What could effect ignition timing while running? Strange problem.

[BadDog]

Last summer, I noticed I had some pinging under load (up a gentle hill in 4th or 5th gear) while driving home from a show in Rochester. When I checked my timing the next day, it was dead on what I last set it to in the early Spring. Took it out for a drive, similar temperature and humidity, no problems at all.

Not long after that, I started noticing that my idle was occasionally high at stop lights. Not much, just a couple hundred rpm. I figured something was sticking, so when it happened once when I was parking anyway, I jiggled various carburetor, throttle, and choke pieces, but no change.

I figured I'd do another minor tune-up, so when I went to do that and was letting the car warm up, I heard the rpm jump a little by itself, and after a few minutes it dropped back down. I can't remember for sure, but there may have been a "tick" sound when it happened. It was definitely an abrupt change, it wasn't gradual.

What I have:

Stock engine internals

Header

Open-style air cleaner

SU's

MSD6A

Pertronix igniter w/flamethrower coil

Pretty new distributor cap, rotor, and wires

Rebuilt distributor approx. 2 years old

NGK BP6ES(?) plugs

The only recent changes made to anything are the open air cleaner (custom modified stock housing) and some new Hella horns. I do think I have a small amount of throttle shaft air leakage. The distributor was purchased from Z Car Source (i think) and I tried checking the vacuum advance and it doesn't seem to stick, but I'm not sure how to do an accurate/thorough check.

The MSD is about 3 years old, and I've had the pertronix and coil for about 10 years, and they've been rock solid except for when one of the magnets came loose in the pertonix setup. Alternator is also about 10 years old.

I once had a wiring harness issue (corrosion) where the regulator(?) hooks in and I had to "jump" one wire across the plastic harness connector so that the voltage gauge wasn't getting pegged.

I'm considering trying to source another rebuilt distributor to rule that out. I've also considered I may have an overall low-voltage situation (dimming lights etc.) and may need to get one of those 80(?) amp alternator setups.

Anyone else have any helpful suggestions? Am I overlooking something obvious? I'd like to get some leads on this before I take her out for the Spring

Thanks all!

[madkaw]

You mentioned charging issues or voltage low, you need to get that right to expect your ignition to work right. Low voltage and electronic ignition don't mix. I'd be checking your voltage when the rpms bounce around. Clean up all your grounds and connections, then go from there.

What type dizzy?

Edited March 18, 2014 by madkaw

[djwarner]

You are focusing on the distributor timing with very little evidence. But since you ask, two things affect timing dynamically: rpms and vacuum. Rpms are sensed by fly weights resisting centrifugal force with a spring. You should be able to verify this is working by measuring the time with a strobe lamp as you rev the engine. You should see the timing advance smoothly with rpms and smoothly retard as the rpms drop.

Vacuum advance can be checked with one of those brake line vacuum hand pumps. With the engine off, connect the pump to the vacuum line at the end that connects to the manifold. Squeeze the pump until you get 10"Hg. If the diaphragm and tubing is good, it should hold that value. Release the vacuum and start the engine and observe the timing with the strobe as you slowly pump up the vacuum. The timing should start to advance above a certain point (varies with model distributor) and retard as you release the vacuum.

Assuming you have a D609-56A distributor, centrifugal advance starts at 550 rpm and advances 9 degrees at 1200 rpm. Vacuum advance starts at approximately 4"Hg and advances 7.5 degrees at 13"Hg. What you are looking for here is not a specific degree of timing, but rather the change in timing as the input is varied.

Fly weight mechanisms are known to hang up when worn. Vacuum diaphragms crack and leak with age. Their mechanical linkages can also hang up when worn.

Edited March 19, 2014 by djwarner

[BadDog]

Thanks madkaw, I will double-check grounds and connections. Stock dizzy (rebuilt).

[BadDog]

Thanks djwarner, that's a very straightforward explanation of how to check the vacuum advance.

Yes, I may be focusing a little too much on the distributor, but that's what my gut is telling me because it seems like my timing is advancing by itself under some set of conditions (temperature-related?), staying that way for an indeterminate amount of time, and then abruptly kicking back to "normal". The rebuilt distributor isn't that old, but maybe there is a problem with the centrifugal advance or the vacuum advance, I don't know where the "tick" sound might be coming from when that happens.

On the other hand, it could just be electrical and maybe my base timing is too far advanced, but under low-voltage conditions everything seems OK and when voltage is "normal" then the idle is higher, pings under load, etc. Changes in the electrical status-quo could account for the abrupt change in idle, but again I don't know where the "tick" sound might be coming from when that happens.

I'll explore both of these avenues some more in the coming weeks. Thank you both for your help!

