39 Plymouth idle/run issues

[39plymy]

Hi all,

First time poster long time reader. OK car now starts and idles well cold. Has new points, plugs, vacuum control, new leads. Leads are in correct order. After reaching a operating temp (5 mins running) it starts to miss and then lacks power and then dies. Random small backfire. I have checked timing  currently 7 degrees approx BTDC.  Will not start again after heat cycle but will when cold. The best it would do is start and run like it had racing lump cam in it, fire almost die and fire etc etc then will die. I have played with air intake screw and that seem to be OK at about 1.5 turn off fully in. Choke is not stuck. Fuel is clean and supplied via a new electric pump out of a recently de-rusted and treat tank, also new fuel lines. Motor is old but has no head gasket issues (good radiator and new water pump) with consistent compression across all cylinders. I am replacing the coil in the next days or so. Suggestions as I am running out of ideas? Perhaps running 91 with lead additive my plug gaps might need to be different to book? Points are set to .20 thou. There is a little play in distributor shaft, would this cause this?,,,, but cannot see why as this was the distributor that was in it when it was running sweet.  The car has not done a lot of work in recent years and was running fine out of storage. Did a refresh and now this happens. (note to self : don't fix unless broken.lol). Cheers Spiny.

[Los_Control]

Welcome aboard .... have you felt the coil when it happens ... is it hot to the touch?

Seems like something running for a few min and then getting hot, and a coil could do that.

Another thought is the new condenser, assuming you changed it with the points ... modern stuff is junk and you need to buy 3 to get one good one .... you might try putting the old one back in.

[keithb7]

Coil is my first guess too.  A failing coil can behave differently when they warm up.

[desoto1939]

According to my Motors manual and tune up data for a 39 Plymouth the timing should be set at TDC or Top Dead Center and not 7 degrees. Also you might want to check the distributo beaker plate assembly. It could be worn and you might have excessive play or movement.  Might want to replace this also to insure that you have a good internals on the dizzy unit. The breaker plate Assembly is an Autolite IGS 3004 which is the upper and lower plate with the points and condenser and all small wires.  I have a few of these if you can not find one.

I have a 39 Desoto and a lot of parts and cross reference catalogs for your car on CD's.

Rich HArtung

Desoto1939@aol.com

[kencombs]

My first guess would be condenser, second coil.  A timing light connected to the distributor cap to coil wire can confirm if you are getting a loss of spark when this happens. 

 

If the refresh you mentioned included a new set of points and condenser, those would be the first suspects.  Lots of bad out of the box, off-shore sourced electrical parts.

[Tim Larson]

I would definitely replace the coil. That solved a similar issue on my '49 Plymouth. As far as I could tell the coil that was on the car when I bought it was original. 

[keithb7]

I bought my last coil on Rock Auto web site. 6V Blue Streak if I recall. For my ‘38 Plymouth. A quick wire change compared to the old stock type. Easily sat in the original mount spot in the firewall.  All fixed up and good to go. 

Edited November 1, 2019 by keithb7

[Pete]

My '39 Plymouth runs best at 3 to 4 degrees BTDC. If you have a timing light, check the advance at higher RPM both with and without the vacuum advance disconnected and plugged.

 

Pete

 

[9 foot box]

   I would check that the gas cap is venting. With only a few gallons of gas, the tank is like a large vacuum tank, if the cap is not venting. Eventually starving the fuel to the carburetor. Condensation had stuck my vent closed, a dab of lube and I pushed in the spring loaded vent at the inside center of the cap.  Start the car with the cap loose, and see if it runs longer.

[39plymy]

ok lots of things to look at here. Replaced coil. No difference.  Had it running last night with a small miss that goes out when raising the revs. I have NGK plugs which many people do not like. I cleaned them and reset to .035" as they were smaller than this, ran even rougher. I have ordered some autolite 303 plugs but not confident that will make the diff. I rechecked that rotor is pointing to number one lead on cap at TDC.  Now here is the funny thing, at rough idle I removed each lead to see if it was a single plug  missing.  I went through all leads and it made no difference at all. I even removed 2 leads (5 and 6) the same time and there was no noticeable change at idle of the misfire. Was going to put a new carby kit in it but the condensor will be my next effort I think?  I will loosen fuel cap but I had the tank out recently and all lines are clean and clear with no rust. She really does not like be set at TDC likes 5 to 10 before but still misses. When the revs are up it runs ok. Plugs were very sooty but not oiled or burnt or wet. It is running  rich as it is fumey when idling.  

