Miss or skip while cruising

[Frankspolara]

Hello everyone -
My car is a 1968 Polara 383 originally 2 bbl. I had a 4bbl intake from a 67 383 laying around and decided to upgrade (replace) using that manifold and an Edelbrock 1406 (plan was to eventually replace the cam, timing chain, lifters).
Here is the issue that developed:
First drive of the day, the car will intermittently and randomly skip - not like a misfire, but enough to rock you during mild/light acceleration. Every once in a while - very infrequent, you'd hear a pop/backfire from carb. The one or two times I was able to look at the tach and steer this car straight, you could see it drop momentarily. can't really hear it at idle but did at one time when it was cold and on fast idle.
After the first drive, if the car sits, it runs great until the next morning. Because of this, it makes troubleshooting difficult and we're quickly running out of summer.
Here is what I have done:

• Prior to upgrade I installed the Pertronix Electronic Ign, cap, wires and plugs. Car ran fine.
• After replacing the carb and manifold is when the problem started.
• Replaced the coil, rerouted and checked ign wires esp 5&7 - all tightly plugged in.
• Replaced the cap again
• Installed a new manifold and gasket (Edelbrock 2186)
• Checked rechecked and varied timing
• Ran without vacuum advance connected and plugged
• Fuel filter
• New kick-down

My thought is something ignition related or burnt valve but do not want to rule out fuel or misfire. I ran the car this morning so it may be fine the rest of the day.

Thanks for your help/advice!

 

[commando1]

Here we go....

 

[CBODY67]

Rule out: burnt valve as that's a constant, non-variable "miss" or power decrease, just more noticeable at lower rpms.

"Kick down"????? Adjustment or the complete rod system? Why might this cause your issue? Is the adjustment "to spec"? It can be easy to mis-adjust the preload, by observation..

Seems like you've already narrowed it down. Item #2 in your list. Reverse that and see what happens. Might also have a fuel pump issue.

CBODY67

 

[Frankspolara]

Rule out: burnt valve as that's a constant, non-variable "miss" or power decrease, just more noticeable at lower rpms.

"Kick down"????? Adjustment or the complete rod system? Why might this cause your issue? Is the adjustment "to spec"? It can be easy to mis-adjust the preload, by observation..

Seems like you've already narrowed it down. Item #2 in your list. Reverse that and see what happens. Might also have a fuel pump issue.

CBODY67

Thanks - the only reason i mentioned the kick-down was because it's an upgrade. When I installed the carb, I changed the kick-down for it to work. When you say reverse item 2, put the original carb and manifold back in? I was hoping to avoid that, can I re-use the same tray gasket? I may try the pump next but if that doesn't work, I'll re-install the original manifold and carb. Just wish I could put my finger on to what's going on.

 

[Big_John]

It sounds like it's running a little lean when cold. Try adjusting the choke thermostat so it opens a little later.

 

[Big_John]

Thanks - the only reason i mentioned the kick-down was because it's an upgrade. When I installed the carb, I changed the kick-down for it to work. When you say reverse item 2, put the original carb and manifold back in? I was hoping to avoid that, can I re-use the same tray gasket? I may try the pump next but if that doesn't work, I'll re-install the original manifold and carb. Just wish I could put my finger on to what's going on.

I'm always hesitant to help when folks start changing parts rather than trying to diagnose the issue. For example, a fuel pump test is pretty easy.... All you need is a hose and a coffee can. A zillion times easier and cheaper than switching a pump... and it tells you if there is an issue.

The problem you are having is very minor and pretty much part of switching to an aftermarket carb. You have to tune it to work on your car. It's not like a factory stock carb where Chrysler did all the homework. It's never "bolt on and go".

 

[Frankspolara]

I'm always hesitant to help when folks start changing parts rather than trying to diagnose the issue. For example, a fuel pump test is pretty easy.... All you need is a hose and a coffee can. A zillion times easier and cheaper than switching a pump... and it tells you if there is an issue.

