SharkSkin

I *think* it would be #2, but I have not fully investigated the various diverter valves used and their operation. It seems that some MY cars want that connected to the front of the throttle body(83, IIRC), some want it connected to the rear. It seems clear that you want the airflow directed to the cats at idle, so an easy test would be to check where it connects to the air cleaner. You should have no air flow to that fitting at idle, and you should get airflow there when revving the engine. The idea seems to be that air should be directed to the cats at idle.

CAShark

Thanks Sharkskin! That makes sense to me.

CAShark

Here you go!

https://rennlist.com/forums/928-foru...-throttle.html

Tom Rathjen

No, not yet.

SharkSkin

Once you have the Mityvac and start doing checks, it should be pretty obvious if the advance pot is good or bad. Post your results and we'll do our best to help you forward.

BTW the docs are worth their weight in gold.

WallyP

Getting the tech info CD set should be a VERY high priority...

Tom Rathjen

Thanks. Hopefully it will come soon. And hopefully I will be able to make time to tinker during my partents 9 day visit coming up.

Ps--it is amazing how predictable the intermittent tach failure has become. I have been driving the car to work the last few days...just for fun. The tach always works initially (after sitting over night or all day), and then dies almost eactly 7 minutes later....going and coming.

PPs--I am starting to doubt that the vacuum advance/retard, whether it is working right or not, is really the cause of my high RPM stumbling/power loss. Starting to make a list of other things to investigate.

Tom Rathjen

Yeeha, the mityvac arrived. Fast ebay shipping! Cool little tool. Now all I need is quality garage time....

SharkSkin

That's the challenge, isn't it...

Tom Rathjen

OK, here's what I found.

With the distributor cap off, the advance mechanism inside the distributor begins to turn at 6 inHg. (Mityvac connected to distributor's advance port.) It's maximum rotation (advance) occurred at about 12 inHg. And was pretty linear in between. I was just eye balling the amount of rotation/advance in this test, but seemed to be up to about 6 degrees max (at 12 mmHG). I could be a bit underestimating.

Also with the cap off, the retard port did nothing. That is, pulling a vacuum on the retard port with the trusty mityvac produced no movement of the mechanism in the distributor. In fact, the mechanism felt "bottomed out" in the initial condition. (i.e., no where for it to retard) The retard port did hold vacuum, however. There was no leak. (And this was also the case with the original advance mechanism that I replaced with the recently acquired used one currently installed.)

With the cap back on and the engine idling, pulling a vacuum on the advance port produced the following net (delta) advance in timing (using timing light and looking where the mark moved):

6 in Hg 2 deg
9 in Hg 14 deg
10 in Hg 15 deg
12 in Hg 18 deg

And, again with the cap on and engine idling, pulling vacuum on the retard fitting produced absolutely no change in the timing.

So....it looks like the advance function is behaving pretty close to what the curves posted ealier show. And the retard is doing nothing at all. (Just as I believe the original distributor advance's retard port did nothing at all.) So, the big question is, should the retard port be doing any thing on this model/year? Or is it a dead/unused/dummy port? It is worth noting that the port on the TB associated with it was originally plugged when I got the car (the one referred to as "1A" in earlier posts), and the retard port left open on the distributor.

Also, I measured the vacuum produced at TB's ports 1A, 1B, and 3. (I didn't mess with 2 just now.) I got the following:

Port 3 (the "ported" vacuum): at idle, nothing. With throttle beginning to open, quick jump to 5 inHg. Max vacuum at open throttle 15mmHg.

Port 1A: 17 mmHg at idle. Increased to 20 mmHg with throttle beginnig to open, then a rapid fall to 0 when opening further.

Port 1B (which I have connected to the charcoal valve): 17 inHg at idle, fluctuates up and down a little with slow increase in throttle, then eventually drops to 0 (but at noticeably more throttle than 1A)

So that's what I've got. My initial opinion is that the vacuum advance and TB ports are all working properly, and I am wondering if my retard is broken (on both the original and replacement) or if it is really not supposed to be functional on this car. And....my suspician is increasing that my stumbling/lack-of-power is not due to ignition/timing.

