Really weird ignition issue...

[PoormanMopar]

Hello,
This car is a 1972 Plymouth Duster:

View attachment 1716369281

Years ago, I swapped in 1979 360 from a van, later I swapped in a reproduction 340 cam from Mopar Performance. It has a Weiand 4 barrel intake and I had a Holley 600 4 barrel on it for most of the time I've had the car. Today I changed to an Edelbrock 750 since I had it here.
I changed the engine bay wiring to a 73-75 style so I could use electronic ignition with stock wiring. It uses all stock components.
I lined up the oil pump drive/intermediate shaft to point toward the forwardmost intake bolt as suggested by Rick Ehrenberg. Supposedly, this puts the distributor in the proper location.
With the engine timing set to 10 degrees BTDC, it idles great and runs well. When I try to connect the vacuum advance vacuum hose, the engine pops, sputters and runs so bad it stalls. Oddly, it does this whether attached to the ported or the manifold vacuum ports.
I did have Holleys on the car, both gave the same response with the vacuum line attached. I swapped on the Edelbrock and while it runs excellent at idle and under a load, if I attach the vacuum line it does the same thing as with the Holley carburetors. I can't even get a timing reading from it to see if it suddenly adds far too much timing.
One more thing....The vacuum advance cannister is touching the oil pressure sending unit. I have the "bell" shaped one on the car which is not correct since those are for cars with actual oil pressure gauges. I can't imagine that the wire terminal for the pressure switch could affect the distributor....is that even possible?

View attachment 1716369283

Hi a thought. Possible vacuum leak on the distributor advance?

 

[Kern Dog]

I rechecked the timing and placement of the distributor. Both are where I like them. I was thinking that the vacuum can was too far clockwise which would be expected if the rotor phasing was way off.
I pulled out the Mityvac.

This is the first time I’ve messed with it. I’ve had it for 12 years. An old timer that got out of the hobby gave me a bunch of stuff and this was in with it all.

I squeezed the handle and can get the dial to rise but it won’t stay. I even pulled the hose and plugged the end of the pump body with a finger and it still drops. I’m guessing the dial is supposed to hold if the end is plugged and the pump isn’t leaking.
Regardless….

M

I just found that the dial will hold if the handle is squeezed and then released a little bit.
The vacuum advance canister in this distributor only pulls to 6-7” and drops. I tested another distributor and it goes to 10” and while it also drops, it does so much slower. I take that to mean that the one in the car does leak vacuum.
It doesn’t explain why the car runs terribly with the vacuum can connected.
The car runs great otherwise. Excellent idle, great throttle response, plenty of power. The car has a 2.76 diff in it but it still feels peppy.

 

[RustyRatRod]

They are both bad. They normally pull in about 10 degrees at the distributor...or 20 crank degrees. Once a vacuum is put on the can, it should remain where it is, rock solid, unless there's a leak. You've diagnosed a bad vacuum canister. Twice.

 

[toolmanmike]

This is what I would do:
I would;
Take the carb off, and drain it.
Check and adjust the float level to the designed level.
Holding the throttle closed to prevent the fast-idle from engaging, flip it upside down, and
With the speed screw, adjust the Primary transfer slots to a little longer/taller, than wide. and from this point on, I would NOT TOUCH THE SPEED SCREW!
I would
Close the Secondaries up tight, but not sticking.
Put the mixture screws back to 2.5 turns out.
Flip it right-side up, and reinstall it.
Test the VA to make sure the diaphragm is NOT ruptured, and put it on the spark-port. Make sure it is truly on the spark-port and not on the choke pull-off port.
If the car has a Brake booster, I would clamp off the hose for now.
I would
>Plumb the PCV to the place provided for it, at the front of the carb, just below the throttle blades.
>Use only thick-walled hose rated for PCV use. I would not plumb the PCV to the back of the intake plenum, nor to any single-intake runner. This is VERY, almost extremely important. This calibrated air-leak is the Idle-Air bypass, and that 340 cam, in a low compression ratio 360, is gonna want that bypass air. In fact, probably it will need even more air but we'll get to that later.
Finally, I would take all the slack out of the accelerator pump linkage.
Next;I would
Reset the static timing to 12*.

