DorkyDan

Old Plymouth Inline Six - Runs bad when WARMED UP!!

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I just got my 201 Cu In inline 6 antique Plymouth running again (1939 P7 Business Coupe).  And now I have a new problem: When it warms up, it starts to run bad.  Smooth running when cold and a bit of choke on.  But, stumbling and rough idle when warm and choke all the way off.

 

Points to note.  I'm using a 6v electric fuel pump instead of the original pump for the fuel. Plus ... Carburator has been rebuilt, with all new gaskets, needle valve, and jets.  New plugs, points, condenser, and cap, and new spark plug wires and boots.

 

Original coil (or old replacement) in the dash style.  Running on no-alcohol gas with Lead Substitute.

 

Anybody got any helpful advice?  I appreciate any and all input.

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Is the heat riser stuck?

 

I had a car that ran bad when warmed up that I eventually traced to a bad coil. Worked okay when cold but apparently had an internal break in the wiring that expanded when warm.

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Check for vacuum leak. Use a length of garden hose , one end to your best ear, search around intake manifold for leak. Also, pull coil wire out of coil a little ways while running and observe spark. Also, make sure spark plugs are not too long, if they are the intake valve will hit them and close the gap. I actually did this one time on a flathead Plymouth.

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Is it flooding or starving? Is the idle too slow?

 

Flooding will give black smoke. In my Dodge Brothers it results from a speck of tank dirt under the float needle so it doesn't close. Is your electric pump (why?) putting the right pressure on the float valve at the carb?

 

Starving: does it run better if you pull the choke slightly? If so, look for vacuum leak and think about carb float level. Does it have vacuum wipers? Check as above and look at the wiper system as well. If you have had the manifolds separated you might not have the mating face on the head on the same plane for both manifolds. If it has a vacuum advance system look at that too.

 

I ran my Dodge Brothers on lead substitute for about a thousand miles. Then it wouldn't run until I replaced the spark plugs. No more lead for me.

 

We assume you have the timing and points gap right.

 

The new distributor cap: does it have aluminium contacts inside and crimped aluminium holding the carbon in the centre? If so, look out for the carbon brush to fall out. I have had that on two such new caps on my Dodge Brothers. Look for caps with copper coloured crimps and contacts.

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Is it flooding or starving? Is the idle too slow?

 

Flooding will give black smoke. In my Dodge Brothers it results from a speck of tank dirt under the float needle so it doesn't close. Is your electric pump (why?) putting the right pressure on the float valve at the carb?

 

Starving: does it run better if you pull the choke slightly? If so, look for vacuum leak and think about carb float level. Does it have vacuum wipers? Check as above and look at the wiper system as well. If you have had the manifolds separated you might not have the mating face on the head on the same plane for both manifolds. If it has a vacuum advance system look at that too.

 

I ran my Dodge Brothers on lead substitute for about a thousand miles. Then it wouldn't run until I replaced the spark plugs. No more lead for me.

 

We assume you have the timing and points gap right.

 

The new distributor cap: does it have aluminium contacts inside and crimped aluminium holding the carbon in the centre? If so, look out for the carbon brush to fall out. I have had that on two such new caps on my Dodge Brothers. Look for caps with copper coloured crimps and contacts.

 

>>  My Response:  ALL very good advice.  I will answer in turn, (and hoping for a final answer soon)  Yes, it does run better with the choke partially pulled out.  But the last time I drove it, it ran SO BAD ... it actually DIED before I could get it home.  Heat Riser?  Doesn't have one.  Distributor cap --- Yes, this one has the COPPER contacts and a carbon button.  Just put in NEW plugs of the same number that came out.  Plugs set at .025 gap.

 

"Timing and Point Gap".  This is a PROBLEM.  I had TROUBLE getting the point gap to stay at .020 per the manual.  I NEED HELP in this area!!  People has said that "All Distributors have a Centrifugal Advance.  I can't FIND any in this distributor.  AND, the backing plate is VERY LOOSE ... only held somewhat in place by the Vacuum Advance arm that rests on a post on the backing plate.  Should there be something MORE substantial HOLDING that backing plate IN PLACE??  That's part of the problem in SETTING the point gap.  Too much SLOP!!

 

On the electric pump ... I had trouble with the car acting like it was running out of gas.  Replacement ORIGINAL pumps were VERY expensive.  The electric pump (6v) puts out an even 5 lb pressure.  No longer "running out of gas".  And, a repair kit cost MORE than the electric pump.  Would prefer "Original" .... if I can get the original rebuilt for a reasonable price.

 

Float level is set properly.  Will check again though and see if there's any debris in the float needle.

 

I also just REVERSED the Coil Connections.  Figured it MIGHT make a difference since I put the battery BACK to Positive Ground as it was supposed to be.  But it's running VERY BAD ... so, might switch it back ... just as a test.  Don't know what difference that makes, if anything.  Any IDEAS about that, or ANYTHING else we've discussed?

 

 

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OK, distributor rebuild time. If the timing wanders around under a strobe light and you struggle to set the point gap, it needs attention. Can you wiggle the shaft with the cam on it? New bushes on the shaft will keep it in the same place, keeping the points gap stable and timing stable. Nothing should be sloppy in there.

 

The top plate in the distributor should be held firmly and rotate on the springs and weights underneath. If not, it needs attention.

 

You say no heat riser: missing? The Motor manual picture shows one, typical '35-'46. It looks like an "automatic" type, with bimetallic spring and a counterweight. So yours could be stuck open or shut or in between.

 

If the float needle is jammed open there will be petrol everywhere. You will know! Suddenly running badly and stopping: float needle stuck open is a possible symptom - exactly what mine did. It just petered out and died and wouldn't start. Luckily it was a nice day to get out and clean it out.

 

Is the idle a little slow?

 

Motor says fuel pressure at the carb. should be 3 to 4 lbs.

Edited by Spinneyhill (see edit history)

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Running good with the choke on, and the throttle in the fast idle position, then stumbling and failing to idle when it is warmed up, may indicate a dirty idle jet.

 

It is possible the idle jet has dirt or sludge in it. There is an easy way to fix it without rebuilding the carb. Carefully screw the idle mixture screw in, counting each turn and fraction of a turn. Do not mash it down hard, just till it bottoms. Remove the screw. Blow air into the hole. Use 2 or 3 puffs of compressed air not a continuous blast. If you don't have a compressor you can use spray carb cleaner, or buy a can of compressed air like they use on computers. Anything to clear out the idle jet.

 

When you are done put the screw back, and back it out the same number of turns it had before. I have done this on several old cars and it almost always works,

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You don't need lead substitute. Your engine has very low compression and will run fine on the cheapest regular. It was made in the days of low lead, or no lead fuel. All Chrysler engines in those days came with hardened valves and exhaust valve seat inserts.

 

If you want to give it a treat add a dash of Marvel Mystery Oil, Redex, Bardahl or your favorite upper cylinder lube. This will lubricate and protect the valves, cylinders, and piston rings.

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