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Engine stumble off idle, tuning problems: fixed

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I just wanted to share the latest problem so that others could benefit from my stupidity experience. I had been chasing a performance problem for a couple of months which had prevented me from driving my 1941 Chrysler New Yorker very often. It had a moderately rough idle, difficult hot-starting, a bad stumble when coming off idle once it was warm, and it kept getting worse over time. It had started stalling when coming off idle once in a while. These fluid drive cars with the throttle guard are supposed to be impossible to stall, so that was the final straw. I had been avoiding driving it because of the lack of power when taking off from traffic lights -- couldn't guarantee it would actually go. I tried a lot of different things -- initially I worked on plugs and timing, cleaning plugs which were still relatively low miles, but admittedly 10 years old and had minor fouling. Tweaked the timing to try to advance it to get a little more power -- that just made it tough to start. Cleaned and re-gapped points, which seemed like they should still have plenty of life left in the wear block and the points dressed up nicely. Nope.

Checked fuel system to see if was starving for fuel in coming off of idle. Had to run the idle speed a bit high to keep it from stalling, and it required a lot of nursing the throttle while stopped in traffic.

I had replaced the fuel tank over the winter, mostly because the drain plug had been frozen in place forever, yet it dripped pretty steadily ... And the tank was not in terrible shape, but I could really never properly drain it easily, and it was pretty nasty on the outside, with some minor dirty stuff on the inside. Replaced scungy fuel line; replaced old fuel filter, realized I was doing the glass bowl fuel filter wrong; re-installed fuel filter; checked the carb, which had only a small amount of sediment in the bowl, but I had thought that a clogged accelerator pump circuit could have been the source of the stumble. Even though I could visually see that the accelerator pump was giving a squirt. Nope, that wasn't it either. Carb is now very nice, but tuning the (2bbl) carb with the idle mix screws was puzzlingly not responsive. Was it fuel flow problem? Did the new gas cap I got (after I left one on the pump last year after a fill-up) vent properly (i.e. prevent a vacuum in the tank from working against the fuel pump)? Nope, gas cap is properly vented for vacuum. Gas caps taste pretty nasty, too. Fuel bowl level? Nope. 

Checked distributor some more. Free play acceptable. Cleaned vacuum advance, it was basically working, if dirty, and the diaphragm was sound and the spring good. Mechanical advance was basically within spec using a timing light. Distributor slightly dirty inside. Condenser checked out okay with a multimeter. It's about 300 nF, for those who want to know. 

Finally went to poke around more with timing and I said to myself, "You know, self, what do you really know about that ignition coil? How long's that been in there?" and I figured I didn't have much to lose checking that out. It does require crawling under the dash to get the coil out, because it's got one of those coils through the firewall with the ignition lead to the coil primary on the back side of the coil inside the car. So I pulled that out, and put the coil on the bench with the voltmeter, and it looked "funny". The primary read unsteadily at between 0.3 and 1.3 ohms, rather than a steady value somewhere around 1.5 ohms. The primary-to-secondary resistance was not the expected 7000 - 10,000 ohms, either. It was mega-ohms or open-circuit. It wasn't really a definitive answer that the coil was bad -- I mean I had checked spark previously, and while it might have been intermittent or missing a bit, it still put out a basically good spark, I thought. One of those cheap spark-gap checkers is a nice tool instead of zapping yourself by holding the lead near the engine block.

But the local auto parts joint had a 6 volt coil in stock for only $20, so I figured I may as well just check that out, for all the time I'd been dumping into this. It required a little gimcrackery to get the ignition lead to the front of the coil in the engine bay. But it just fired right up and I could not believe how smooth the idle sounded. Quick check to tweak the timing back to TDC. I let it warm up for several minutes, come off the fast idle cam on the choke, and it runs much nicer now. Drove it for a half-hour on a warm day, and it just pounces off the idle now. Whole different car, seriously. Drove it straight up the hill that has the stop sign in the middle of it and no hesitation or stumble now. Where did all this power come from? It's still a heavy car and underpowered by modern standards, but it drove like I remember it running. Practically leaps off the line. Finally. Now re-check the carb tune, and it is very much more responsive to carb adjustment, and the idle speed can be adjusted all the way down to the 500 rpm given in the manual without any trouble. 