The problem you are having is very minor and pretty much part of switching to an aftermarket carb. You have to tune it to work on your car. It's not like a factory stock carb where Chrysler did all the homework. It's never "bolt on and go".

Thanks - You're right - that is the only thing I haven't tried. The only thing is I tried with a carb from a "donor" car just for kicks and it still did it. I am also going to test the fuel pump but again, after the first initial drive of the day, it's perfect.

 

[jct]

change out your fuel pump push rod while you're at it

Hughes Engines

 

[Davea Lux]

The Edelbrock carbs these days tend to run a little on the lean side. As the misfire is at road speed, you probably have a main carb function that needs to be fattened up. Check the fuel delivery first to be sure you are getting adequate fuel as noted above. Metering rods and jetting may need to be changed.

See the Edelbrock Tech Brochure #0033 on how to do this.

Dave

 

[CBODY67]

I mentioned the fuel pump as I had an issue with one on a non-Mopar. The first time I noticed it, I'd started it and let it run to warm up. I noticed the rpm start to decrease, so I pumped the pedal to keep it going. Then all was fine.

BUT it resurfaced on the freeway, after miles of driving. Pumping the pedal wildly while cranking would get it to start, then die, a few times before it would run long enough to put it in "D" AND quickly drive off. That's when I learned that the cigarette lighter didn't work, which meant my bag phone in the rear floorboard was useless to call somebody.

A new fuel pump fixed it. Just after I got the new pump installed, I noticed that it ran smoother and better than ever before. No fuel pump pushrod on that particular engine.

The original Edelbrock AFB Tuning Manual was about 50 downloadable pages. It mentioned using a stopwatch to clock various accelerations, then go richer or leaner for best performance. A time-consuming "adventure", it seems.

Was the whole kickdown linkage changed? To what? Or just the two upper pieces between the carb and pivot point at the rear of the motor? Just curious. I know that when I upgraded my '67 383 4bbl (AFB) to a Torker 383 and 9801 TQuad, I needed an extension for the long rod, which Edelbrock sold. Just curious.

As for changing the intake gasket again, I think I'd probably re-use it, BUT put the 6-Pack paper gaskets I mentioned earlier between the existing gasket and the manifold, using a thin layer of high-temp black silicone on each side of them, when cured, use them between the gasket and the manifold. Torque gently and to spec, in sequence. As an upgrade, remove the existing gasket and put the same silicone in the beads of the gasket, removing excess with a Mr. Gasket gasket scraper. This makes it a Print-o-Seal style gasket that should seal better than metal-on-metal, I believe. Just a shadetree thing I came up with for better gasket sealing AND easier clean-up should replacement be needed.

Thanks for the additional information.​

Please keep us posted on your progress.

CBODY67

 

[Frankspolara]

I mentioned the fuel pump as I had an issue with one on a non-Mopar. The first time I noticed it, I'd started it and let it run to warm up. I noticed the rpm start to decrease, so I pumped the pedal to keep it going. Then all was fine.

BUT it resurfaced on the freeway, after miles of driving. Pumping the pedal wildly while cranking would get it to start, then die, a few times before it would run long enough to put it in "D" AND quickly drive off. That's when I learned that the cigarette lighter didn't work, which meant my bag phone in the rear floorboard was useless to call somebody.

A new fuel pump fixed it. Just after I got the new pump installed, I noticed that it ran smoother and better than ever before. No fuel pump pushrod on that particular engine.

The original Edelbrock AFB Tuning Manual was about 50 downloadable pages. It mentioned using a stopwatch to clock various accelerations, then go richer or leaner for best performance. A time-consuming "adventure", it seems.

Was the whole kickdown linkage changed? To what? Or just the two upper pieces between the carb and pivot point at the rear of the motor? Just curious. I know that when I upgraded my '67 383 4bbl (AFB) to a Torker 383 and 9801 TQuad, I needed an extension for the long rod, which Edelbrock sold. Just curious.

