a NAPA VC1765 or equivalent in any other brand name and all should be stamped "B20" on the mouting bracket.

If you have a L-68 (or L-36) through '67 without K-19 I believe that these engines have OE full time vacuum advance, but L-72 and L-71 do not, and I'm not sure about L-78.

The other C2 engine that can benefit is '63 FI, which has ported vacuum advance (it went full time in '64), but the VAC must be changed to a VC1810 or equivalent (stamped "B28"), but I understand that they are no longer being produced. The '63 L-76 has full time vacuum advance, but the OE 201-15 VAC will cause idle stability problems since the Duntov cam only generates about 12" idle vacuum and the 201 VAC needs 15.5 to be pulled and "locked" to the limit.

Duke

[Stuart F.]

Re: 67 3x2 Vacuum Advance Enhancement

Duke;

Sorry to depart from the original thread here, but I believe you have provided him what he needs.
About the 63 L-76, which I have, I recall we tossed this subject around some about the vacuum advance recently and I concluded that I might be wise to switch to a #236 in place of the #201 in order to smooth out my idle a bit. I don't know if it was my conclusion or if I read it into your advice. I now have the parts on hand and am about to make the change this weekend. Should I proceed or not?

Stu Fox

[Duke W.]

Re: 67 3x2 Vacuum Advance Enhancement

My '63 L-76 suffered chronic idle stability problems in the early years. About 1966 I figured out that the 15.5" OE 201 VAC was the cause because it "dithered" with the mere 12" of idle vacuum with the OE Duntov cam, which was the cause of idle destabilization and stalling.

The dithering VAC caused a loss of total idle timing creating a feedback loop - idle speed dropped which lost more vacuum and vacuum advance until the engine would just stall.

At the time I installed the 236 8" VAC in '66 I also installed with '64-'65 centrifugal weights and springs, which considerably increased low end torque and the whole deal also improved fuel economy.

I've discussed this issue a number of times before.

Duke

[Stuart F.]

Re: 67 3x2 Vacuum Advance Enhancement

Duke;

Thanks. Sorry to bother you again on this old subject. I just got excited when I read your mention of it in this thread. I long ago reworked my mechanical advance and installed the Petronix II electronic ignition conversion. Actually, I don't really have an idle problem perse, certainly not to the degree that you had to deal with. But, I always strive to make things better if I can w/o compromising the basic esthetics of the car. I run the 3:36 rear with the close ratio 4-speed, of course. I was experiencing some low speed stumble when starting out from a stop, specially when cold. I actually solved some of that by putting the correct throttle return spring on in place of my modified white spring (yes, I was fooled all these years by that). I've also had a choke pull-off problem, but believe I have resolved that as well, thanks to the valuable advice from you and a few others.
Have a great weekend.

Stu Fox

[Duke W.]

Re: 67 3x2 Vacuum Advance Enhancement

If you still have a OE 201 VAC you should check that it actually still meets spec.

Also check vacuum @ idle speed. The Duntov cam will pull about 12" at 900.

Then put dial back timing light on the tab at idle. The total idle timing should be about 16 plus whatever initial you are running, and it should be steady.

All engines with full time vacuum advance should have a VAC that keeps the diaphragm pulled to the limit at no less than 2" less than typical idle vacuum. I call this the "two-inch rule" and have written about it on the Board many times.

Duke

[Stuart F.]

Re: 67 3x2 Vacuum Advance Enhancement

Duke;

Thanks. I will proceed on that basis. One other item on my project list for the weekend is to pull the valve covers and do a dry set on the valves. I am sure we all have our favorate way to do this. I gave up on hot running sets when I was in my teens, but I still have the clips in my tool box. I prefer to do them cold and have for 45 years with good results. I believe we each develope our own "Feel" and should probably stick with it for the sake of consistancy. Having said that, I am sure I should probably do the valve adjustment first before doing any vacuum checks as I recall the settings can affect the vacuum readings by an inch or two.

I'd be interested in hearing what your methods are for doing a setting of the valves on your L-76.

Thanks again;

Stu Fox

[Duke W.]

Re: 67 3x2 Vacuum Advance Enhancement

Email me through the Board and I will send you a tech paper on how to adjust SB mechanical lifter clearances with a very specific procedure and recommneded settings for all OE mechanical lifter cams including all the whys and wherefores on why my recommended clearances are different from GMs.

Of course, you must be absolutely positively sure that you have an OE mechanical lifter cam and which one it is.

Duke