Rough midrange

[38plymouth]

I'm having an issue with my engine. When it's idling in neutral and I rev the engine it starts breaking up at about half throttle. If I hold the engine at that speed it runs rough. It runs well at low speed and if I punch it seems to run well. It's only in the midrange. I don't really notice it when driving but it does seem underpowered lately. I rebuilt the carb last year and I tried adjusting it and it has no effect. I also checked the wire in the distributer and that seems fine, I put shrink tubing over it just to be sure. I'm thinking of trying the points and condenser since I have never changed them. I'm wondering if anyone has any thoughts. The engine is stock except for a split exhaust manifold with duals and an aluminum fenton head.

[John Reddie]

I had a '48 Plymouth once and it was doing that. It turned out that the breaker plate in the distributor was loose. It would idle fine but became uneven when reved up slightly. Good luck to you.

John R

[P-12 Tommy]

It wouldn't hurt to at least check your points and re-gap if necessary. 0.20 gap. Matter of fact, you should check everything in the dizzy.

Tom

[John Reddie]

Yes, where you say the points have been in there for a while, I would change them with the condenser. Worn contacts can greatly reduce performance. Also maybe the cap and rotor.

John R

[Don Coatney]

Have you removed and looked at your spark plugs? That is always the first thing to do.

[38plymouth]

Have you removed and looked at your spark plugs? That is always the first thing to do.

I checked the plugs and they look great, they have maybe 1,000 miles on them.

I'm hoping it's just the points.

[james curl]

If the points are ok in low RPMs and High range RPMs then it is not the points. It could be the timing, but most likely be one of the advance methods employed in mid range. I would suspect one of the transition jets in your carburetor is not flowing the proper amount of fuel to substain the rpms and is going to a lean condition and is having trouble transitioning from idle to full throttle.

[John Reddie]

A lean mixture is a good possibility. You could try pulling out the choke slightly and see if it smooths out. If it does, that it is a lean problem.

John R

[38plymouth]

You said you put heat shrink on the distributor wire. If it's the one inside the distributor, it may be too stiff to allow the braker plate proper advance/retard movement. Just a thought.

I checked this also and it seems ok. I plan to replace the wire when I pull the distributer out.

I'm starting to wonder about the carb now. I rebuilt it last year and I think the problem started last year. I was messing with the mixture screw and I think the problem got a little better. I'm going to adjust the carb a little more when I get time.

[greg g]

The mixture screw only effects the idle mixture. Once off idle its setting is moot.

Ascertain whether your accelerator pump is projecting a healthy stream of fuel into the carb. Also when you rebuilt your carb did you check the operation of the intermediate step up valve inside the carb? This circuit is operated by maifold vacuum (or rather lack there of) to enrich the mix after the accelerator pump shot to sustain acceleration of power during climbing a hill.

High vacuum at idle and steady state running, keeps this valve closed. As vacuum decreases a spring pushes up against the weakeed signal raising th taperd needle out of a jet, to icrease and maitain an enriched mix until the vacuum signal returns to normal closing the jet.

Take off the top of the carb and you can locate the valve on the opposite side fo the float bowl from the acc pump. you will note a brass assembly of parts. You chould be able to press down on this assembly and it should move down freely agaist slight pressure of the spring. If is doesn't move freeley or is not at the top of its stroke with the top of the carb removed this might be your problem.

Also there is a small hole in the bottom of the carb base. This hole needs to line up with a slot in the base to manifold gasket. If not aligned the vacuum signal will be lost. This will cause the circuit to provide an enriched mixture at all times.

Good luck

[38plymouth]

The mixture screw only effects the idle mixture. Once off idle its setting is moot.

Ascertain whether your accelerator pump is projecting a healthy stream of fuel into the carb. Also when you rebuilt your carb did you check the operation of the intermediate step up valve inside the carb? This circuit is operated by maifold vacuum (or rather lack there of) to enrich the mix after the accelerator pump shot to sustain acceleration of power during climbing a hill.

High vacuum at idle and steady state running, keeps this valve closed. As vacuum decreases a spring pushes up against the weakeed signal raising th taperd needle out of a jet, to icrease and maitain an enriched mix until the vacuum signal returns to normal closing the jet.

Take off the top of the carb and you can locate the valve on the opposite side fo the float bowl from the acc pump. you will note a brass assembly of parts. You chould be able to press down on this assembly and it should move down freely agaist slight pressure of the spring. If is doesn't move freeley or is not at the top of its stroke with the top of the carb removed this might be your problem.

