72 Timing Advance

[Rod McCormick]

[Patrick H.]

Re: 72 Timing Advance

Rod,

What vacuum can is on the distributor? and have you checked the advance curve?

Also, try plugging the vacuum take off on the back of the intake so that all vacuum systems are disconnected and the intake is "sucking in" no vacuum. See if that helps. The surging sounds like it could be from a vacuum leak.

There are at least 2 350/200 carbs for auto cars in the Driveline right now. I'm not sure why the Holley went on, but I always mistrust "professionals" who swap the Q-jet for a Holley "just cuz."

Patrick

[G B.]

Re: 72 Timing Advance

How do you know the timing mark on the balancer actually indicates true top dead center? Did you check this with a piston stop tool or just eyeball it by watching the rocker arms?

[Mike Swanson]

Re: 72 Timing Advance

I have the exact issue with my 67 L79. I thought the balancer had slipped so I tore it apart and checked it against a new one. Both the same. I have been through all the steps. Locationg TDC with a piston stop(halfway between both marks) located TDC exactly at the position on the balancer but it will not run whan I set it at 10* BDC per spec. In order to get it to run properly, my balancer line is at ~12 oclock position.

It runs great but I want to get this resolved so I guess next step it to pull the timing cover and check cam index to crankshaft.

Good luck, I will be following this thread closely.

See my post and other responses back in April. Search on C2 L79 Harmonic Balancer.

Mike

[Mike M.]

Re: 72 Timing Advance

Rod,
My educated guess? The distributor drive gear is installed backwards. The mechanic may have rebuilt the distributor and put the gear on wrong. The dimple in the gear should line up with the metal output leg of the rotor.
Why this guess? I rebuilt the distributor in my '72 350/200 and installed the gear backwards. Had to time it as you stated just to get it to run. Finally figured out my goof.
Mike

[Rod McCormick]

Re: 72 Timing Advance

Mike,

The drive gear is one that I would have never figured out. I just thought they were symetrical - but apparently not. I think I'll try testing the vacuum 1st; probably this weekend. If that checks out OK, then Ill check the gear. If both fail, I guess I'll have to pull the timing cover.

[Rod McCormick]

Re: 72 Timing Advance

I took out the #1 spark plug, rotated the engine by hand, and inserted a small screw driver to feel TDC. Then, looked at the timing marks. They lined up pretty darn close to perfect - certanly not off much, if any.

[Rod McCormick]

Re: 72 Timing Advance

The vacuum can looks original as do the timing curve pieces and springs. But, how do I tell if they are stock or not?

[Patrick H.]

Re: 72 Timing Advance

Rod,

Remove the distributor cap. On the bracket portion of the distributor's vacuum advance facing up are some numbers. What are yours?

Now, take a vaccum pump (or a small hose leading to your mouth) and apply vacuum to the advance can. Does it hold vacuum? If not, then it's useless and could be a portion of the problem.

I went through a lot on Dad's 72 before I figured out the dist advance can was so poorly matched to the engine that it was causing a host of similar problems.
Else check the gear on the bottom as mentioned below.

Patrick

[Patrick H.]

Re: 72 Timing Advance

Jerry,

Have you ever truly seen one that really was not lined up?

Just curious, that's all.

Patrick

[G B.]

Re: 72 Timing Advance

I've found the balancer mark way off on about 10 of the sixties cars I've checked. The mark wasn't really "off" on those balancers, it's just that the wrong balancer had been used for the engine timing tab.

Chevrolet moved the timing mark on the balancer several times after 1967. I timed some trucks in the seventies by pointing the light straight down behind the water pump. There are many Chevy balancers out there with a factory TDC mark as much as 10 to 30 degrees different from the stock location for a midyear small block. There's nothing wrong with these balancers if they are used with the right chain cover timing tab.

I think that old wive's tale about the outer balancer ring "slipping" (rotating) is a bunch of crap. Outer rings do come off, but they don't simply rotate on the inner hub. I have found the factory balancer mark to be off a couple of degrees though. I believe that's probably within production tolerances. You'd never feel that little difference when you were driving.

You're not the first person to question my credibility on balancer marks. I had an FI customer who flat refused to leave his timing alone after I reset it correctly.

He had a post-'68 balancer and a pre-'69 timing tab. His balancer mark was off by exactly 10 degrees. I put a black "X" over the balancer scribed line and painted a new line in the correct place 10 degrees away. Well, this Ironhead never accepted the new mark. He reset his timing using the old incorrect scribed line, and then tried to cure the car's poor performance by changing my calibration settings on his FI unit! He ended up with a car that would barely run. He finally called me and announced that "something had gone wrong with the fuel injection" after he had "fixed" the timing. Just shoot me.

[Patrick H.]

Re: 72 Timing Advance

No, no, NO.

I was NOT questioning your credibility. I just cannot recall anyone who ever really witnessed a "slipped" outer ring. I figured that if anyone might have it would be you, so I asked.

I was trying to learn from your years of experience.

Patrick

[Joe L.]

Re: 72 Timing Advance

Jerry-----

I know of only one timing mark change for small blocks. That occured for 1969 and later small blocks of all displacements and configurations. So, there's the "pre-1969 timing mark" and the "1969 and later timing mark". The timing mark affects both balancer and timing tab, though. So, these components cannot be "mixed-and-matched". I don't think that GM ever sold or used a bolt-on timing tab for pre-69 small blocks. The aftermarket might, though.

[Tom H.]

Re: 72 Timing Advance

I might be just unlucker than everyone else, but the balancer on my 72 200 hp was original when I bought the car, and after two months of running, it did slip. You could put a timing light on it and sometimes you could see the mark just where it was supposed to be, and sometimes it was gone. The car ran smoothly,just no timing mark visble. It took me a week to figure it out, because the two pieces stayed together evenly. It was moving several inches inside the outer ring. When I took it off, you could look straight on and see where it slipped. It would slip off the mark, and back on. The rubber had let go, and would allow it to move around. I doubt that this is your problem, but it can happen.

[G B.]

Re: 72 Timing Advance

I should have learned my lession years ago about relying on my personal experiences when posting on this board. I'll document the specific applications for the "behind the pump" timing tabs before I ever comment this subject again.

I also remember a "through a tube" timing tab located north of the mid-year position, but again I digress. I'm recalling Chevrolet trucks, funky displacements, and non-Corvettes I'm ashamed to have repaired. Even worse, I have no home movies or credible witnesses to back me up.

Rather than take this any further off topic than I already have, I'll shut up now and return to my home planet....er, I mean go to bed.