As for changing the intake gasket again, I think I'd probably re-use it, BUT put the 6-Pack paper gaskets I mentioned earlier between the existing gasket and the manifold, using a thin layer of high-temp black silicone on each side of them, when cured, use them between the gasket and the manifold. Torque gently and to spec, in sequence. As an upgrade, remove the existing gasket and put the same silicone in the beads of the gasket, removing excess with a Mr. Gasket gasket scraper. This makes it a Print-o-Seal style gasket that should seal better than metal-on-metal, I believe. Just a shadetree thing I came up with for better gasket sealing AND easier clean-up should replacement be needed.

Thanks for the additional information.​

Please keep us posted on your progress.

CBODY67

Thank you - entire kickdown changed with a Lokar kit, so original kickdown replaced from the tranny to carb. Just ordered up a new fuel pump - it'll be here on Tuesday. I thought I had it fixed last night - short trip, doing it again - adjusted the carb less lean - could hear a miss when it was idling but nothing like what was happening while driving. I took it out for a cruise last night, ran flawlessly - such a nice car.
Started the car early this morning - sunshine and cool. Fired right up - let it warm up for 15 seconds or so, backed out of the driveway started down the road - boom, right after the two - three shift. Maybe just a one time thing but once I was cruising, there it went again and again etc, now fully warm - so random and intermittent.
The car actually sounds like it is shutting off, just for a split second - real abrupt, not like a misfire which I don't think would be as abrupt. Does not stall or hesitate, just this random miss - best way to describe it. Turned the interior lights on thinking it may be electrical - they don't even flicker when it happens.
May bite the bullet when it cools and re-install the old 2bbl carb and manifold, but why did it run so good last night? Wouldn't it happen all the time? can almost guarantee that after I let it sit this morning for an hour, it will run fine. Almost at my wits end!!

 

[CBODY67]

If you can wait for the fuel pump to arrive, don't change anything until after you get the new one installed.

Idle screw adjustments only affect idle and "transition fuel", which is probably from about 1200rpm and lower. No affect once the main system starts working, when the fuel comes from the venturi rather than the idle needle hole or transition slot just above it, in each primary throttle bore (4bbl).

How many tanks of gas have you run through it since this started? Just curious.

Thanks for the additional comments,
CBODY67

 

[Frankspolara]

If you can wait for the fuel pump to arrive, don't change anything until after you get the new one installed.

Idle screw adjustments only affect idle and "transition fuel", which is probably from about 1200rpm and lower. No affect once the main system starts working, when the fuel comes from the venturi rather than the idle needle hole or transition slot just above it, in each primary throttle bore (4bbl).

How many tanks of gas have you run through it since this started? Just curious.

Thanks for the additional comments,
CBODY67

I ran through two tanks beforehand - since June. Ran about half tank since this happened.

 

[CBODY67]

So the gas probably isn't the problem, then. With a "common" tank from before and after the "upgrade".

Thanks,
CBODY67

 

[LeBaron1973]

Have you considered reversing the changes you've made? Just seems that problems which occur after changes, may disappear when original parts and settings are restored.

Hope you soon return to happy mopar motoring.

 

[Frankspolara]

Have you considered reversing the changes you've made? Just seems that problems which occur after changes, may disappear when original parts and settings are restored.

Hope you soon return to happy mopar motoring.

Yes I have considered - mentioned in an earlier post but hoping not to. I let the car sit all day today and figured I'd take it out for a quick jolt this evening with my son - see if it would do it. Ran like a champ. The only thing I tried was installing a brand new ballast resistor. Could that be possible? I'll find out tomorrow for sure. Afterwards, all I have is the fuel pump coming on Tuesday.

 

[300rag]

... The only thing I tried was installing a brand new ballast resistor. Could that be possible? ...

That won't do it. It either works or it doesn't.

 

[Frankspolara]

I took the car for a four mile drive this morning before work and it ran perfect. This is the first time that it ran good in the morning since I started having the issue. Will try again this evening and tomorrow morning and will keep everyone posted. I would much rather know what caused this issue than miraculously having it disappear!!

 

[mr. fix it]

Thanks for the update

 

[Big_John]

Cold and damp this morning in the finger lakes. The choke was probably on a little more letting it run richer.