Tom Rathjen

OK, I did this. Vacuum on the retard port did not move it, even after it had been pulled off the stop by a vacuum held (by pinching hose) on advance port.

SharkSkin

That sounds really strange. It may take 11in/Hg on the advance port to get it to overcome the spring and move off the stop, but then it should take very little to offset it via the retard port since the spring is helping to push it the other way.

Are you sure it didn't slip and end up back on the stop before you started pumping on the retard port?

Also, with the vac hoses connected, what is the timing at 3500RPM? Switch the hoses on the distributor and try again. Please post both readings.

I'm kind of going back to first principles here, because the details in the WSM are fragmented and confusing. So, with your base timing set to spec without the vac hoses, connect up the hoses and see what the total timing is. If it seems reasonable within a few degrees, it's time to look for other causes for the stumbling. The fact that your timing was going too far advanced then backing way off made the vac advance sound suspect, since your test of the centrifugal advance checked out. The change in timing would definitely cause stumbling IMHO. Let's see if that behavior has changed with the different vac pot, and see what happens with that same test after swapping the hoses.

If that ~17° reduction in advance is no longer occurring and you still have a stumble then we can consider your timing good enough and move on to look at other possible causes.

Re: Vac ports:

It looks like 1A should go to the retard port on the dist.
1B should go to the diverter valve(Ref. "Service Info Tech 1981.pdf", P.9 -- no mention in 1982 book).
Port 3 should go to the advance port on the dist.

Tom Rathjen

I tried the "pinch off vacuum in the advance port" test at several vacuum levels...10, 15, 20 inHg. For any vacuum above 5-6 inHg, the mechanism did move off the stop. And it did hold the vacuum through the test, because when completed and I released the pinched hose, the mechanism was observed to settle back to the stop. And it was easy to see that it was not loosing the pinched vacuum and creeping backwards toward the stop, because of the relative position of landmarks. There was no reaction to any amount of vacuum applied to the retard port at any of these advance port vacuum levels. I also experimented with whether or not applying vacuum to the retard port had any affect on how much vacuum it took on the advance port to begin advancing it off the stop. It didn't. It always began advancing at about 5-6 inHg, with or without vacuum on the retard port.

That original problem of advancing, and then backing way off, was corrected when the hoses were hooked up correctly. Originally, the advance was not connected to the ported vacuum on TB, but to one of the others that had vacuum at idle. (So, at the very least this thread led to identifying and fixing that hose routing problem! Which was huge.)

I don't recall exactly what the timing/advance is at 3500 with the hoses attached, but it is about 35-40 BTDC. And I don't recall at all what it is at 3500 with the hoses detached. (It is set correctly at 23 BTDC with hoses detached at 3000 RPM.) I can test those cases later, but it's too late tonight to make a lot of noise. (Neighbor complained once.) I got sidetracked this evening by discovering a leaking coolant line which then had to be replaced. (Interesting how things like that keep popping up now that the car is starting to be run more after it's long dormancy.)

But here is some news:

I tried the cat converter test. I hooked up the mityvac to a line coming off of the front of the intake manifold (one that was a little easier to tie in to than the brake vacuum)....see picture. At idle, it was about 15 inHg. It stayed at about this level until about 3000 RPM. Then, it rapidly dropped off to about 5 inHg at max throttle (which was about 4K RMP). Now, it did stay steady at any given engine speed. But the vacuum very definitely dropped off at high RPM. So...if I understood your description of the symptoms of a bad/clogged cat conv (and if the line I tapped into is appropriate for this test), then perhaps I found the problem?

SharkSkin

All good news, don't worry about 3K vs 3.5k. I just picked a number that ensures that the centrifugal advance is maxed. It sounds like you're done fooling with the timing for now -- Don't rule out going back later to fine-tune it if it pings, or if you want to go for more advance -- but it doesn't sound like it's a problem anymore. I'm still puzzled by the no-op retard port(maybe it really IS retarded ), but leave it for now.

Just to make sure I understand your description -- you were gradually increasing RPM, and seeing the manifold vacuum remain relatively stable, then without any drastic change in the throttle position, it rapidly dropped to 5 in/Hg? That definitely sounds like a plugged exhaust.

First, go tell your neighbor that you are going to make a lot more noise, but just for a couple of minutes and you're done. Offer to take him for a ride if it will grease the skids. Disconnect the exhaust behind the cat. Repeat the test. If it still does the same(possibly at a slightly higher RPM) then your cat is plugged. If the problem goes away then some other part of the exhaust is plugged. If you have the car nosed-in to the garage, you can mitigate the noise a bit by putting a piece of plywood behind the car -- probably good for a few dB.

