Timing Jumping

[Steve K.]

Hello All, I am new to the forum and could use some help. I just purchased a 66 roadster with a L72. I was initially experiencing overheating so I first inspected the timing to find it jumping 20 degrees back and forth around TDC. I replaced the cap, rotor, points, plugs and wires but this did not help. I pulled the distributor and sent it to the shop. It is tight with new springs and the proper amount of shaft end play. Still jumping but slightly better. It use to be transistorized but someone gutted the distributor and put in points. The old module is still bolted to the drivers side fender well. I then replaced the timing chain and sprockets with a new set but still jumping. I have been able to fix the over heating by replacing the coil with a newer coil and resistor (I think it was related to the resistor) but the timing is still jumping around. I have set the timing by sound and feel but do not like this method since I could be off. The engine runs great and has plenty of power, no knocking or run on after shutting it down. I also purchased a new timing light still no difference. I have worked on car for a very long time and never seen this much variation. Any help would be greatly appreciated.

[Timothy B.]

Re: Timing Jumping

Steve,
If you can read the #'s on the vacuum advance can post them on the forum. Your timing problem may be a incorrectly matched vacuum can to the engine idle vacuum and it is changing timing with vacuum. I don't know if L72 has ported or full time vacuum to the advance can but you should be able to tell by idling the engine and removing vacuum hose and feel for vacuum.

If the vacuum advance can is not fully deployed at idle this can definately cause your engine to run hot.

[Steve K.]

Re: Timing Jumping

Timothy,

Thanks for the quick response. I can get the numbers and post them but I have run this test with the vacuum disconnected as well and the timing still jumps quite a bit. The shop also changed out the vacuum dashpot.

[William C.]

Re: Timing Jumping

What speed are you running the engine for this check?

[Terry F.]

Re: Timing Jumping

If it is literally jumping up and down while using the timing light??? You might have a broken harmonic balancer. When the rubber gives out, the outer ring will jump back and forth causing the same problem. Good luck, Terry

[Jim T.]

Re: Timing Jumping

Since you are new to the board and a new 66 Corvette owner, if you searched the archives on vacuum advance you can read a lot about it and the heating problems using ported vacuum advance.
The vacuum advance unit that was installed by your distributor rebuilder may not be the right vacuum advance unit for your 66. Vacuum advance units for small block, base, and higher performance engines as well as the big block lower hopsepower and high horsepower engines use different vacuum advance units.
Full time vacuum will definitely allow an engine to run cooler in stop and go traffic and another benefit is better gas mileage.
Ported vacuum does not supply vacuum to the vacuum advance unit when your engine is at idle because of its source.

[Timothy B.]

Re: Timing Jumping

Steve,
If the timing is changing at idle I would think it's idling on the curve meaning the springs are allowing the centrifugal curve to start at or below idle speed. Check the archives on how to check/verify exact TDC mark on your balancer, and try to set timing for total timing at approx 34-36* This has to be done by reving the engine so be careful.

You can also remove the centrifugal springs from the posts so the advance comes in with just a little reving and this makes it easier. Use your balancer timing tab and rotate the engine and put a 36* mark on the balancer so you have a mark, rev the engine slightly and set to the 36* mark.

Let the inital timing fall where it will and don't worry about it unless the engine pings. What is engine vacuum at idle as this will help us determine the correct vacuum advance control for your engine. You should actually check idle vacuum with the IDLE timing at approx. 25* but let us know what vacuum is at idle and we may be able to tell you if the advance control is correct and the correct one for your engine.

[Steve K.]

Re: Timing Jumping

Jim and Tim, Thanks for the reply, I have to say that this is the best forum I have ever been on. I have never seen so much support!! I have read several good articals about timing including vacuum advance. And I will take these suggestions into consideration. I have also searched this forum and others regarding this topic but non have shed light. The problem is that the timing is erratic at idle while the vacuum is disconnected and plugged, it jumps back and forth about 20 degrees which make setting the timing with a light impossible. I have tried the trick of reving the engine to enable all the timing (3000 rpm) but the harmonic balancer under the timing light has an apperance as if it is slowly rotating. I have never seen this before. I am going to investigate the suggestion of the harmonic balancer rubber being damaged. If this does not work than I am baffled.

The engine is not overheating anymore since I changed out the coil and resistor but the timing is still the ?

Thanks!

[Steve K.]

Re: Timing Jumping

Terry, I check the harmonic balancer an it is fine. I have tried different resistors too but patient is still unstable. I just do not understand why the timing mark would jump around so much. I am going to try to set marks ever 45 degrees and check each wire to see if they all jump around.
Thanks!!

[Steven S.]

Re: Timing Jumping

I'm just kinda talking out loud here so if this is a dumb question/idea then please excuse me, but since most timing lights now clip over the outside of the plug wire, and if you are running the braided wires and clipping out over the braided area, could that give the timing light an inconsistent signal thus causing your readings to be all over the place? Like I said, just an off the wall idea.

[Donald L.]

Re: Timing Jumping

One possible cause not discussed is the condition of the points plate. If it is worn and loose the timing will be inconsistant.

[Ken A.]

Re: Timing Jumping

First thing I would do is try another timing light.

[Steven S.]

Re: Timing Jumping

He did say he purchased a new light and tried agian.

[Thomas H.]

Re: Timing Jumping

I usually stay away from these trouble shooting threads since there are many members with greater knowlege then I, but......

"I replaced the cap, rotor, points, plugs and wires but this did not help"

Have you tried a new condenser (capacitor)? Make sure it's ground is good also.

Let us know what ends up fixing this problem....

Regards,

Tom

[Steve K.]

Re: Timing Jumping

Guys,

I appreciate all the replys, they are all great suggestions but I have covered these is great detail. I have tried a set of non-shielded ignition wires and there is no difference. I started with my old Sun timing light and then bought a newer Digital timing light, no difference. I am using original style OEM plugs and did put in a new set. When I sent the distributor out my guy did repair the slop in the point plate. Polarity of the coil is good too. Everything is tight in the distributor. There was no change when I did change out the points, condensor, cap, rotor and plugs. I definitely will post the solution when I figure it out. I am also open to any other suggestions. Thanks to all for your support!!