Also there is a small hole in the bottom of the carb base. This hole needs to line up with a slot in the base to manifold gasket. If not aligned the vacuum signal will be lost. This will cause the circuit to provide an enriched mixture at all times.

Good luck

I will check this out. Thank you.

[38plymouth]

The mixture screw only effects the idle mixture. Once off idle its setting is moot.

Ascertain whether your accelerator pump is projecting a healthy stream of fuel into the carb. Also when you rebuilt your carb did you check the operation of the intermediate step up valve inside the carb? This circuit is operated by maifold vacuum (or rather lack there of) to enrich the mix after the accelerator pump shot to sustain acceleration of power during climbing a hill.

High vacuum at idle and steady state running, keeps this valve closed. As vacuum decreases a spring pushes up against the weakeed signal raising th taperd needle out of a jet, to icrease and maitain an enriched mix until the vacuum signal returns to normal closing the jet.

Take off the top of the carb and you can locate the valve on the opposite side fo the float bowl from the acc pump. you will note a brass assembly of parts. You chould be able to press down on this assembly and it should move down freely agaist slight pressure of the spring. If is doesn't move freeley or is not at the top of its stroke with the top of the carb removed this might be your problem.

Also there is a small hole in the bottom of the carb base. This hole needs to line up with a slot in the base to manifold gasket. If not aligned the vacuum signal will be lost. This will cause the circuit to provide an enriched mixture at all times.

Good luck

I pulled the top of the carb and the step up valve seems fine. I also pulled the carb and the hole lines up with the gasket.

I pulled the distributor and replaced the points and condensor. The ones from napa listed for 35-38 were wrong. The 39 and up worked. No change.

I still have the same problem. When the engine is running at approx. 1500 rpm's it pops out of both tailpipes. I tried pulling the plug wires one at a time with rubber handled pliers and leather gloves and I got shocked when I tried pulling the first one so I gave up that idea. Should that have happened? The wires are around 8-9 years old but I've only driven it a few thousand miles.

I'm also wondering about a lean condition. I think this started around the same time I put the aluminum head on. The plugs are a dirty brown to off white color. I tried closing the choke a little and it might have helped a little bit. It could be wishful thinking though.

I'm very frustrated now.

[Robert Horne]

One of my vehicles I had a problem with past 1500 RPM, was that the distributor shaft was stiff or did not spin easy, causing the timing to get delayed. Plug wires 8 or 9 years old, maybe new ones would not be a bad idea. Time is the enemy on any type of rubber.

[greg g]

Take a look under the hood after dark with the enigne running. If you can see blue sparks jumping around the spark plug wires, it time for a new set. Tractor supply has 6 clinder universal sets for tractors for under 20 bucks. Several folks are running them with good results.

Check the wire in the distributor that connects the coil terminal tot he points. Some time the insulation will wear allowing it to ground against the body of the dizzy when the breaker plate moves. Also from all the flexing over the years, the wire strands can fatigue within the insulation either breaking the circuit or causing less current flow through the remaining strands.

Also is your breaker plate moving freely and effecting spark advance.

[38plymouth]

Take a look under the hood after dark with the enigne running. If you can see blue sparks jumping around the spark plug wires, it time for a new set. Tractor supply has 6 clinder universal sets for tractors for under 20 bucks. Several folks are running them with good results.

Check the wire in the distributor that connects the coil terminal tot he points. Some time the insulation will wear allowing it to ground against the body of the dizzy when the breaker plate moves. Also from all the flexing over the years, the wire strands can fatigue within the insulation either breaking the circuit or causing less current flow through the remaining strands.

Also is your breaker plate moving freely and effecting spark advance.

I checked the wires last night and didn't see any sparks.

Both wires inside the distributer look great but I suppose I could replace them to be sure. The plate seems to move freely and I checked the vacuum advance with a vacuum tester and that seems fine.

[38plymouth]

I did some more searching and now I'm wondering if the pcv system I added to the car is causing the stumble. I read a few threads where others experienced a vacuum leak caused by the pcv that causes a lean condition at midrange. I will test this new theory out tonight and at least eliminate it as a possible cause.

[38plymouth]

Well I think I got it tonight. I swapped the plug wires for a used set I had in the garage and that seemed to help. I'm going to pick up a new set soon and put them on.

