bouncy timing mark...

[Kevin N.]

hey guys...i'm trying to help a friend with a 66-425 hp t.i. coupe. we've checked the archives and found nothing. we have a problem with the timing mark jumping around approximately 6-8 degrees when putting the timing light on it. idle is a tiny bit rough inspite the normal lope. after about 3,000 rpm it runs great. very responsive when reving it up after 3,000 rpm's. the motor is newly rebuilt, so the timing chain is not the problem. the compression is 155-165 across the boards, so there is nothing bent as far as valves are concerned. the distributor has very minimal slop in the end play. the distributor is stabbed right. we've been told some areas to address are...vacume advance, springs and weights under the rotor, harmonic balancer (seems solid, though), the t.i. itself, plug wires too close to each other (hard not to be under the shielding or not), fouled plug (got new 1's) or bad plug wire (new, too). it seems that the majority of us backyard mechanics tend to lean towards the springs and weights under the rotor. is there anyting else we can do and check? thanks in advance.

[John G.]

Re: bouncy timing mark...

Does the timing bounce with the vacuum disconnected??
John G.

[Kevin N.]

Re: bouncy timing mark...

hey john...yes it does. when we unplug the vacume line from the advance there isn't any rpm drop. the line has vacume, but we haven't put a gauge on it to see how much yet. the carb was just rebuilt. when the motor's turned off, i pull the vacume line from the carb and suck through it and the plate in the distributor moves, but a very small amount. any suggestions would be greatly appreciated.

[Duke W.]

Re: bouncy timing mark...

I am assuming you still have what is believed to be the original and unmodified TI system.

What is the number stamped on the VAC mounting bracket? Does it meet spec?

Is the centrifugal mechanism clean and free? Is it OE or modified? Measure the centrifugal curve with a dial back timing light.

As built, all L-72s have ported vacuum advance so there should be no vacuum on the VAC signal line at idle. Converting to full time is a good idea, but may require a change from the original or installed VAC.

Was the engine rebuilt with the OE cam or something else?

What is the idle speed and manifold vacuum at that speed?

It sounds like the VAC might be nearly frozen, but we need to cover all the bases.

Duke

[Kevin N.]

Re: bouncy timing mark...

duke...1. system is completely oe stock to the car (L72). 2. i'll get the # from the bracket asap. 3. not sure till i get the #. 4. completely clean and moving freely. 5. seems oe. 6. don't have dial back light. 7. all rebuilt to oe specs. 8. we have had idle speed anywhere between 600-1,200. didn't ever check vacume at those times. still had bouncyness at any idle speed. there will be a delay in response as my friend lives in florida and i'm in california. the car never ran right, but we decided it was a misinstalled distributor. we restabbed it and now it runs better, but it has this timing problem. unfortunately we never checked it with a timing light before we restabbed the distributor to see if it had this problem then. it just ran to bad to put a light on it then. the car ran great when he first bought it (6-7 years ago) it did alot of sitting. ran it a few times a year. sounds like a good possibility that the vac (like you mentioned) is frozen up maybe from lack of driving. i'm guessing it's something reallly simple, but we just can't see it. i'll get back here as soon as i get those numbers from the bracket. thanks...

[Kevin N.]

Re: bouncy timing mark...

I am assuming you still have what is believed to be the original and unmodified TI system.

What is the number stamped on the VAC mounting bracket? Does it meet spec?

Is the centrifugal mechanism clean and free? Is it OE or modified? Measure the centrifugal curve with a dial back timing light.

As built, all L-72s have ported vacuum advance so there should be no vacuum on the VAC signal line at idle. Converting to full time is a good idea, but may require a change from the original or installed VAC.

Was the engine rebuilt with the OE cam or something else?

What is the idle speed and manifold vacuum at that speed?

It sounds like the VAC might be nearly frozen, but we need to cover all the bases.

Duke

duke...the number on the mounting bracket is MS 360-12. i don't have any publications to see if that's to oe specs or not...

