1947 D25: Weird Wandering Idle Speed

[bamfordsgarage]

Hello all — my trusty D25 has developed an intermittent idle problem, and I'm hoping the hive mind here can point me towards the cause...

 

This is a maybe-today, maybe-not-today issue. The car starts, idles and runs smoothly most of the time, but occasionally lapses into a period of variable, rough idle that wants a higher idea speed setting and is still prone to stalling. The plugs are recent; the plug wires, distributor cap and condenser are many years old,. I just swapped in a NORS vintage 6-volt ignition coil.

HERE is a short video illustrating the issue. Key points:  0:11 and 0:24 revs ramp up and down; 0:48 revs ramp up and down on the tach; and 1:02 revs ramp up and down on the vacuum gauge.

 

I suspect the carburation/fuel system.  The vacuum gauge registers 11 inches at rough idea, 15 inches at smooth.

 

All comments and suggestions most welcome!

 

[DJ194950]

Voltage checks while running are in order I think!  Is all in order?

 

DJ

[Sniper]

15 inches at idle is pretty low< i get 21 which is pretty good. 

 

https://www.motor.com/magazine-summary/mastering-the-basics-reading-a-vacuum-gauge/

[9 foot box]

   My guess would be a valve or valves sticking. You could de-carbon the combustion chamber with an application of Sea-Foam. I’m at 4550’ and have 17 in/Hg. That can be decreased with a timing adjustment either way. Rick D.

[soth122003]

Looking at that picture, I think the car is shivering.LOL

!. Check the timing. Set according to the manual. no fluid drive set at 0. fluid drive 2nd mark before 0.

2. Using the vacuum gage, set the idle mixture and idle speed. You should get between 18-22. Set to highest possible then back of 1-2 to eliminate ping. (just did mine from a carb change and am at 18) If the gage is flucuating, check the manual for symptoms of what is wrong. There is a chart in there for vacuum flux.

Once the basics are set recheck the timing. If the problem goes away with this, should be good for a while. If not move to the distributor. Check the points for gap and wear. check for worn wiring. Check the rotor and cap for carbon or arcing marks. 

 

Joe Lee

[bamfordsgarage]

Thanks all for your replies.

 

I think maybe I haven't got my key points across:

 

1. The idle speed varies up and down like a slow wave (peak and trough over a period of a few seconds) — this is a little hard to tell from the audio, but is clear when one views the tachometer and vacuum gauge at 0:48 and 1:02 in the linked video.

 

2. The issue is intermittent... Yesterday going for breakfast, no problem: starts, idles, runs like a champ. A couple hours later on a Home Depot run, the engine won't idle, wants to stall, needs throttle blips to stay running. It's like two different cars and I don't know which one is in the garage this morning.

 

While I'm no mechanic, this behaviour does not sound consistent with stuck valves (did 'em this spring) or loose wiring/carbon tracks etc. I would really like a better sense of the potential cause before digging into individual components.

 

PS: To clarify for Soth122003, mine is a Canadian-built D25, so essentially a Plymouth P15 with fancy name plates and no fluid drive.

 

[Kilgore47]

Vacuum leak?  It seams low.  If you have a PCV set up the PCV could be sticking.  Vacuum wipers?  Pull the hose to the wipers and plug the hole and see what happens.  Do the same with the vacuum advance.  Do the old spray carb cleaner at the base of the carb trick and see if that makes a difference.  

12 hours ago, bamfordsgarage said:

I suspect the carburation/fuel system

A carb cleaning couldn't hurt.  While you're at it blow back through the fuel like to clear the pick up in the tank.

[LazyK]

14 hours ago, bamfordsgarage said:

I just swapped in a NORS vintage 6-volt ignition coil.

I would start by checking or replacing the coil.

[greg g]

Does the coil get hot?  Points clean and gapped?  Carb to intake nice and snug. Throttle cable bound up, choke fully open?  Does pulling the choke closed a bit effect the symptom?  Carb to manifold clear of any wetness? Voltage to coil proper and steady?

[soth122003]

Didn't see the vid link till just now. First, when was the last time you cleaned the carb? Could be a piece of debris is in the carb floating around and affecting the idle circuit. Pull the top of the carb and check for debris.

2. Do you have a vacuum advance on the distro? It sounds a lot like my car did when I put in the new carb. (just last week) I did not have the advance hooked up and the idle would wander and try to stall. Once I hooked up the advance the car smoothed right out and worked great. The diaphrams in the advance might be going out. 

2a. Where is the vacuum gage line connected? (Probably not your issue just curious)

3. Your idle (when running smooth) is a little high. Idle should be 450 to 500rpm. your vid shows 600-700rpm which may be why your vacuum gage only reads 15 at max. 

 

Based on the video, I would check the carb float bowl then check your vacuum advance. My bet would be the advance since you stated this just started happening. also since you are not changing the idle yourself, the advance or the distro (worn bearing causing a wandering points gap) is the only thing I can think of in your situation that can cause these symptoms.

 

Joe Lee

[desoto1939]

also pull the dizzy cap and also the rotor. Then check the breaker plate for any movement side to side. The BP might need to be replaced because of getting wornout.

 

They do need to be replaced every so often. I have spares and then rebuild my used BP with NOS components to have as a fresh one in my tool box plus you also get to have the small extra wires that are in the dizzy.

 

Rich hartung

Desoto1939@aol.com

[kencombs]

I've encountered similar symptoms when the centrifugal advance springs are broken.   Plate just sorta floats from advance to non or someplace in between.

[bamfordsgarage]

Again, thanks for all who have contributed to the thread. Many worthwhile suggestions, particularly as the ignition and fuel systems have been largely neglected over my many years of ownership. A deeper dive into both is forthcoming this winter.

 

In the meantime, I continued my quest for the Magic Bullet... gotta be something easy and quick I can do without all that work, right? In my experience, Sudden Onset can mean Something Just Changed and if we're lucky it's Something Easy to Fix.

 

I consulted with a local club member, a retired independent mechanic who cut his teeth on '50s-70's cars. Sounds like fuel he said, possibly a blockage that drifts in and out of trouble. 

 

Recommendation #1: pull the top off the carb and look for junk in the float chamber... tried that, clean as a whistle.

 

Recommendation #2: pull out the mixture screw and give a shot of air with a rubber-tipped gun to hopefully clear any blockage in the idle circuit or whatever...

 

Bingo!  Engine now starts, idles, runs like a champ. I have two test drives under my belt and about to leave for the third. If all stays well, great. Otherwise, if the problem returns I'll roll up my sleeves and approach it in a more thorough manner.

 

Again, thanks to all for your comments and suggestions. Much appreciated.