[Jetaway]

Could be separate issues. In your first post you said you heard the pinging while going up a slight hill in high gear coming back from a show but didn't the next day in similar weather conditions. But it wasn't clear if the test drive included a grade. If not, I wouldn't be surprised, the uphill ping just suggests you were running right on the edge of a too-lean mix.

You wouldn't have A/C would you?

You may have a throttle return valve that slows down the return to idle. Like the vac advance, it has, or mine does, at least, has a rubber seal that could wear out. That might explain the not returning to idle.

Now the jump up / fall down idle is a puzzler. Well, it could be the throttle return valve or the vac advance suddenly losing engine vacuum, which would increase the rpm's at idle. A stretch, for sure, but if I had to go one way or the other, I'd go with a vacuum leak somewhere rather than something electronic. If nothing else, should be cheaper to investigate.

Chris

[BadDog]

Update: re-cleaned battery terminals and grounds, put some protectant on the posts, and replaced the terminal clamps.

Checked my plugs and remembered that I opened up the gap wiiiiiide to like .75 after I installed the MSD 6A. I thought maybe it was misfiring occasionally so I put new plugs in gapped to factory specs to eliminate that. Idle is steady, but still changes occasionally.

Noticed the other day that after hard breaking, the idle dropped to almost stalling at around 500rpm, and then recovered after half a minute. Hmmm maybe that's lean ping under load, not timing.

I am leaning toward the lean ping idea, and I do have a little more lean pop than normal (i like to adjust the carbs so I have a tiny but of lean pop when engine braking from higher rpm, but now I'm getting pop between shifts sometimes). I tried opening the çarbs up quite a bit, in 1/2 turn increments trying to get rid of the ping under load. Now they are at 5 turns, about 2 more than "normal" for my car. The ping/rattle under load is better but still there, and I am now running very rich (it stinks ). I have a vacuum tester on the way, but it hasn't arrived yet. I think I definitely have a vacuum leak somewhere, I can hear it whistling sometimes. Afraid its the throttle body bushings and there's nothing I can do about that right now.

Jetway, my car had "original" A/C but its long gone. So are the related valves etc., as well as the old smog equipment.

[240260280z]

check for timing scatter and function by using a timing light and hand revving the engine from idle to 3500rpm.

Weak springs in the distributor can cause ping and uneven idle.

[BadDog]

No evidence of timing scatter, it looks to be rock solid.

I decided to go back to the basics and check my carb balance and fuel mixture. Its been a while since I watched my z therapy tuning video, and my VCR is out of commission, but this page reminded me of things I had forgotten about: Technical Information Page ..... Carburation

One issue was that I had forgotten was that I should loosen up the throttle linkage and balance screws when balancing the carbs. The technique recommended there for checking mixture by raising the lift pin (I didn't even know that was there!) in the underside of the dashpot mount is where it clicked for me. That's when I found that my front carb needed over 2 full turns out more than my rear carb needed. I found that my front carb's float level was way lower than the rear. All these years I've been adjusting my fuel mixture knobs the same amount, and then just test driving to look for issues. I forgot to check them individually for mixture, so I've probably been running with my rear carb way more rich than my front for years now.

I still have to do something about my float levels, but this helped so much its not even funny. No more lean ping. Great throttle response, and the engine is smoother at high rpms now. I had my idle suddenly change on me while I was tuning, but that was before I found my fuel level imbalance. Now I'm thinking that change had to do with the front bowl starving for fuel, and then getting more, so the idle would go up and then down again after some revving. I tried to bend my float tabs accordingly, but the bowl gaskets are really old and I don't have anything to make a site glass out of, so I'm not ready to adjust them multiple times in trial and error.

[240260280z]

some useful twin su tips:

you can compare plugs 2 and 5 for a quick front, rear carb functioning/ fuel mixtures

Norm's Method

you can compare load sharing of front and rear cab at idle by holding up the front carb piston (disables it) and causing the motor to chug-chug-chug-along on only the rear 3 cylinders. Repeat but hold the back carb piston up and run on the front 3 cylinders. Adjust fuel so that both run equally crappy

repeat the 3 cylinder running at 3000 rpm as a check

My Method (a variation of Norm's)

For more precision repeat the front and rear carb disabling and monitor the a/f. Set the jets so that it is the same at 3000 rpm I think it will be ~26:1 to 29:1 on one carb at idle.

[BadDog]

That's the other weird thing, my plugs almost always read about the same. Might be due to the hot coil and MSD box....

[Unkle]

That's the other weird thing, my plugs almost always read about the same. Might be due to the hot coil and MSD box....

This might sound stupid , but what about the effects of a bad batch of gas ?

Your original post suggests a possibility of a winter gas issue .

Plugs will still be brown from previous use with good gas , masking poor octane consistency . Ever run Seafoam product ? I don't know , my 2 cents .

Edited May 26, 2014 by Unkle