c2 timing moves after initial setting

[Robert B.]

I have a 1966 327/300 4 speed roadster with factory/ac. I am having issues with my timing moving. If I set the initial timing to 6 degrees BTDC then drive the car it retards anywhere between 4 to 6 degrees.

Heres what I have tested. My timing light is good and the timing mark is steady. I have changed my vacuum to test. It is a the NAPA correct type can and advances 15 degrees at 16 vacuum. Mt car had steady idle vacuum of 21. Both the old VC and new VC are good. Centrifigul
advance and distributer are original. The distributer provides 30 degrees of advance at about 5000 RPM's. Springs and weights are clean and don't stick. Distributer has correct bushing play and has been checked and in spec. Dwell is steady at 30. Distributer bolt is also tight and holds distributer.

Any ideas would be great.

[Bill I.]

Re: c2 timing moves after initial setting

Bob, are you setting the timing with the A/C off? That's the way you should. Bill.

[Robert B.]

Re: c2 timing moves after initial setting

Bill,

Yes a/c off and advance plugged. Intital idle set at 750 rpms.

Rob

[Bill I.]

Re: c2 timing moves after initial setting

Bob, the other thing that concerns me is the total advance, it should be around 36 at a much lower RPM. I don't know. Hopefully, John Z. will see this, he KNOWS dists. Bill.

[Duke W.]

Re: c2 timing moves after initial setting

30@5000 is the max centrifugal spec for the OE distributor.

The idle timing should be checked at 500. You may have to set the normal idle higher to keep the engine from stalling when the compressor is engaged.

Check for wear in the breaker plate hole where the VAC rod engages. It should be snug and the breaker plate should have no angular play.

Pull vacuum repeatedly on the VAC and verify that the initial returns to the proper setting each time vacuum is removed. It could be a defective VAC.

Duke

[Timothy B.]

Re: c2 timing moves after initial setting

Also, check that the engine is not idling on the curve. Usually the centrifugal advance does not start until about 900 RPM on 300HP but the springs may have been changed and centrifugal starting early so 750 RPM is not a good timing RPM. Try to time at 500-600 RPM with vacuum advance plugged and leave it unplugged to see if the timing changes.

Check that base plate to see if any free movement when vacuum advance can is plugged. 21" vacuum is high, are you sure about the test equipment/timing light in good working order.

[John H.]

Re: c2 timing moves after initial setting

Remove the cap and rotor, install a stout rubber band around the weights, button it back up, and check your initial timing again, and make a note of it. Then remove the rubber band and check the initial timing again - if it shows more advance without the rubber band in place, you're idling "on the curve", which shouldn't start until 100-200 rpm above the base idle speed.

[Harry S.]

Re: c2 timing moves after initial setting

Is the performance the same before and after it retards, If yes, check your harmonic balancer, it may be slipping.

[Donald O.]

Re: c2 timing moves after initial setting

If the cam gear and chain are the originals, I'd be highly suspicious of them.

Don

[Robert B.]

Re: c2 timing moves after initial setting

Thanks everyone. I will try the list of items in the post tomorrow an let you know what I find. Test equipment is good for sure. All other ideas I will recheck.

Thanks,

Rob

[Robert B.]

Re: c2 timing moves after initial setting

I did a few tests. It appears the distributer base does not move. I reset the car and reset the timing at 8 degrees @ 500 rpms and then drove the car without the vacuum advance. After the test drive I rechecked the timing it was advance to 12 degrees. I then reset the timing back to 6 degrees and drove it again. When I rechecked the timing it was at 10 degrees. At this point I quit for the day. I will redrive and check today to see if it moves off the 10 degree mark. If it doesnt move today I may be ok. So it appears to be advancing 4 degrees. It does not detonate at 10 degrees. Is this too high? This would give me 40 degrees of total advance as my distributer does give the correct 30 degrees of advance.


I also checked the breaker plate, it has a little play, but not much. Vacuum advance works fine and does not hang up. But since I tested it without the vac advance I sort of eliminated this as a problem.

It must be something in the distributer. I have no play side to side and no high rpm miss.

Any thoughts?

[Duke W.]

Re: c2 timing moves after initial setting

Keep in mind that one degree of radial play in the breaker plate is two degrees at the cranshaft. Also check the breaker plate for wobble. It should fit snuggly on the housing and not show any significant wobble.

Use a small screwdriver to tweak the breaker plate via the point adjustment window while the engine is idling and see how much that effects timing.

It may not detonate at 10 degrees initial because full centrifugal is not achieved until 5000 revs, but most of it is in by 4000.

Also start checking dwell angle to see if it is stable. If the dwell changes, timing will change. It's possible that the points adjusting screw does not have enough tension and is moving around. If dwell decreases the timing will advance and vice versa.

A "slipping balancer" can also cause this problem. Though rare, it is more of an issue with the SHP balancer. Run a test with the initial at six. Drive the car several cycles and see if it keeps slipping or if it just slips a few degrees the first cycle and then remains stable.



Duke

[Donald O.]

Re: c2 timing moves after initial setting

I did a few tests. It appears the distributer base does not move. I reset the car and reset the timing at 8 degrees @ 500 rpms and then drove the car without the vacuum advance. After the test drive I rechecked the timing it was advance to 12 degrees. I then reset the timing back to 6 degrees and drove it again. When I rechecked the timing it was at 10 degrees. At this point I quit for the day. I will redrive and check today to see if it moves off the 10 degree mark. If it doesnt move today I may be ok. So it appears to be advancing 4 degrees. It does not detonate at 10 degrees. Is this too high? This would give me 40 degrees of total advance as my distributer does give the correct 30 degrees of advance.


I also checked the breaker plate, it has a little play, but not much. Vacuum advance works fine and does not hang up. But since I tested it without the vac advance I sort of eliminated this as a problem.

It must be something in the distributer. I have no play side to side and no high rpm miss.

Any thoughts?

Sounds more and more like the plastic coated teeth on the cam gear being worn beyond its intended life

Don.

[Richard M.]

Re: c2 timing moves after initial setting

Distributer bolt is also tight and holds distributer.

Any ideas would be great.

Robert, Just as an aside, you may want to disconnect the tach cable at the distributor as a test also. Our 327/300 '63 coupe did a strange thing a while ago. The tach gear in the dist jammed up, and turned the distributor, even though the clamp bolt was very tight!

Rich

[Robert B.]

Re: c2 timing moves after initial setting

Thanks, I will do some more tests.