Timing

[Christopher R.]

Just finished timing my '62. The timing seems to jump around. About 2-4 degrees at 1000 rpms. Maybe a little more at higher rims.

Is this normal?

[Ed H.]

Re: Timing

Chris,
Is the distributor both mech and vacuum. If so, it could be a fluctuation in the advance vacuum or the mech springs or the advance weights. Either will cause a change at various engine speeds. The other possibility could be the timing pulley. Over time these units can come loose and give you
a various timing reading. All can be repaired.

[Gene M.]

Re: Timing

No, is vacuum bouncing around too?

[Terry D.]

Re: Timing

Also check your distributor, is the dwell jumping around also?

[Christopher R.]

Re: Timing

The distributor has both the usual Chevrolet vacuum and mechanical advance mechanisms. When I checked the timing, the vacuum advance was disconnected.

I didn't check the manifold vacuum when I did the timing. But that's a good idea. I'll do that later when I go back to the car.

The harmonic balancer was rebuilt a few years ago. I should probably check that the timing mark is accurate. But I'm pretty satisfied that the ring is not moving on the balancer.

The car runs badly, and I'm trying to eliminate possible causes. I replaced the spark plugs. The old ones looked pretty good. Looked like the car is running rich. But not too bad. I did a compression test. All cylinders were around 150. And the plug wires checked out OK.

What got me was the jumping around of the timing. I'm trying to determine if this is normal. Or, should that mark be dead nuts on every time?

[Duke W.]

Re: Timing

You need to tell us something about the engine configuration. Does it have an OE engine? Which one? Does it have an OE equivalent camshaft? Idle vacuum at specified idle speed? Number stamped on the VAC? It would also help if you tell us the start and max points (RPM) for the centrifugal advance. Does it meet OE specs? They're in your service manual.

Spark scatter can be caused by excess distributor end play, worn shaft bushings, or by the centrifugal advance starting at below idle speed. Estimate end play and post the number. Side play should be imperceptible.

As previously mentioned also state dwell variation between idle and max RPM. The acceptable variation is no more than two degrees.

Duke

[Joe L.]

Re: Timing

The distributor has both the usual Chevrolet vacuum and mechanical advance mechanisms. When I checked the timing, the vacuum advance was disconnected.

I didn't check the manifold vacuum when I did the timing. But that's a good idea. I'll do that later when I go back to the car.

The harmonic balancer was rebuilt a few years ago. I should probably check that the timing mark is accurate. But I'm pretty satisfied that the ring is not moving on the balancer.

The car runs badly, and I'm trying to eliminate possible causes. I replaced the spark plugs. The old ones looked pretty good. Looked like the car is running rich. But not too bad. I did a compression test. All cylinders were around 150. And the plug wires checked out OK.

What got me was the jumping around of the timing. I'm trying to determine if this is normal. Or, should that mark be dead nuts on every time?

Christopher------


The instability of the timing mark is not normal. My guess would be that it's the result of a worn timing set. Of course, it could also be caused by some of the other things that have been mentioned but my bet is on a worn timing set. Your engine, if original, did not use a nylon tooth cam sprocket which is a major cause of timing set problems in 1966+ small block engines. Nevertheless, the timing sets do wear over time and use.

[Terry D.]

Re: Timing

Christopher
Let us know what you found!

[Christopher R.]

Re: Timing

Thank you, gentlemen. Will do. I haven't been able to get back to the car. Hope to do so tomorrow.

The car runs like crap. I'm trying to eliminate possible causes. I did a compression check. All cylinders were 150 lbs., +/-5 lbs. All the plug wires checked out OK. All the old plugs looked pretty good. Running a little rich. But not so much, in my opinion, to be the cause for running poorly. I changed the distributor from a dual point centrifugal only advance (985) to a generic 62-74 tach drive one with a vacuum advance. I want the vacuum advance for driving on the street. When I went to time it afterwards, I noticed the timing jumping around a bit. The timing was done with the vacuum advance disconnected. The timing was done at around 800 rpms. I know that might be too high, and I'm probably getting some centrifugal advance in there. But I can't make the engine run lower than around 800 rpms. I stopped after the timing to ask my original question before proceeding farther.

The 327 engine has been rebuilt. Probably has only 2000 miles on it. The pistons are 9.5 to 1 compression ratio. The cam is the 327/350 hydraulic one. Bob Kunz did the carburetor. The intake manifold is the aluminum '62 340 HP one (3799349). Only unusual thing I see with the engine is that the oil pressure seems very high. I attribute this to a new engine and the 15W-40 oil in it.

