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Username Post: ZZ4 vacuum advance        (Topic#359380)
Posts: 10

Age: 74
Loc: Amarillo, Texas
Reg: 02-14-17
01-05-20 10:49 AM - Post#2783742    

I am looking for a reasonable explanation as to why the ZZ4 is equipped w/a "specially curved " distributor w/a vacuum advance and instructions not to hookup. I have always believed vacuum advance, sourced from manifold vacuum, was primarily used to bolster fuel mileage and since my 34 Chevy sees significant road miles, fuel mileage is a prime concern. What am I not understanding here?

34 Master 5 wdo, 64 Corvette roadster, 64 4x4 Suburban, 72 C10 Highlander,

Edited by PHROG on 01-05-20 02:03 PM. Reason for edit: No reason given.


Valued Contributor
Posts: 3717

Reg: 04-15-05
01-05-20 03:22 PM - Post#2783758    
    In response to PHROG

Because GMPP "tuned" the engine ON A DYNO, and NOT in a vehicle on the street. This gave the way more used than should be "top fuel" curve for an engine that does not want it.

The 69120 ZZ vacuum advance is no different in operation, nor end use than any other tuned for the application vacuum advance, and with ONE very simple change/addition, can be made to work very well for the ZZ series engines.

The key to being allowed to use the advance with full manifold vacuum is to stop down the number of vacuum advance degrees, and change the other settings that the dyno forced unwitting GMPP to eliminate it. SIMPLE stuff, just not much common sense, and GM had the answer right in front of them.

Ever take a close look at a GM factory stock points distributor vacuum advance, and its aftermarket replacement? If so, what is missing on the aftermarket advance?

Answer, a simple rubber tubing bushing on the pull pin, to regulate just how many degrees of crankshaft timing is given. After market replacements don't have the bushing, and stocker's this old, the stop is long rotted away.

The lack of the stop brings countless posts on "When I put the vacuum advance hose on the fitting, the timing goes from 12 degrees I put in as initial, all the way to 35 degrees, what goes?".

Well, the bushing stops down the degrees to work the way the advance should, without it in place, disaster.

Question, just why do you want to add a simple section of rubber hose tubing, when for a couple of bucks more, you can get a stop fitted that will never rot away, mounted in the right place, and, is ADJUSTABLE? No brain'er, ask for the vacuum advance modification package, it is totally FREE, NO ADS, just complete info on fixing it right, the first time.

Ask here, gmvacuumadvancemodificati

You will be amazed at how easy to do, and low cost it is to do it the right way.

Frequent Contributor
Posts: 2006

Loc: Lake George, NY
Reg: 11-11-15
01-05-20 04:33 PM - Post#2783763    
    In response to IgnitionMan

Great info.

Member #271 Senior Chevytalk Moderator
Posts: 25079
Loc: Austin, TX 78748
Reg: 05-09-00
01-06-20 04:55 AM - Post#2783795    
    In response to Shepherd

I agree with Ignitionman. My ZZ4 is set to 8 degrees. It runs on ported vacuum. In my '83 C10, it has managed up to 18.5 MPG @ 70 MPH. A 700R4, 245/60R15 tires, 3.08 gears, dual LS1 cooling fans, and bed cover combo work good. City average is 13 MPG if I keep my foot out of it. It can turn ~1950 RPMs all day. The most important thing I've learned from long distance driving is to run it at 195 degrees. Anything cooler and it gulps fuel.

My plan is to remove the distributor and send it in for modifications. I'll leave the timing mods to an expert.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles

Valued Contributor
Posts: 3717

Reg: 04-15-05
01-06-20 05:47 PM - Post#2783850    
    In response to IgnitionMan

The ZZ distributors are easy to fix.

Do the vacuum advance mods I outline in the package, set the advance degrees stop for 8 to 10 degrees.

Set initial to 12 degrees, add vacuum advance to full manifold vacuum NOT PORTED. Once on full manifold vacuum, idle timing will be between 20 to 22 degrees, just right.

Mechanical advance: the degrees, 22, of mechanical advance is good, the speed of the stock mechanical advance was slowed down way down to compensate for the ported vacuum advance, and the really bad second acceleration timing curve the engines do not like.

To fix the mechanical advance speed, change springs only to get curve to start 100 or so rpm's above out of gear idle speed, limiting mechanical speed to between 2,800 and 3,000 rpm's.

Reference, stock ZZ mechanical advance starts about 1,300 rpms, limits just at 5,500 rpms, YES 5K plus rpm's. Stock mechanical speed os set way too high to compensate for the addition of useless ported second acceleration curve the engine doesn't like in the first place.


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