MYSTERY FLATHEAD 6 ENGINE IDENTIFICATION ADVICE NEEDED

[PT81PlymouthPickup]

I restored a 1939 Plymouth PT81 pickup almost 40 years ago. I believe it had a 201 or 218 engine from the factory, but when I acquired the truck the engine had been replaced and the identification numbers had been milled off. I was told at the time some professional re-builders did this? Anyhow, this engine was worn out, so we rebuilt it. Bored, oversized pistons, rings, all new bearings, etc. It has performed pretty well. I've tinkered with the truck over the years and have made several upgrades including T5 trans, 12 volt conversion, Jeep posi differential, front disc brakes with split master cylinder, but never any engine modifications. Lately, I noticed the engine seemed to falter a bit at about half throttle and seems to lack the power it once had. Still Idles very smooth. The engine has less than 2,000 miles on it since rebuild and still had the same points which I had filed a few times over the years, so I figured the likely culprit of the mid rpm faltering? I pulled the distributor, installed new oem Mopar points, condenser, and rotor, set the gap and reinstalled.  I still have the same issue. I misted the ignition wires with some water while running to see if the wires might be leaking at higher rpm, but they did not seem to be. I'm uncertain as what to check next? I doubt spark plugs would go bad with only a couple thousand miles on them even though they are old? I've been considering installing either Langdon's mini HEI or Pertronix ignition. But here's the weird thing! I did a little research on my distributor #IGS-4003-1 which looks like it came in early 1935 vehicles? From what I've read the truck should have had a IGS - 4103 distributor. So now I'm wondering if the engine was an earlier one? There are some casting numbers on the block. Perhaps someone knows how to identify from these? (1119729-21) I'm hoping to get some advice to resolve the faltering issue and also wondering if I were to install a Pertronix ignition, should I use the distributor I have? I'm leaning towards Langdon's HEI because it uses off the shelf components and Langdon's price is only slightly higher than the Pertronix, but seems like a much better value when you consider you're getting the entire distributor.  Opinions, comments, and advice welcomed and appreciated.

[Plymouthy Adams]

with all the modern stuff added and changes made from stock I cannot see how this car is restored....but I do appreciate the Chrysler alternator in place.....?

[Plymouthy Adams]

PS as a side note on the distributor....while you can run either model distributor you MUST buy your tune up part based not on the year of you vehicle but the very model of the distributor use....there are issues in spark gap if mismatched rotor and cap are utilized.  

[PT81PlymouthPickup]

14 minutes ago, Plymouthy Adams said:

with all the modern stuff added and changes made from stock I cannot see how this car is restored....but I do appreciate the Chrysler alternator in place.....?

Yes, you are right, it no longer qualifies as "restored". I did "restore it as pristine as I knew how back in the 1980s and found it was a bit of a pain to share the road with modern vehicles. It's a bit safer and more fun to drive now. I think of it as a resto - rod. I do have all the original parts and made certain not to make any modifications that could not be easily reversed. So, I hope purists don't get too angry at me? I also try to keep it as much Mopar as I can. I appreciate the engineering and skills of some of the street rod guys, but I must admit I get furious when they cut up a perfect vintage vehicle. Seems like a crime!

[Los_Control]

With the choices you offered, I guess I would choose Langdon for simple reason, you can keep your old one wrapped up under the seat and drop it back in if the hei fails on the side of the road.

I have no personal experience with either brands, just what I read from others .... some have gone years with no issues while some go weeks or months and experience  failures.

 

My choice would be to go with the slant 6 conversion, then if something fails you can buy parts from just about any local auto parts.

 

I kinda have this feeling that the distributor ignition is not your issue, since you changed things and problem did not change at all. I wonder if it may be some crud built up in the carburetor ... 2k miles in 40 years it is basically parked all the time?

[Plymouthy Adams]

I was not knocking your build please accept my apologies if I offended.  I find your build very realistic in the fact that it can go out on the road in todays traffic and play well with others.  This is important.....maybe sometimes I get a bit sensitive to the word "restored".   We had three level of repair based mainly on availability where time was the main ingredient for return to use.  Restore, Refurbish and Repair to work.  Restore was back to original including finish, Refurbish was to replace parts that were marginal and beyond the adjustment allocations and then repair to work was the best alignment one can do in short order for operation knowing other work is warranted as soon as possible.  Many take the less involved "repair to work" and call it restored.  I like to use the term refurbished, it that it operates as intended but with only necessary new parts.  Some call it personalized but to me that is usually meant to describe in appearance inside or out.  Me, my cars get "Timmizied" as my one buddy likes to say.....he refers to my shop as Timmy Town.....works for me...I don't get hung up on such monikers.

[PT81PlymouthPickup]

2 minutes ago, Los_Control said:

With the choices you offered, I guess I would choose Langdon for simple reason, you can keep your old one wrapped up under the seat and drop it back in if the hei fails on the side of the road.

I have no personal experience with either brands, just what I read from others .... some have gone years with no issues while some go weeks or months and experience  failures.

 

My choice would be to go with the slant 6 conversion, then if something fails you can buy parts from just about any local auto parts.

 

I kinda have this feeling that the distributor ignition is not your issue, since you changed things and problem did not change at all. I wonder if it may be some crud built up in the carburetor ... 2k miles in 40 years it is basically parked all the time?

