Warm & hot idle/mixture screw problem

[Pete]

Hi all. My ’39 Plymouth touring sedan has a strange thing going on. I searched here on this forum and also elsewhere online, but have not found the info I’m looking for. Apologies in advance for the long post.

The problem affects warm and hot idle. With the engine is warmed up I set the mixture screw by backing it out until I get max vacuum, which is 20.5 lbs. I have the idle set a bit high at about 700 - 750 rpm for drivability. This doesn’t seam to affect the off-idle port as seating the mixture screw will stall the engine. Lowering the idle speed does not change the vacuum reading. The problem is much worse with Ethanol fuel.

However, at this mixture setting when I put my ear near the tail pipe it sounds pretty unsteady like it has a lean stumble. If I back the mixture screw out another 1 to 2 turns that slowly clears up until it purrs like a kitten. The vacuum is still at 20.5 lbs. At this point the mixture screw is out 3 to 4 turns from seated. That seems to be way too much.

The car has its original 201 engine, and I installed a correct pro rebuilt carb about 2000 miles ago. I lowered the carb float by 1/32 inch from spec to account for Ethanol fuel expansion. I pulled the plugs and they look good with just a very light coat of light gray. The engine runs well at any speed above idle. I occasionally add MMO to fuel tank. The heat riser valve is not working, so I have it locked closed. The ignition system has not been modified and seems to be working fine. The car starts easily (except after a heat soak) and I’ve never heard the engine ping. When the engine is warmed up I spot checked the engine and the radiator with a laser temp probe. All seems normal. I’ve had the cooling system gone over by a local old-time radiator pro and all looks good.

I think the car has been running too lean at idle since I got it. If I run it at or near idle for a long time like in a parade on a very hot day the car has fuel problems. My understanding is running an engine too lean causes it to run hot, which is why I been looking at this issue. Just last weekend I smoothed out the idle by backing out the mixture screw and haven’t yet put many miles on it to seem if this clears up the problem. The car seems to be running really well.

My planned next step is to take the spare carb that came on the car and give it a good cleaning, rebuild it, and see if that makes a difference.

Please let me know your thoughts on this. Is having the mixture screw that far out bad for the engine? Is it wasting fuel? Most of all, is this normal? Do any of you need to do this? Should I just leave it as it is and be happy?

Thanks in advance,

Pete

 

[Pete]

In my post above I meant 20.5 inches of vacuum, not lbs. Haven't had my coffee yet...

 

 

[Dartgame]

You might look at timing as well.  A retarded timed engine will run hot. Are the valves lashed correctly ? I might try rebuilding the other carb to see if that changes anything. 

[kencombs]

The high idle speed probably accounts for the need to open the idle fuel needle.  The butterfly is open enough that the 'normal' 2 turn or so open does not flow enough fuel.  Easy enough to check.  Lower the idle to 500-550 range and readjust the fuel mix. As far as lean causing overheating, it will, but only lean while running under load.  Lean idle just doesn't sound as good but not enough fuel is being used to overheat. Over heating a low speeds is usually due to low airflow over the radiator.  If you're not concerned with originality, a small electric fan in front of the radiator can help, or use a blade that moves more air, at the loss of some HP.

[Pete]

HI Ken,

 

Thanks for the reply. Better fan flow sounds like a good idea. I don't want to go with an electric fan at this point as I want to keep my original generator and regulator. Where would I find a fan/blade with better airflow? From a different Mopar flathead? After market?

 

Pete

 

[kencombs]

1 hour ago, Pete said:

HI Ken,

 

Thanks for the reply. Better fan flow sounds like a good idea. I don't want to go with an electric fan at this point as I want to keep my original generator and regulator. Where would I find a fan/blade with better airflow? From a different Mopar flathead? After market?

 

Pete

 

Heavy trucks, later AC equipped cars, industrial applications or heavier cars are potential sources for a better fan blade.  some of the later A engines may have the same bolt pattern and be useable.

[greg g]

Try setting your timing using the vacuum gauge. First set the idle to 400 to 500 rpm. Then with the dist nut loosend enough to turn the dist. rotate it to the highest steady vacuum reading.  Readjust the idle speed and idle air screw to get the smoothest 500 rpm idle.  Snug the dist and go for a ride.  Listen for any spark knock  or ping under brisk acceleration or under load like accelerating up a hill in third.  If you encounter ping, retard the timing till it disappears. Tighten the distributor.  Readjust idle speed and idle mix to attain a smooth 500 rpm idle.  Then you should be set to your type of gas, height above see level, and the way you drive.

[Pete]

Hi Greg,

 

Thanks -- good stuff. Do I need to disconnect the vacuum line from the carb to the distributor, or is that taken care of by setting the idle to 400-500 rpm?

 

Pete

 

[greg g]

The vacuum signal from the carb to the dist is negligable at idle. So no need to disconnect.