1972 Chevrolet Service Manual valve adjustment

[Terry M.]

Re: 1972 Chevrolet Service Manual valve adjustment

Terry,
Here's an interesting point, the car's initial timing based on the engine placard is 4 BTDC. The car best at 10-12 BTDC. When I was playing with the distributor the other day and reset to 4, it ran poorly so I reset it 10-11 and the stake marks line up. I don't recall the exact words I used when I talked to MacNeish but it was something along the lines of "I want a good running street car, no drag racing, no high speed/high rpm runs etc". Just a good running street machine. I didn't know enough (still don't) about optimum advance curves for various applications.

I don't recall the numbers, because it was long ago, but I found the chisel marks on my car lined up at a different point than the emissions tag said also. Interesting points.

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

Honestly what you need to know is if you used a 90 degree rotation on the valves or 180 degree rotation on the valves... if the later, readjust the valves - using a 90 degree rotation after adjusting the 2 noted valves.

I definitely used the 90 degree rotation schedule because I made my own damper tape and marked it in 90 degree increments.

[James G.]

Re: 1972 Chevrolet Service Manual valve adjustment

OK then I think you have a fuel related not a spark related problem...

part throttle breaking up or intermittent hesitation just sounds like Holley primary fuel circuit.
I think that if you are running a fuel with any ethanol you should change your 68's out to 70's, maybe 72's which is what 70-71's have... SEEMS like a LEAN STUMBLE,
Maybe your accelerator pump is not giving a full shot, IF you have checked that it is a steady stream , then what does it do when in the problem area stumble hesitation, and you goose it a little, thus throwing a douse of raw fuel into the throttle bores, does it pick right up? OR continue to stumble?

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

As an added question, would a dose of 100 low lead avgas prove or disprove the ethanol/gas question? I mentioned I have access to avgas and have considered it but at $5/gallon haven't pulled the trigger on that yet. Of course, with 500 +/- miles per year the cost is neglible.

[James G.]

Re: 1972 Chevrolet Service Manual valve adjustment

at 500 miles a year the cost IS negligible... which is the conclusion I came to myself and I drive mine 2000 miles a year or more.
However your car is jetted much leaner than the previous two years. My original fuel pump failed after probably 1000 miles of me driving, but not bad, not enough to really notice except in hind sight was the cause of irritating idle problems, they went away immediately upon replacing, still need to send it off.

[Gary S.]

Re: 1972 Chevrolet Service Manual valve adjustment

While reassembling the engine components, I decided to pull the primary bowl off and look at the primary jets. They are definitely 68s but I haven't taken the secondary bowl off yet.

Secondary's are 73.