Tom Rathjen

Yes, I think that describes what I observed pretty well. I gradually/slowly increased throttle, and the RPM gradually increased accordingly. The vacuum reading stayed at about 15inHg pretty constantly (a little bit of wiggling) until RPM began to exceed 3K. Then it began falling off pretty quickly, like down to about 8-10 in HG over the next couple hundred RPM (about 3200), and then down to around 5 or less inHg somewhere between 3500 and 4000 RPM. I would like to repeat the test (when I am not so worried about finishing it quickly because of noise). What manifold vac should I see as RPM increases in the 3-4K range?

Yikes, running the car without the muffler connected. All the neighbors are going to complain. I may need to go out in the country somewhere for this. Hmmm.

Just got word I am being sent to Florida for a few days. So I will have to get back on this at the end of the week.

Thanks!!

SharkSkin

Manifold vacuum is load-dependent. Higher load, lower vacuum reading. If you are increasing RPM gradually it should stay high, ~15in/Hg. If you slam the throttle plate open at any point it will drop momentarily even on an engine with an open exhaust, though if you are already at a high RPM when you do so the drop will be less pronounced.

You may be able to do this at a friend's house, or a local shop. Maybe even do it in the garage with the door closed, but do it quick and open the door ASAP to get some fresh air in. For example, there is a small independent muffler shop near me -- good guys -- they would gladly do it for me. Or you could throw parts at it. I prefer to get a positive diagnosis before changing parts, wherever possible.

Tom Rathjen

Back from Florida.

I just repeated the original test (cat still connected to the exhaust pipe and muffler), but took more time and did it more carefully. And now I'm not sure that it isn't doing exactly what it is supposed to (after reading the post above). When I increase the throttle very very gradually, I get the following:

1500 RPM, 18.5 inHg
2000, 18
2500, 17.5
3000, 17
3400, about 16
4000, about 12.
I could not get it above 4K at max throttle. And it was starting to misfire (or stumble) at max throttle.

Yes, when I rapidly increase the throttle, it drops to 4 or 5 inHg. And then gradually rises to the numbers above.

I'm surprised I am not seeing the more drastic drop above 3K like I thought I saw Sunday evening when doing this test. I guess I was not opening the throttle as gradually as I thought, since I was kind of rushing due to the noise at night.

So, darn. I thought I had a real smoking gun, but now I'm not sure. I definitely don't want to start throwing parts like a new exhaust system at it (or going through the pain of replacing exhaust components) without better diagnosis.

Also....I messed around a little with the AFM. I did not take it off and test it yet. But, I took the air filter out and used a dowel to try opening the flapper valve manually at various engine speeds. In all cases, forcing the valve open just a bit more made the engine speed drop and in some cases stall. I'll still pull it out (easy) and test for smooth resistance change across its full range, but I'm thinking this quick test seems to show that it is working right.

PS--one that that I have not thought to mention in all this is that the "stumbling" at high engine speeds is much worse under load. That is, when I rev the engine to 3K and above while idling in the garage, it is pretty smooth up to about 4K. But when driving, especially up a hill, the stumbling is very pronounced beginning just above 3K. And there is a real lack of power.

SharkSkin

Interesting. OK, so it doesn't seem to be plugged exhaust(That's why I asked the more detailed questions). Do check the AFM and inspect the wiring. I think it's pins 6 & 7 you care about, and the wires lead to the same-numbered pins on the control box.

Some other possibilities -- fuel flow(See if fuel pressure drops around 4K RPM), I assume you've checked for arcing plug wires(fireflies in the dark), another thing you might want to look at is the distributor again. IIRC it has a mechanical revlimiter, if it has a broken or weak spring it could cause your ignition to cut out early. Put a timing light on the coil wire and rev it to 4K. It should give you a very fast flashing light, almost indistinguishable from an "always on" light. Does the light continue to flash at the same rate when the car begins to stumble or does the light stutter?

Have you checked the coil ground? Also inspect around the coil terminals for signs that the oil is leaking out. I don't have the WSM handy, but IIRC the coil primary should check out to around .3 - .4 ohms, the secondary about 5-10K ohms. These measurements are from the center high voltage terminal to the side terminals.

A real quick, shadetree stone-axe check is to have someone activate the throttle(or use a piece of cord ) to the point where it's stumbling. If you smell raw gas at the tailpipe, it's an ignition issue. If not, it's a fuel issue. This test used to work a lot better in the pre-catalytic converter days but is still worth a try IMHO.

You have an 02 sensor, right? Is the wiring good, and does the sensor check out?

Tom Rathjen

What does "IIRC" mean? (Some forum lingo I'm still learning?) Are you saying that the distributor has a rev limiter in it that cuts off the ignition? That's interesting. I'll try that test of putting the timing light on the coil wire and reving it till it stumbles.