It's the distributor that I'm wondering about. I'm not sure of its provenance. I think I rebuilt it when I installed the Pertronix on it. But now I'm not sure. I thought I might be on to something when I saw the timing jumping around. I'll put a vacuum gauge on it tomorrow when I play around with it some more. I'm also going to run the engine in the dark with the shielding off and mist some water on the plug wires. In the meantime, I have another distributor. I'm trying to find somebody with a distributor machine that can check out that distributor.

I'm not an intuitive mechanic. If I take my time, I can swap out parts. And that usually works for me. This time, I'm trying to proceed methodically.

[Duke W.]

Re: Timing

The L-79 cam needs a 12" B26 VAC to pass the Two-Inch Rule. What about end play and side play?

The distributor is easy to rebuild as long as the shaft bushings don't need replacement, and you don't need a distributor machine to set it up. A dial back timing light will work just fine. The first thing you need to do is replace the existing VAC with a B26 is one is not already installed, and it should be connected to a full time vacuum source. Then determine the centrifugal curve.

Search for a Dec. 2012 thread started by me - L-79 dist. blueprint overhaul... set it up a described and the engine should run very well with no spark scatter.

Also suggest you do a Web search for my 2012 San Diego National convention tuning seminar, which explains how to set up the spark advance map for any engine configuration.

Typical L-79 idle behavior is 14-15" @750 with a slight lope.

Your high oil pressure is likely due to the installation of a "high volume" oil pump when it was rebuilt, which is a common error. The 15W-40 oil has nothing to do with it at moderate to high ambient temperatures.

Duke

[Joe L.]

Re: Timing

Your high oil pressure is likely due to the installation of a "high volume" oil pump when it was rebuilt, which is a common error. The 15W-40 oil has nothing to do with it at moderate to high ambient temperatures.

Duke

Duke-----

Not necessarily a high volume pump but a high pressure pump.

All high volume pumps are also high pressure pumps (i.e. have high pressure relief spring installed).

Standard volume pumps may either be standard pressure or high pressure. No L-79 was ever originally equipped with a high pressure pump.

[Duke W.]

Re: Timing

True, but it seems common for "engine rebuilders" to install HV pumps, apriori, which as you say always come with a high pressure relief spring on the order of 60 psi.

I'd say that on 90 percent of rebuilds the OE pump is okay to use if disassembled and inspected, which his a very simple and short job. About the only way they can be damaged or significantly worn is if they ingest debris.

Duke

[Christopher R.]

Re: Timing

Got back to the car today.

The distributor has 21 degrees of advance at 900 rpms and 31 degrees at 3000 rpms. These readings were taken with the vacuum advance disconnected. The readings were done with a dial back timing light. The readings are steady at 3000 rpms. They jump around 2-4 degrees at 900 rpms. I'm going to remove this distributor and substitute it for another one. I'll send that other one out first for calibration.

Big concern is the lack of vacuum. I mean the gauge reads only 2-3 inches of mercury. Hardly moves off the stop. I either have the mother of all vacuum leaks; a bad gauge; or, I'm not getting a signal from the port in the back of the carburetor.

I'll have to come back to this later. I have other responsibilities for the next week. Thank you for your help with this.

[Jerry W.]

Re: Timing

thanks Duke...that's a new word for me

A priori......relating to or denoting reasoning or knowledge that proceeds from theoretical deduction rather than from observation or experience.
[COLOR=#878787 !important]"a priori assumptions about human nature"[/COLOR]

synonyms:

theoretical, deduced, deductive, inferred, postulated, suppositional"a priori reasoning

[Joe L.]

Re: Timing

Got back to the car today.

The distributor has 21 degrees of advance at 900 rpms and 31 degrees at 3000 rpms. These readings were taken with the vacuum advance disconnected. The readings were done with a dial back timing light. The readings are steady at 3000 rpms. They jump around 2-4 degrees at 900 rpms. I'm going to remove this distributor and substitute it for another one. I'll send that other one out first for calibration.

Big concern is the lack of vacuum. I mean the gauge reads only 2-3 inches of mercury. Hardly moves off the stop. I either have the mother of all vacuum leaks; a bad gauge; or, I'm not getting a signal from the port in the back of the carburetor.

I'll have to come back to this later. I have other responsibilities for the next week. Thank you for your help with this.

Christopher-----


The first thing I'd do is check the vacuum directly from the intake manifold. If that's what you meant by "the port in the back of the carburetor", then you're using a vacuum source that should supply full engine vacuum. You should be able to feel substantial vacuum just putting your thumb over the port. If you truly have 2-3" of vacuum, I'm surprised the engine will run, at all.