[38plymouth]

Check your plug wires at the cap. I had a bad miss once that drove me crazy, it was caused by the plug wire working itself out of the cap. The rubber part was still on the cap but the wire itself popped out of the socket and I couldn't see it by looking. I changed to a different set of wires that locked into the cap better.

[greg g]

Is the base of the carburetor wet when the engine shuts down?  Do you have proper fuel flow?  Have you tried blowing air back through the fuel lines to the tank.  Disconnect the fuel line at the fuel pump inlet and blow some 10 to 15 psi compressed air back to the tank. Remove the gas cap and listen for a good air flow and bubbles at the tank filler. Is there an inline fuel filter some where that needs to be changed.  In the fuel pump there may be depending on what p.ump you have a brass screen filter between the suction side and the pressure side.  Is it clean?  Also, go get some new plugs AC 46 or Autolite equivalent.  Champions are junk these days and with all this run then stall business they are probably fouled. Even if you have an autilites put fresh one in.

[keithb7]

You started pulling plug wires and there was no noticeable difference in engine RPM?  Interesting.

What can you can say about your valve settings? Too tight clearances, then when they heat up, they get a little bigger. Then the clearances get tighter. Then valves are not closing all the way. Not sealing. Then you have dead cylinders. Sparkplugs would be firing, but there would be no power in that cylinder.  Sounding like a miss. Later the engine cools down, the valves shrink again, and they seal again. Until the next time they heat up.

 

Have you any compression test results? Hot and cold?

Edited November 4, 2019 by keithb7

[Sam Buchanan]

I'm assuming you have carefully set the idle mixture.......

[John Reddie]

I would also check your distributor cap inside for carbon tracks or cracks between the electrodes if you have not done so. Good luck.

John R

[Plymouthy Adams]

11 hours ago, 39plymy said:

ok lots of things to look at here. Replaced coil. No difference.  Had it running last night with a small miss that goes out when raising the revs. I have NGK plugs which many people do not like. I cleaned them and reset to .035" as they were smaller than this, ran even rougher. I have ordered some autolite 303 plugs but not confident that will make the diff. I rechecked that rotor is pointing to number one lead on cap at TDC.  Now here is the funny thing, at rough idle I removed each lead to see if it was a single plug  missing.  I went through all leads and it made no difference at all. I even removed 2 leads (5 and 6) the same time and there was no noticeable change at idle of the misfire. Was going to put a new carby kit in it but the condensor will be my next effort I think?  I will loosen fuel cap but I had the tank out recently and all lines are clean and clear with no rust. She really does not like be set at TDC likes 5 to 10 before but still misses. When the revs are up it runs ok. Plugs were very sooty but not oiled or burnt or wet. It is running  rich as it is fumey when idling.  

 

 

at this point, you have not stated and all is assuming you still running on 6 volts...but with it heating up and not restarting I would ask if  you are running 12 are you going through a dropping resistor.  Next is the plugs, are you running a resistor plug and needed to go to .035 or are your plugs standard and you trying to jump a gap your coil is not wound to for secondary voltage.  IF your points are used and you are gapping them at .020 you could be way off many degrees of dwell.  If your dwell time is too short, your coil will not reach saturation and your secondary voltage will be weak..combined that with a larger gap plug you are compounding the problem....

 

go back to your basic tune up and assure all you have is proper, set correctly even though you say it is...check again...things are not always as they seems...remember this is by your own admission a self induced problem thus the need to verify your replaced parts and your work.  ALSO and of great importance is DID YOU GET THE RIGHT TUNE UP PARTS.   this can only be assured if you ordered/bought your parts but the number on the distributor tag.  Mismatching tune up parts leads to extreme electrical gaps inside the dist (cap and rotor) and will drive you nuts......and lastly, if you put in new points, did you get both the springs placed properly for the correct tension so to prevent point bounce.   

 

 

[soth122003]

You should do a compression check cold and hot, then If I were you I would use a vacuum gage to check your engine out and set the timing with that and then check with a timing light. The procedures and the results are listed in the service manual and I think that will go a long way in finding out what the condition of your engine is in.

 

Joe Lee