I'll also test the AFM with an ohm meter as soon as a can. That should be easy, and will be good to cross that off the list if it checks out.

Yes, it does have an O2 sensor. The O2 sensor light comes on in the dash when the engine is first started and then quickly goes out. So I have assumed it is working properly. I have not checked the wiring closely, though. I could do that.

I put in a new coil in the process of getting the car running. This was a new part, not used, so I think it is good.

I have not seen any arcing from the plug wires. They all look in really good shape. I'm sure they are not the original equipment; someone must have replaced them.

What is a good method for testing for fuel pressure drop at high RPM? I put in a new fuel pump a short time ago, since the old one was making a noise. But I have been wondering if the problem could be the fuel regulators.

Thanks

mark kibort

remove the 02 sensor, in case its bad. you dont need it and it will idle a little higher without it.

the voltage at idle should be about 4.7 volts off the hot wires of the AFM, near 8 at WOT.
are the coils good, or showing signs of arcing on the terminals?

all you can reallly do with the fuel is put a gauge on the rail.

I feel your pain on this one.

SharkSkin

IIRC = "If I Recall Correctly". Any time you are confused by an acronym check the "New Visitor" link at the top of the forum. There is a set of acronyms and their meanings in there.

I'm not sure when they started putting the revlimiter in the control box instead of the distributor, but yes -- the early years had a mechanical revlimiter in the distributor. Either way, the test I described stands a good chance of revealing a "loss of spark" issue. It's easier for some people than others to detect variations in flashing light at high frequencies. You could also try it on a plug wire.

Checking the AFM off the list will help, as with many tricky problems it comes down to a process of elimination.

The O2 sensor is not cheap, it's worth checking. Voltage should be ~0.45v when the engine is running at the proper mixture, ranging from ~2v to ~8v(Lean to rich, respectively).

OK, we'll assume the coil is fine. Also the ballast resistors that power it must be OK, or you would be seeing different symptoms.

Plug wires OK, check.

Roger at 928srus.com has a fuel pressure gauge that attaches to the fuel rail, either as a permanent installation or for testing only. I don't know the price offhand but I don't think it's particularly expensive. This would be more than adequate for the rough troubleshooting you are doing now, though there are fancier, more accurate setups out there for testing(not permanent mounting).

The debate goes on in some circles as to whether or not it's a good idea to mount one permanently, as doing so adds a potential point of failure and high pressure fuel spraying in the engine bay is no joke.

SharkSkin

Mark, when you use the plural form of "coil" I wonder if your advice is applicable to the early car. In any case, I'm guessing you didn't notice that he mentioned installing a new coil.

Regarding the O2 sensor, are you recommending he simply unplug it as a troubleshooting step or remove it completely? If the latter, I'm not sure it's safe to say he "doesn't need it". Maybe it depends on whether or not you know the guy that does the smog inspection. Maybe it's more important than you think, especially for non-track use.

WallyP

One free check on the ignition system:

The amount of voltage required to fire the plugs increases as more fuel/air mix is allowed into the cylinder, since the mixture is an insulator. The more insulation you pack in the plug gap, the higher the voltage required to fire thru the insulation.

This means that it is possible for the ignition system to fire the plugs at starting, and at low throttle openings, but to have serious misfiring at higher RPM and load levels.

One quick check is to temporarily close the plug gaps to 0.020" - 0.025". If the engine runs better, you need to fix the ignition.

Tom Rathjen

Wow, thanks for all this. You've all given me enough to keep me busy for a while. I'll see how much I can get done today and tomorrow and report back.

And thanks for the reference to the new visitor link for abreviations. That's awesome.

Tom Rathjen

Well, it didn't take long for me to find the next suspicious thing.

I took the AFM off, and tested across pins 6 and 7. And the ohm readings jumped around. Full closed was about 120 ohms, and full open was about 600 ohms. But in between it was not at all smooth. I took off the cover, and gently cleaned the rheostat surface. I could see a number of "bumps" along the arc, one about every 10 degrees or so. When the rotating probe touched those bumps, the resistance would drop to about 120 ohms. And in between, it would be about 600 ohms. It appears that these bumps coorespond to connection points on the printed circuit board, and that perhaps the continuous resistive surface that should be running all along the arc and over these connection point bumps has been rubbed off or something. Anyway....it sure doesn't seem right. About what should the resistance at full open and full closed be?

Also....the only other thing I have done so far today is do the check with the timing light on the coil wire and on each of the plug wires and reving it. None of these tests showed anything unusual. The strobing seemed as it should, even when the engine started to stumble at the full throttle and high RPM.