Now, listen carefully;
>the distributor drive gear cares not one whit to where the drive-slot is pointing. Not one tiny bit.
Every time you re-index it;
to make it work at the new location, you will simply reclock the cap to get a tower above the rotor, at the firing point of the reluctor, and rewire the cap as may be necessary, and so, the engine will run exactly the same as before you reclocked it. This is a Big waste of time, unless you just want it to be like the factory
>There are only three reasons to use the factory..... clocking. They are;
1) cuz when you do, and your cap has a "1" beside one of the towers, that tower is supposed to send the spark to the #1 cylinder, and
2) the custom-fit factory plug wires will all fit. and
3) if another DA mechanic works on it, he won't charge you an hour to re-clock it to; "well, that's where the factory put it, so they must have wanted it there for a reason" ; which reason I have already explained; just go find a new mechanic! That is all.
Go ahead everybody, prove me wrong. Criminy I figured that out over 50 years ago.
ANY TOWER, can be used as #1, but it better be the one directly above the rotor when the #1 cylinder is at TDC compression! That is all there is to it.
If it should happen
that your Vcan is hitting the Oil sender, then the daymn thing is up too high. If you have room, and if you need to, you can just re-clock the distributor to the next tower, and re-clock the wires, but leave yourself enough twisting-room to be sure you can get 36* advance at 3600rpm.
Now,
I have seen distributors, not factory ones, that when you set the static timing to 12*, the rotor-tip is quite far from a tower, and when the advance comes in, the rotor runs away from the correct tower, and the spark goes to the wrong cylinder.
And I have seen pick-ups wired in wrong polarity, that you can make idle, but as soon as the flyweights begin advancing, the spark goes berserk.
And I have seen reluctors installed on the wrong-way rotation index pin, which indexes the rotor too far out to lunch..
So there's lots of ways to get your ignition system messed up, but clocking to the oil-pump-drive slot is NOT one of them.
And BTW, the electronic ignition distributor does not require nor care about electrical grounding, so long as the pick-up remains isolated.

Ok, so, assuming the following:
> the t-slot sync is close, and
> the wet fuel level is correct and stable, and
> the coil is correct for the system and wired correctly,
> the ignition system is working as designed, and
> the PCV is working and is plumbed correctly, and
> all the air that the engine is gonna be seeing, is gonna be coming into the plenum past the throttle blades, and
> the valves are all closing and opening in the right order and at the right time, and sealing, and
>The Secondary System is known to be working,
Then
Put some gas in the bowls thru the bowl-vent, and splash a couple of teaspoons into the primaries, then put the gas pedal down half way, hold it there until the engine revs up, and CRANK IT!
The engine should lite right off!
It may rev up and then die, I would kindof expect that with an open choke. Just dribble another two teaspoons of gas down the primaries, and repeat. The second time, the mechanical fuel-pump should have finished filling the bowl, and it should run. Just sit there for a half a minute and keep the revs up at around 1500>2000. This will call the VA into play, and if yur lucky, that will bring in another 12>15 degrees timing, for a quick warm up.
After the engine has smoothed out and burned off all the fuel I dumped into the intake, now you can slowly bring the Rs down towards idle.
If the engine stalls, you will be tempted to increase the speed-screw; DON"T DO IT.
Instead, just give it more Idle-Timing.
Don't worry, at idle, your 340-cammed 360 will gladly accept tons of timing, maybe over 30>35*. just get it to idle so it can warm up. Use the fast idle cam if you have to, but do not touch the speed-screw!

Ok, after the engine is up to temperature,
and the lifters have adjusted themselves;
Kick off the fast-idle, and start backing off the Idle-Timing, towards 12*.. This number is arbitrarily chosen. You can set anywhere between say 5* and 18*. But
The right Idle Timing and T-slot sync, will be whatever timing it takes
1) to get rid of any tip-in sag, and
2) does not bang excessively into gear, and
3) gets you about 750 rpm in Neutral, and does not
drop more than 100 rpm into "Drive".
Next;
Adjust the mixture screws for best lean idle.
If this takes more than 2.5 turns out, then the engine wants a lil more fuel from the transfers.
If the mixture screws want to be at less than 2.5 turns, then the transfers are already too far open.
Adjust the speed screw in increments of 1/2 turn, until she gets happy and the rpm stabilizes..... but not more than 1 turn from the starting position.
If the engine is still rich, chances are that she wants more Idle bypass air. But, PROVE IT FIRST; check the fuel level and pressure; a high fuel level will manifest as a rich idle. If you think it's rich, PROVE IT FIRST.
I don't think the factory 340 cam, with the correct PCV valve, will want bypass air; but if it does; first prove the system is sucking air.
Then having proved that she's rich at the Synchronized slot exposure, and it's not a bad fuel-level, Ok;
I would just drill a small hole in each Primary throttle blade, beginning at 1/16" inch. I put them just back from the front edge, near to between the transfer slots and the idle discharge ports. Then test it.
I would go no bigger than 3/32.
When you do this, the idle-speed will immediately jump up; so, put the speed screw back to where it started. Then go back to "the right Idle-Timing" as above. The whole idea is to get the rpm down, to reduce idle power, to prevent the dreaded harsh transmission engagement, while simultaneously eliminating tip-in sag.
Ok now,
just so you know; I am confident that I could tune your combo, with a correctly installed 340 type cam, to idle at 750rpm in Neutral/Park, at between 5>12* of advance, with NO more than 100rpm drop into gear at factory stall, which will not have any tip-in sag.
And now, so can you
Finally, unclamp the brake booster, let it charge, then make sure the idle speed and quality has not changed.