What's the moral of the story? Well, "99% of carburetor problems are electrical," that's for sure. And the coil check was simple, but the annoying firewall installation of the coil discouraged me from checking it earlier. Have to stop getting suckered into checking and fixing things just because they're easy and accessible. I think I'd do almost anything to avoid crawling under the dash. And I also should have trusted my ear when it was telling me that it was missing intermittently (and not on any one cylinder). Smooth running is a good thing to pay attention to.

But also, I want to thank the forum for being here and full of questions and for the people who have the good sense to answer "Just keep at it, you'll figure it out. Track it down, these cars aren't rocket science." Of course, I went down some blind alleys based on irrelevant questions...

Now I have to make the coil install look a little prettier. There are a few people on the web who have the original style 6 volt coils with the primary lead on the back, but boy, do they want serious money for them. Cheapest I saw was $140, and it's NOS, so who knows how sound it is and how long it'll last? Others want $200. I'm gonna stay with the off-the-rack coil, I reckon. 

 

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Glad to see you got it running!  I hadn't driven my '50 Plymouth Special Deluxe because I had to replace 2 brake lines and the master cylinder after already replacing all the wheel cylinders and brake pads.  Now the brakes work properly.  I bought the car in the spring and the brakes were terrible, you had to pump them and it still took it a while to stop.

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Glad to hear it! I reaplaced the coil on my '49 Special Deluxe and am going to get the rest of the engine tuned up within the next couple of weeks along with new tires.

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This is good information to remember. Thanks for doing the “R&D” for the rest of us!

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The firewall mounted coil with the reinforced ignition wire to the rear of the coil was used up to the late 40's as far as i know, and I've seen replacement coils with a separate triangular bracket that allows the new style coil to mount in the original spot although the wire has to be passed thru the firewall and new bracket to the front.............I'd be extremely hesitant about using a NOS coil unless it came with a 100% refund garanttee of its performance and quality.....some NOS stuff is worth chasing.......some not so much..............good to see you have had a win............andyd

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Sounds great.. I have had a similar experience with weak coils..  Here is another tip area to tune up.  the distributor itself. I sent mine off to George Asche for a tune up, checks springs, advance, vacuum  cars run like a top.

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Sounds like me. Tuesday I was going to get my heart meds in my 48 Plymouth SD and about a mile from the house it just died. I mean quick died, no stumble or put-put just nothing. I'm on the back road heading for the US highway and no traffic till it died. Then of course cars coming out of no where. Anywho,  got it parked with some help from passerbys, call my wife she comes and gets me. Got to the base in her car get my meds then wait till my friend gets off work and tow it home. Now, I'm thinking what could be the problem. Pretty sure it's electrical. So coil or the little wire in the distro comes to mind. The next day I pull the distro to check it out. Wires look good, the condenser looks good but has corrosion on the side and the coil is the Flametrower from the Pertronix setup that had died, but the coil was good so I kept it in. When I felt the coil on Teusday it was warm but not hot, so was probably good. Then I got fixated on the condenser. AutoZone $6 the next day, Amazon $10 bucks and 2 days. Rock out for the same item was $3.50 and 3-5 days at &2.50 shipping so I bought 2 and was going to wait. Then Thursday I thought well let's see if it is the coil. So I cleaned up the POINTS, put the distro back in and put a spark checker between the #1plug and plug wire. Used my starter bypass rig and Boom I getting spark. So I pulled the spark checker and the starter rig and tried to start the car and like magic it fired right up. Turned out the points were not lined up and just barely making contact and then had carbon build up stopping the whole shebang. 

 

The point (no pun intended) to the story is for some reason the points never even entered my thought process as a possible problem, I had 2 spare coils and thought I had a condenser (but no it disappeared somewhere) the little wire that can rub and wear the insulation off and ground out the system were what I focused on. I figured out why today after reading this post, Points rarely get talked about here. Got quite a few threads about the little wire and a few about coils and condensers because of cheap Chinese manufacturing , but the only real mention of points kinda deals with Pertronics and electronic ignition from the slant sixes. So as an aside always check the contacts and gap of points as a possible source of potential starting and rough running problems. I also found out that the hardest tool to find in your tool box is... a points file. That thing likes to hide when your looking for it.

 

Any way that's my story and I'm sticking to it.

Joe Lee

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Andy Bernbaum shows a picture of the coil bracket for firewall mounting on his electrical parts page but there is no listing for the part. If you google andy bernbaum coil bracket you will see a few parts listed .. I bought one from andy a few years ago so not sure why no longer listed, have sent them an email.

Alternatively try this link 

https://dcmclassics.com/electrical/576-le-183-coil-mounting-bracket-firewall-style.html

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