no worries Niel that why we are all here to learn and help each other out. I can't begin to think how much "other stuff" I learned to hear about how things work. most of the time it's just good to have an understanding of most things.

well, I back to too looking at the mag I think. I had the pony running and pulled the #1 plug wire out of the mag cap and nothing changed. so I changed the mag cap but that did not seem to help any. I try to change the mag with the one-off my 4600 but it did not seem to fit. I have put a new set of auto light 295 plugs and check them with a meter to make sure they were non-resistance. tested the plug wires with a meter as well I do have compression on that cylinder but I wonder if there something blocking the air intake as I never flooded this cylinder on the pony which can be easily done. I am at a loss right now I going to seek local help to re-check my mag. my friend across the line has a very nice mag tester but dam broader is still closed due to B.S.

I would say the heads and plugs should be fine as long as you get compression and clearance; heads on the pony were a lot alike for a lot of years. If you are concerned about the plug heat range run it for a few and check the plug color. White is to hot and lean where brown red is normal and black it is rich or cold plug (nutshell). I will say you are asking the right questions, I know that because I'm about 6 months behind you on my 9K n 8T, well 8 months to a year now cuz of the "Damndemic"...
So I am keeping a close eye on this thread

Can't say if you might have the same problem... but I just did this and caught it before it became a problem... I put too much oil in the pony oilbath airfilter which chocked out the pony engine..DUH. I'm telling on myself because after rereading you comment it hit me in the head like a D9G
Good luck

well, it's running better but still not right. a friend thought the valves might be gummed up so as a last-ditch effort we used a can of intake/combustion chamber cleaner while it was running. it did seem to help I think I got enough power now that I could fire the diesel if I had to get it started but would really like to find out what wrong.  the heads on the motor were 135-155 F exhaust runners were within about 50F of each other using one of the inferred heat guns I don't feel it very good but maybe then my hand touching to see if there a difference.  hears a video of it running today. 

FatDan is definitely on the right track...you want the spark plugs tan, brown...etc...

-do you have any spark, voltage delivered to the plug?? use a plug..test light...unwilling participant/ guinea pig....etc...
-squirt some fuel (gasoline) in cylinder...insert plug, crank..pull....what happens...??

-if you have air to the cylinder, like you say....something has to happen...

especially if spark timing is anywhere correct...

-air, fuel, compression, spark (at correct time).............doesn't something happen??

I have a similar issue with the pony on my D2 5U, one day it just decided it wasn't going to fire off properly on both cylinders, I tried everything, cleaned the carb twice, cleaned and checked the mag timing twice, new plugs, new spark wires, flushed the fuel lines in both directions 3 times, pulled the heads off, then started swapping in another magneto, and a different carb, even ran it without the carb!!

Never did get it right, but did get it better, it sounds like yours, fires off on #1 cylinder first or second pull of the rope, but takes 30 seconds or more before #2 cylinder starts firing, and that can be very uneven, after about a minute #2 will then run normally. This problem drove me nuts for days, even had 2 other smarter Catheads help me try and sort it out, without success.

My old Cat Mechanic buddy 2,000 miles away suggested it might be a valve gap issue, and I should pull the top manifold cover to check the gap hasn't closed up completely on the inlet and exhaust valves. I haven't done that yet, because I have a nasty habit of not being able to stop reconditioning a old Cat, and I know this pony could use a rebore and a set of rings if I open it up, which is a much bigger job, once I get the pony running OK, it fires off the diesel quickly, so I'm putting off opening up the top of the pony, but maybe you should check your valves.

I have been chasing a one cylinder miss on a D4 pony. I finally found the air cleaner was plugged up. As soon as I loosen the oil cup it took off.

thanks for your info and suggestions. Mike glad I not alone yes it does start very easily most of the time it only needs 1/2 a rope pull. I am at the point do I keep fighting it or take it off and tear it apart. I would hate to as it feels to have very good compression the most of out any of my pony's on my other equipment. I can't tune it through compression stroke with a hand on the flywheel. I been running with an air filter off but do agree that there could be something in the intake. if it was mouse nest I should have sucked it into the cylinders by now.