Happy HotRodding

PS
1) if the reluctor is wired in reverse polarity, you can set the idle timing just fine. But like I said, as soon as the flyweights start advancing, the spark will go crazy, You can see it on the timing light. The strobes will jump back and forth and sometimes never show up at all. I have never seen this associated with anything else than wrong polarity. The pick-up belongs to a reverse rotation distributor, like a BB. Either get a correct pick-up, or better yet get two, one for spare.
Alternatively, cut your pick-up wires and solder them together in the correct polarity, but, seal them well from air and water., then shrink-tube them to prevent vibration from breaking them beside the solder-joints. Ten buy a just one spare correct polarity pick-up.
How can you tell which one is correct?
Well from Mopar they use color-coded wires. AFAIK there are three of them; grey, violet, and one other I forget the color of. You want the one with one violet wire. I cannot speak to aftermarket pick-ups.

2) E-core coils work well on CDI amps. Not so good on standard Mopar ECUs. The best daymn coil I ever got was the big Accell square top.

3) If you have a five-pin ECU wiring harness, you can run either/or the 4-pin or the 5-pin ECU with no other changes except, the dual-pin ballast is no longer required, altho it hurts nothing to leave it in the circuit, as it just dead-heads at the ECU.
4) If your fuel filter fills up with air, check your rubber jumpers. If you have clamped them with screw-clamps, they need to be doubled on each side , with screws staggered 180 degrees; ESPECIALLY when using EFI hose, as the rubber will bunch up under the screws.
Yes you can tighten the single clamps, but you will just end up squeezing the rubber thru the slots in the bands and eventually, they will leak anyway.
5) no your 340 cammed 1979 360 , does not need a 3/8ths fuel-line, on the street. but having one is Not a bad thing lol.
6) your 360, if an iron-headed lo-compression engine, will come alive with an old-style 2800 convertor. FABO guys are saying that modern convertors are more efficient, and so, even higher stalls are on the table. This will depend a lot on what rear gear you are running, and even on what transmission.
7) That 340 cam, on a 114 LSA, is a big pressure pisser. It worked in a hi-compression 340, but not so much in a low-compression 360 automatic. The cylinder pressure tanks, maybe down at 120psi, and then the bottom end goes away. But it sounds real mellow, out the pipes, and , with a hi-stall or a a manual-trans, guys are not complaining too bad. Mostly I guess cuz they've never run in the window of 185>200psi, lol..............
That's a whole nuther engine!

Jesus AJ. You"ve outdone yourself.

 

[Kern Dog]

Yeah!
while all help is appreciated, he sure went to the moon and back on that one.
This may have come down to rotor phasing as stated by @1WildRT .
I compared these two. First is the one in the car.

I set the reluctor gap back when I installed this distributor. This is the #6 firing position.

I spun it around to #1 and put the rotor on.

Then marked the rotor tip position on the distributor body. Look at the difference between the two. The left one is one from the shed. Right side is from the car.

There is 1/4” difference in rotation between the two. I could probably figure out the degrees that is with some measurements and math but here are the pictures.

The right distributor was 1 1/2 from that housing screw to the right. The left distributor was 1 1/4”. When I moved the rotor on the left to match that 1 1/2” number, here is the reluctor wheel.

I’m going to stab in this distributor and report back.

 

[RustyRatRod]

Jesus AJ. You"ve outdone yourself.

lol

 

[Valman]

It doesn’t explain why the car runs terribly with the vacuum can connected.

I think it does explain it! You’ve diagnosed a vacuum leak on the vac canisters.

Where the arm comes out the back of the vac canister, there’s a hole. With the engine running, spray some brake, carb cleaner, starter fluid whatever in the hole. Put the nozzle right in the hole. Any change in idle indicates a leak.

