Nathan,
Is there any way you could post a video on youtube or somewhere that could let us listen to the pony as it lugs down and get a sense of the warm up etc? We all might learn something. Did mud daubers maybe get into the intake and partially plug it? Does it have good compression? I'm just throwing crap at the wall in hopes it may stir another idea as what might be the problem. Sounds you have worked through most all the suggestions.

I had an 8U pony run sluggish, backfired, etc.
Turned out to be a bad exhaust valve spring.
Might not be a bad idea to pop the heads off and rotate engine to see if everything is opening/closing.
Good luck to ya. JM

Neil wrote:

Ok good that's a positive test done. Do you have access to another known good carb? That would be a second test to conduct. Someone might be able to post you one to try temporarily. I could - it'd be coming from NY so it could be a bit pricey

That's an over the top kind offer Neil. I think I will figure this out between the parts I have here.I have a couple carbs but they are also kind of wild cards. I have only been in the old Cat game for one year. I had my 7U last winter but it has an electric start pony so i could start it even with a sloppy set up and it never pulled the main super strong either. Honestly I didnt know these little motors were capable of spinning the diesel effortlessly until I went through my newly acquired 5T in pursuit of a no start issue. Turned out being the mag gap was .010 rather than .020. When I had it running right it was like the sky broke open and beams of radiant light shone down on me 😁👍

Yes they kind of are little troublemakers in a way but I am determined to know what makes them go. They've been starting old diesels from 5 to 25 Liters displacement in hot and cold weather for 85 years so it is possible. Tonight I found that my solder repair on the (I dont know what it is actually called) metering tube? was not 100% I shot brake clean through it and there is a small are leaking so that cant help. Going to fix that up and check the rest too this evening.

Couldn't get the metering tube thing soldered in the right spot so I ended up using another one that I also needed to solder but that went ok. I needed to swap throttle bodies because that tube wasn't happy going into the original one. So now I'm using a complete different carb. This one is from a newer 7U tractor and has a different style choke plate that has a valve built into the plate to let it run once it fires and the main jet adjustment is different too. Going to try it out once it is along enough in the day that I'm not going to upset our neighbors.

Well I tried my hand today but didnt have the success I was after. I first tried a carb off my 7U which is more or less the same but they do have minor differences. Well, that carb was leaking externally. Not sure where from. I didn't spend much time on it. It did kick a few times but I decided to go back to the old carb.
I put the old carb back together using the metering tube from the 7U carb that I had tried earlier in the morning. That tube is in good shape and has never been soldered. I blew it out and it appeared all good. Using those parts, it was fairly difficult to start. It would kick a few times every pull but that was it. Finally it went and it ran fairly evenly. I had the idle screw out a half turn and the main jet out one turn as per book. Previously with the old metering tube and these settings it would start either one or two pulls everytime. Anyways, I engaged the drive and it was not pulling as I hoped at all. Even with a decent amount of heat in the engine it took a long time to be able to run it with the choke fully off and I had to ease into the revs to get it to take. Finally after playing with the metering screw, I got it to pull the main under compression maybe 90% of rated speed. Enough to start the main but it was all that it had. Before it would turn it at rated speed even without the throttle being pushed in all the way. It had power in reserve. After a couple hours I was all pony'd out. Sorry for the novel. Any help is appreciated!

ours did that too. was suggested to us by the cousin to check the shaft play on the carb excessive play allowing it to draw air past the shafts leaning the mixture and it changing as the shaft vibrated. he suggested he could mill the shaft bores and make new bushings to take up the wear.

another idea could be to add orings to the shafts to form a better seal have done this on holley and webber carbs in vehicles it worked.

we also found we had to leave a bit of choke on to get it to run right. and if the leavers were left in the position it ran last time it would start easier usually on the third or 4th pull. touch em and be there for ages. the uncle reckoned they had a nickname as wiscussin engines wishing it would go cussing cause it wont.

we found letting the pilot run until the main intake was warm to the touch made starting the main easier i dont know if all d4 had this but our has a second indent for full fuel at starting you open the rack and then it would go a few degrees more it seemed to start easier that way too.

Hello , I operated many D4 , D6 D7 and D8 in the sixties and seventies . I found out that cold starting was often a problem . In many cases the starting engine was hardly powerfull enough to overturn the main . That problem was solved on a day when a collegue poured in ( by accident) gas with a higher octane value . From that moment on I always used super gas in these engines with low temparatures . On the D6C the result was incredibly good . I cannot explain why these engines ran better with a gas of a higher octane value , they are side valve engines and normally they do not need that gas because of a low compression ratio . You can always have a try , no cure no pay ! Succes , the Netherlands , johan7

johan7 wrote:

Hello , I operated many D4 , D6 D7 and D8 in the sixties and seventies . I found out that cold starting was often a problem . In many cases the starting engine was hardly powerfull enough to overturn the main . That problem was solved on a day when a collegue poured in ( by accident) gas with a higher octane value . From that moment on I always used super gas in these engines with low temparatures . On the D6C the result was incredibly good . I cannot explain why these engines ran better with a gas of a higher octane value , they are side valve engines and normally they do not need that gas because of a low compression ratio . You can always have a try , no cure no pay ! Succes , the Netherlands , johan7

Thank you Johan for your input. I do run premium 91 octane in all my gas motors around here because they usually see infrequent use and the ethanol in the lower grade fuels absorbs moisture and degrades quicker. I tried the pony yesterday afternoon. It was just at freezing temp 0c and it kicked at two pulls and ran at four. Not bad. Not like the one pull it was before but I can live with it. There other two problems that now exist are that the pony is weak on power and also the over enter pony drive clutch pops out even though I tightened it up as much as could without it being a full on two handed affair to lever the clutch over. Now I'm afraid I might be pulling thr pony drive, which means draining the coolant and lifting the pony off its base. Not the time I wanted to get into that.