Yes, it's been parked way too much. Life's been too busy. Even so, I have maintained it fairly well and kept fresh fuel in it. I took the top off the carburetor and it looks pristine, although it might be wise for me to take it apart and blow out passageways?  I've recently have more time to devote, so I'm hoping to get everything ship shape and drive it more.  I too like the slant 6 conversion except for it requires mounting a module somewhere with additional wiring. The purist in me does not want to drill any mounting holes.

[HotRodTractor]

Installing a Pertronix ignition (or slant six) by itself might not solve your underlying issue - particularly if your issue is that the advance mechanism (both mechanical and vacuum) are not working correctly. 

 

Going with a setup from Langdon gives you an entire distributor that is supposed to have an ignition curve that works well. 

 

Another option is to have your distributor rebuilt by someplace like Advanced Distributors. I have yet to use them, but they do solid work and have experience with flathead mopars. They did the distributor on the Belgium Dodge Brothers car that ran the Peking to Paris race last summer. 

[PT81PlymouthPickup]

16 minutes ago, Plymouthy Adams said:

I was not knocking your build please accept my apologies if I offended.  I find your build very realistic in the fact that it can go out on the road in todays traffic and play well with others.  This is important.....maybe sometimes I get a bit sensitive to the word "restored".   We had three level of repair based mainly on availability where time was the main ingredient for return to use.  Restore, Refurbish and Repair to work.  Restore was back to original including finish, Refurbish was to replace parts that were marginal and beyond the adjustment allocations and then repair to work was the best alignment one can do in short order for operation knowing other work is warranted as soon as possible.  Many take the less involved "repair to work" and call it restored.  I like to use the term refurbished, it that it operates as intended but with only necessary new parts.  Some call it personalized but to me that is usually meant to describe in appearance inside or out.  Me, my cars get "Timmizied" as my one buddy likes to say.....he refers to my shop as Timmy Town.....works for me...I don't get hung up on such monikers.

Absolutely no offence taken! 

[PT81PlymouthPickup]

8 minutes ago, HotRodTractor said:

Installing a Pertronix ignition (or slant six) by itself might not solve your underlying issue - particularly if your issue is that the advance mechanism (both mechanical and vacuum) are not working correctly. 

 

Going with a setup from Langdon gives you an entire distributor that is supposed to have an ignition curve that works well. 

 

Another option is to have your distributor rebuilt by someplace like Advanced Distributors. I have yet to use them, but they do solid work and have experience with flathead mopars. They did the distributor on the Belgium Dodge Brothers car that ran the Peking to Paris race last summer. 

Yes, I agree. I did check the mechanical advance visually and seems to be in good working order and the vacuum advance also meets the distributor spec. when checked with timing light (about 11 degrees max. at higher rpm)  I'll take a look at Advanced Distributors.

[Merle Coggins]

You seem to be focused on an ignition issue. Have you verified that you are getting full throttle travel? Have you checked the carb for possible blocked main jet, or sticking metering needle plunger? 

Edited June 16, 2020 by Merle Coggins

[PT81PlymouthPickup]

1 hour ago, Merle Coggins said:

You seem to be focused on an ignition issue. Have you verified that you are getting full throttle travel? Have you checked the carb for possible blocked main jet, or sticking metering needle plunger? 

Thanks!  Full throttle travel is fine, but I have not inspected the jet or metering valve yet.

[Tooljunkie]

Valve adjustment? Just a thought. 
i put an electric fuel pump on my truck as part of my fixing it to run,and it ran noticeably better compared to gravity feeding i was doing previously. 
a weak fuel pump,a crack in the fuel supply line also comes to mind. 

[PT81PlymouthPickup]

1 hour ago, Tooljunkie said:

Valve adjustment? Just a thought. 
i put an electric fuel pump on my truck as part of my fixing it to run,and it ran noticeably better compared to gravity feeding i was doing previously. 
a weak fuel pump,a crack in the fuel supply line also comes to mind. 

I replaced the fuel pump about a month ago with one I purchased from Rockauto along with a new filter and hoses. Seems OK, but I wondering if the Chinese that built it sabotaged it somehow? Lols! Seriously though, isn't a shame we can't make anything here anymore considering the quality of the stuff the U.S. used to be capable of producing.

[PT81PlymouthPickup]

1 hour ago, Tooljunkie said:

Valve adjustment? Just a thought. 
i put an electric fuel pump on my truck as part of my fixing it to run,and it ran noticeably better compared to gravity feeding i was doing previously. 
a weak fuel pump,a crack in the fuel supply line also comes to mind. 

I don't think it's the valves, but I am going to adjust them this coming fall or winter. It's a bit of a project adjusting those buggers. Should be done running and hot and the only way to really do that on mine is pulling the inner fender.

[PT81PlymouthPickup]

On 6/16/2020 at 12:38 PM, Plymouthy Adams said:

PS as a side note on the distributor....while you can run either model distributor you MUST buy your tune up part based not on the year of you vehicle but the very model of the distributor use....there are issues in spark gap if mismatched rotor and cap are utilized.  

Thanks! Your comments got me to thinking. I have determined that my distributor housing is listed for earlier (1935-36) cars, but the breaker plate is the correct one for my 39 truck. Obviously, some of these parts interchange. The cap and rotor are listed the same for the distributor I have and also the one that's supposed to be in the truck. I'm totally confused by this dizzy stuff. I've found some on eBay that have the correct numbers but different letter and dash #s. For example;  1939 Plymouth PT81 should have distributor IGS-4103A-1 or IGS-4103B-1. I found one #IGS-4103 on ebay but with no letter or dash #. I was hoping someone on P15d24 might explain what all these variations might be and how important is it to have the right one for a particular vehicle?