 

[Kern Dog]

Boom. Done.
The distributor on the left is in and the popping/stumbling is gone.
What a weird one though. I’ve never dealt with this before.
Thanks guys!

 

[Kern Dog]

I think it does explain it! You’ve diagnosed a vacuum leak on the vac

Well… that or that the rotor/reluctor phasing was so far off, the semi functioning vacuum advance pulled the breaker plate enough to widen the gap even further out of whack, inducing misfire or crossfire.

 

[RustyRatRod]

Did you actually diagnose anything or just swap parts?

 

[Kern Dog]

Did you read post #30 ?

 

[Valman]

There is 1/4” difference in rotation between the two. I could probably figure out the degrees that is with some measurements and math but here are the pictures.

Cut a circle out of some thin cardboard, about the same diameter as the distributor body. Put a 1/2’’ hole in the middle so you can slide it over the shaft, under the rotor.

You can then mark your rotor and cap post positions on the cardboard and measure the angle.
You’ll need to drill a hole in a cap to check the rotor phasing - Post #15.

 

[RustyRatRod]

Did you read post #30 ?

Yeah but that says nothing about why the vacuum advance was not working correctly.

 

[Valman]

You can spray into the vent hole to check for a vacuum leak.

 

[Valman]

Maybe need to sacrifice one of your caps to check rotor phasing, and order one of these…

 

[Kern Dog]

Yeah but that says nothing about why the vacuum advance was not working correctly.

Without having a properly working Mityvac, I don't think that I can trust the readings I get. Maybe I am using this one wrong? It had a receipt in the instruction booklet dated 1998.
Besides, if the newest distributor runs fine with the vacuum advance connected, that is some improvement.
Yes, I'd rather be the guy that can diagnose rather than the guy that just replaces parts until the car runs right. Sometimes it is a lot easier to "shotgun repair" when you have 10 spares at your disposal.
Still, yes...I do want to learn.
If the first distributor vacuum advance were actually leaking and not working, connecting the vacuum line to the carburetor would have just resulted in a vacuum leak. This was not that. The engine revved up and down, popped and stumbled and wanted to stall out. To me, that indicates a misfire situation.
If I'm seeing this wrong, tell me why. I leave room for the possibility of missing something.

 

[RustyRatRod]

Without having a properly working Mityvac, I don't think that I can trust the readings I get. Maybe I am using this one wrong? It had a receipt in the instruction booklet dated 1998.
Besides, if the newest distributor runs fine with the vacuum advance connected, that is some improvement.
Yes, I'd rather be the guy that can diagnose rather than the guy that just replaces parts until the car runs right. Sometimes it is a lot easier to "shotgun repair" when you have 10 spares at your disposal.
Still, yes...I do want to learn.
If the first distributor vacuum advance were actually leaking and not working, connecting the vacuum line to the carburetor would have just resulted in a vacuum leak. This was not that. The engine revved up and down, popped and stumbled and wanted to stall out. To me, that indicates a misfire situation.
If I'm seeing this wrong, tell me why. I leave room for the possibility of missing something.

Get a long piece of vacuum hose on the advance can and suck on it and watch the timing with a light. I'm sure you know how to suck.

 

[Kern Dog]

Not on dudes, no. I have no interest in that...not sorry to disappoint you or anyone else either.
In case you missed what I wrote at least 3 times, the old distributor is out and another one is in and working.
I can test the vacuum advance mechanism with the distributor in hand but what you seem to be skipping over is that the rotor position is different between the first and second distributors.
How can you dismiss that as the problem?

 

[Bewy]

I cannot see a 'rotor phasing problem'. If the phasing is wrong, the spark goes to the WRONG cyl. The OP says the engine has a great idle & great throttle response. You would not expect that if 8 cyls were getting spark at the wrong time....

Looking at the phasing pics of both dists in post #30, I do not see much difference, given the method used to measure the difference. The rotor tip has a considerable amount of rotational movement, relative to the pick up generating the spark. It moves with the centri weights & with vac adv.

 

[RustyRatRod]

Not on dudes, no. I have no interest in that...not sorry to disappoint you or anyone else either.
In case you missed what I wrote at least 3 times, the old distributor is out and another one is in and working.
I can test the vacuum advance mechanism with the distributor in hand but what you seem to be skipping over is that the rotor position is different between the first and second distributors.
How can you dismiss that as the problem?

It was a joke, dork. Ok. Whatever buddy. I'm tired of trying to help when effin MORONS refuse to do the SIMPLEST of things to check something out. I never said I dismissed a damn thing, either. ALL I SUGGEST WAS YOU CHECK THE VACUUM ADVANCE WHICH YOU'RE OBVIOUSLY TOO HARD HEADED TO DO. So have fun with it.