[William C.]

Re: bouncy timing mark...

the original vac advance was a "355". specs are 15 degrees advance at 12 inches of vacuum. The 360 is similar but gives 12 degrees of advance at 12 inches of vacuum. Neither should cause the issue you are seeing unless you have a vacuum lead or a sticky valve. What is the hot idle vacuum at idle?

[Duke W.]

Re: bouncy timing mark...

The specs I have for the '66 L-72 OE VAC from both Corvette News and the AMA specs are 0 @ 6", 12 @ 12", degrees measured at the crankshaft, so it's always been my understanding that the "360-12" was OE on L-72

Given age, many of these original VACs have failed either due to a broken rubber diaphragm or internal corrosion. They can be static measured with a Mighty Vac or equivalent to see if they start at 6" and end at 12" by watching the breaker plate move. Then they can be checked on the engine with a Mighty Vac and dial back timing light to see if they deliver proper advance for a given manifold vacuum

There is not direct replacement available, but the best fit is the NAPA VC1765, which is 0 @ 6", 16 @ 12". This VAC, which is equivalent to the OE 355-16 will also work if the vacuum advance is converted to full time with the OE cam, which should idle at about 14" @ 900.

All current replacement VACs are made by Standard Motor Products. They are sold under different brand part numbers and cost about ten bucks. In the case of the VC1765 or the same part in any other brand box it will be stamped either B20 or B26. They both have the same specs within manufacturing tolerance.

Duke

[Kevin N.]

Re: bouncy timing mark...

the original vac advance was a "355". specs are 15 degrees advance at 12 inches of vacuum. The 360 is similar but gives 12 degrees of advance at 12 inches of vacuum. Neither should cause the issue you are seeing unless you have a vacuum lead or a sticky valve. What is the hot idle vacuum at idle?

william...i will get my friend to get the reading asap. that might take a few days as he is real busy with work...

[Kevin N.]

Re: bouncy timing mark...

duke...i'll have my friend pick one up to replace the other and see what happens...

[Stuart F.]

Re: bouncy timing mark...

Any thought towards having a intake vacuum leak (carb mounting or manifold)?

Stu Fox

[John G.]

Re: bouncy timing mark...

I may have missed something, but I believe kevin replied that timing bounces even with vacuum disconnected, which would lead me to look toward mechanical timing. jmho.
John G.

[Kevin N.]

Re: bouncy timing mark...

Any thought towards having a intake vacuum leak (carb mounting or manifold)?

Stu Fox

stu...i don't believe an intake leak would make the timing mark jump around, but i will have my friend check that out...

[Stuart F.]

Re: bouncy timing mark...

Kevin;

Your initial description of your friend's problem left open a lot of possibilities being as how the engine had just been apart and put back together. That's why I mentioned vacuum leak as a possible source. Over the years I have seen many different (recent reassembled) engine running problems which have been traced to all kinds of potential causes, but all were due to something that was changed during the reassembly, i.e. twice I have seen engines running erratically due to the assembler (professional) installing the cam one tooth off. I have seen many improper carb base gaskets used that you'd swear looked ok, only to find they were the source of a minor vacuum leak and the only way to confirm it was by trying another one in the vein of covering all bets.

Now then, has your friend located his problem as yet?

Stu Fox

[Kevin N.]

Re: bouncy timing mark...

stu...as of yesterday he still had it. i apologize to you guys. the motor has been rebuilt, but it was 10-12 years ago. very few miles, but not recently fresh. it always ran good, but something was a little off. that's why we restabbed the distributor and that helped alot. we never checked the timing with a light before we stabbed it, so we don't know if it was bouncy before the restab. we did, however, this time have the carb redone. maybe, like you said, the gasket could be bad and leaking air. i'll tell him to try a new one just in case. i'm still betting on either the weights or springs under the rotor. possibly the vac itself. just grasping at straws i guess. i wish someone here has had this problem so he could say "this is the problem, do this." i'll keep ya all posted...