And....I discovered during this troubleshooting that my fan/air-pump belt broke. So off to the store and planning to fix that before doing more troubleshooting.

Happy Saturday!

Dennis Wilson

The retard is desiged to pull back any centrifugal advance so testing it may be difficult or impossible without the engine running at a high rpm. You might try manualy holding the plate advanced then pull a vacuum on the retard to see if it tries to move back. Also, if the retard diaphram holds a vacuum, it should work properly if the linkage is not binding and the vacuum line is connected to the correct port.

Dennis

SharkSkin

OK, sounds like the vac pot is OK, since you are not finding any leaks. Also, it seems that this design has a spring holding the actuator against a stop. This means that to test the retard portion, you'll have to attach a short hose to the advance side and pump it down until it moves a bit, say to 10" vacuum. Then pinch off this section so that the pot stays in that position when you disconnect the mityvac. Now move the mityvac to the retard port and pump it down; you should see the actuator move back the other way. You'll probably find that it's OK. At least now you know, having tested it.

Another possibility is a plugged cat, which you can also test for with the mityvac. Dennis already mentioned this, but here is a detailed diagnosis procedure. The symptoms are usually just a smooth dropoff in the amount of power, but now that you have the right tool for the job it's an easy test. Connect the mityvac to manifold vacuum, the port on the plenum for the brake booster is a good place to connect it. Rev the engine to 3K-4K and observe the vacuum. You should have a steady reading. If the reading falls off toward atmospheric pressure with the RPMs steady, it's a plugged cat or other exhaust component.

Did you try the lightbulb test to make sure you have constant power to the coil?

Another thing you can check is the air flow sensor. You may need to pull it out for this test, I'm not sure. You need to be able to move the barn-door flap through its full range of travel while checking for a smooth change in resistance on pins 6 & 7. If this car has rarely seen 4-5K RPM the variable resistor in there may be dirty at one end of its travel, affecting the signal to the brain. You can also check this at pins 6 & 7 at the brain, but do so with the battery disconnected so you don't accidentally frap out the ohmmeter.

Another possibility is that your injectors are not capable of flowing enough to run at that RPM. A pint of Berryman's B-12 poured into ~1/4 tank of gas will usually clear out slightly clogged injectors. Every 6 months or so I do the pint - 1/4 tank and run it till the low fuel light comes on, then fill up.

Tom Rathjen

I did both of these things (testing the retard port with the mityvac at high RPM, and also with the cap removed and holding the mechanism in an advanced position)...and got no response at all. Also...I don't quite understand how the retard function could be something intended at high RPM, because the vacuum from the TB port drops of to 0 at all but idle and barely open throttle. Right?

I didn't do this yet. But...doesn't testing the retard port at high RPM do essentially the same thing? (Since the advance is "advancing" at high RPM) I'll do the above procedure to be sure, but I don't think I will see anything.

I'm out of time for a while (daughter basketball game, and then dinner party), so I will get back on this later. Anxious to test the cat converter as described. Also, I did not do the light buld test yet, because that was something I was going to do to troubleshoot the stalling (which has been fixed). But yes, that sound like a good thing to do to see what the ignition is doing at high RPM. And I have messed with the air flow sensor before, but it deserves another look.

Thanks! Back later.

SharkSkin

Now that it's clear that the pot is held against a stop by a spring, It's also clear that there will be no visible effect from pumping down at the retard port unless it has already been pulled off the stop due to vacuum at the advance port.

At high RPM, it is the mechanical advance(weights & springs under the rotor) that is advancing your timing. If the pot is still against the stop at this point applying vac to the retard port will do nothing. The way your system is designed, it's a balancing act based on the vacuum difference between the two ports.

Dennis Wilson

There are two types of timing advance. Centrifugal and vacuum. If you are going 60 mph and let up on the gas, the vacuum advance backs off but the rpm of the motor will keep the ignition advanced due to the rpm of the engine. If there is no retard function, this could cause misfiring. i.e. the retard device moves the timing back to the same as when the engine is idling but no further.

Dennis