 

[RustyRatRod]

I cannot see a 'rotor phasing problem'. If the phasing is wrong, the spark goes to the WRONG cyl. The OP says the engine has a great idle & great throttle response. You would not expect that if 8 cyls were getting spark at the wrong time....

Looking at the phasing pics of both dists in post #30, I do not see much difference, given the method used to measure the difference. The rotor tip has a considerable amount of rotational movement, relative to the pick up generating the spark. It moves with the centri weights & with vac adv.

I didn't see it either, but I'm gonna let him wallow in it. He just made the list. I'm tired of being ignore and down right argued with just to recommend someone make a COST FREE CHECK on something, so to hell with it.

 

[Bewy]

Mate,
Go & have a beer....

 

[Kern Dog]

It was a joke, dork.

I know. I responded to it as such.

Ok. Whatever buddy. I'm tired of trying to help when effin MORONS refuse to do the SIMPLEST of things to check something out. I never said I dismissed a damn thing, either.

I didn't see anyone mention rotor phasing but Randy/AKA 1 Wild R/T, a guy that has not been wrong on anything that he has told me.
You tried but seemed stuck on the vacuum leak issue, one that has neither been confirmed nor denied because I don't trust the tool to get an accurate reading. Yes, the condition I started looking at seemed related to vacuum advance but what if it works fine but revealed a flaw elsewhere? You didn't leave room for that, you got stuck on the vacuum advance and potential leaks.

ALL I SUGGEST WAS YOU CHECK THE VACUUM ADVANCE WHICH YOU'RE OBVIOUSLY TOO HARD HEADED TO DO. So have fun with it.

When you repeatedly run into problems with people, maybe the problem isn't the other people, but you.
You can make suggestions but you can't expect people to do exactly what you want. If you continue to do that, you will be disappointed.

Again, I am not without fault and am certainly capable of letting good help slip right by me. Words and terms get thrown around and some of them stick to memory, some don't.
Correct me if I am wrong on this one issue:
The black lines on the distributor housings represent where the rotor sits at what will be #1 cylinder. This is with a reluctor point in line with the pickup.

Now, the #1 distributor is from the shed. The black paint mark is 1 1/4" from that screw.

The #2 distributor is what was in the car. Note that the line is 1 1/2" from the screw.

That difference means something. Now look closer at the notch in the top of the distributor shaft. It isn't in line with the reluctor point like you'd expect. This orients the position of the rotor.

What if the improper rotor phasing was barely allowing it to fire before the vacuum advance and once the hose was connected, it moved the breaker plate enough to misfire? That is possible from what I understand.

 

[Kern Dog]

I have seen distributors, not factory ones, that when you set the static timing to 12*, the rotor-tip is quite far from a tower, and when the advance comes in, the rotor runs away from the correct tower, and the spark goes to the wrong cylinder.

What about a factory distributor with an improperly positioned reluctor wheel?

I think you should rule out everything else first such as the distributor vacuum.

Yeah, you think that I should. Your opinion was considered.

I believe going after rotor phasing "right now" might send you down a rabbit hole. I'm not saying that might not be a problem. I'm just recommending attacking one thing, fixing it and moving on.

I was "attacking one thing", the rotor phasing but you didn't like that I didn't focus on the vacuum advance first.

I rechecked the timing and placement of the distributor. Both are where I like them. I was thinking that the vacuum can was too far clockwise which would be expected if the rotor phasing was way off.

It turns out that the rotor wasn't phased correctly as I thought.

I didn't see it either, but I'm gonna let him wallow in it. He just made the list. I'm tire of being ignore and down right argued with just to recommend someone make a COST FREE CHECK on something, so to hell with it.

Here is another angle:

The notch sits 180 degrees from the rotor tip so as you can see, the rotor tip does not line up with the reluctor wheel.
If this doesn't matter, I'd like to know why.
I switched to the other distributor and it runs like it should now. How is that bad?

 

[Phreakish]

I beleive the reluctor wheel alignment can be changed by flipping (editted to add: no it can't) it or engaging the other slot in the reluctor. I forget all the details now, but the first dist I pulled apart took a lot of trial and error to get back together correctly.
I have also seen the reluctor gap change with vacumm applied. Worth 5 minutes with some feeler gauges and the mightyvac to look into.
Also, I believe you can get a rebuild kit for the mityvac. The rubber guts are probably turning to dust and causing the erratic behavior.