I also tore the carb apart because I found a few vacuum leaks by spraying carb cleaner on it when running. I found the little rubber gasket in the step up valve tangled in the spring so the valve wasn't working right. I also found some of the gaskets didn't fit real well and I had two different leaks. I used a little gasket sealer and the old gaskets and got it sealed up.

It definitely runs better now and I road tested it and got the timing close. I really need new wires to fix it right, the used ones are not that great either.

Thanks for all of the tips and ideas. I never would have figured out the step up valve problem.

[P-12 Tommy]

That'd for sure a good thing:)

Tom

[38plymouth]

After several frustrating weeks I FINALLY got the problem fixed. I put on new wires, plugs, points, condensor, tore the carb down, played with timing and searched for vacuum leaks.

After all of that I still had a bad surge or rough midrange when driving. It idled fine and ran fine at full throttle but it cruised terrible. I started thinking that it was running too lean and I remembered that when I rebuilt the carb the last time I set the choke butterfly so that it was straight up. It was angled partially closed before that. I went out and started the car last not and revved it until it started running rough and then I slowly closed the choke. That was it, it ran great. I adjusted the cable and took it for a test drive and it drove great. I'm guessing that maybe adding a split exhaust and aluminum head changed the mixture requirements for the engine. Should I do something with the jets now or leave it the way it is and drive it?

[greg g]

Closing the choke (artificially restricting airflow thereby enriching the air fuel ratio) on an egine running at operating temp usually indicates an air leak (artificially leaning the AF ratio) after the carburetor. this would indicate an air leak between the carb and the intake ports. This might be at the manifold block interface, between the carb and the manifold, or any where along the line from the manifold to the motor if your car has vacuum wipers. A cracked wiper motor hose or connection can raise hell with an engine.

Have you done a vacuum gauge test??? Does it run better if you leave the vacuum gauge attached to the manifold and drive it that way???

[38plymouth]

I have one of those vacuum tools and I checked the wipers and they hold vacuum. I searched everywhere for a vacuum leak and besides a couple of small ones on the carb that I fixed there doesn't seem to be anything else. I even took off my pcv system and tried it without it but I found no difference.

[Plymouthy Adams]

running the choke "partial" is another method to draw fuel from a secondary source to enrich a mixture..more than not this case here..with the engine mods you mentioned you could be a bit lean at speed..you could try a larger main jet and back off the choke..if it improves...well, tweak it in...this is often part of the tradeoff when modifying..you got to match the conditions of all changed components..

[greg g]

The motor hold ing vacuum is not the issue, the issue is the line between the manifold and the wiper motor. That neest to attach firmly at each end and be free of cracks.

I was speaking of a vacuum gauge, not a vacuum pump.

A vacuum gauge will tell you if you a pulling sufficient vacuum at idle, should be 18 to 21 inches steady on a healthy engine, and by watchig the needle if its not steady, it can tell you a great deal about the condition of the engine and its systems.

http://www.secondchancegarage.com/public/186.cfm

If you get it going will you be coming up Sunday for Mopar Madness??? Cny Mopar associatioin show and swap at longrbrach park at the west end of ONONDAGA lake.

http://www.cnymopar.com/2010Show.htm

[38plymouth]

The motor hold ing vacuum is not the issue, the issue is the line between the manifold and the wiper motor. That neest to attach firmly at each end and be free of cracks.

I was speaking of a vacuum gauge, not a vacuum pump.

A vacuum gauge will tell you if you a pulling sufficient vacuum at idle, should be 18 to 21 inches steady on a healthy engine, and by watchig the needle if its not steady, it can tell you a great deal about the condition of the engine and its systems.

http://www.secondchancegarage.com/public/186.cfm

If you get it going will you be coming up Sunday for Mopar Madness??? Cny Mopar associatioin show and swap at longrbrach park at the west end of ONONDAGA lake.

http://www.cnymopar.com/2010Show.htm

I meant that I hooked the pump to the line that goes to the wipers. I was afraid there was a leak in the wiper motors or line in between. I checked the vacuum of the engine running and if I remember correctly it was steady at about 20.

I would love to come to that show but unfortunately it's graduation party time and I have 2 to go to in Rochester that day.

[38plymouth]

I just successfully completed a head change and now that I'm running the engine without a nasty spark plug leak I discovered I still have this miss or rough running engine. I have changed or adjusted everything, man this is frustrating.