I had a 4020 LP that gave me fits for awhile. I
don't remember much about the tractor prior to the engine overhaul as I overhauled it immediately after purchasing the tractor and that has been 10 years ago. Seems that I was able to get the tractor started and operational at the dealership prior to the purchase. During the trial and purchase I didn't realize that the weep holes on the side of the block were there to tell you when either your water or oil "wet sleeve" seals were leaking. You guessed it, they were leaking. I found out during the overhaul that someone had replaced them previously and put the wrong seal against the wrong fluid. One is red and one is black. One is made for water/ethylene glycol, and the other for petroleum products (crank case oil). They had deterioriated and started leaking. I found that the after market JD service manual was helpful and I did as it said both during the overhaul and during the subsequent "trying to get it running right". I couldn't get it to run and take throttle advance and all of the things mentioned in your correspondence. I agree with having the air inlet properly terminated. I did an overhaul of the regulator, thinking that might help; even called the company (that made the regulator) in CA. and asked their advice. LP instructions in the service manual were quite extensive and good but the problem persisted. Nothing solved the problem. It just didn't make sense. In short, I was looking in the wrong place for my problem. During the overhaul, I did as the manual said. Part of that process was the setting of the engine to TDC prior to setting up the distributor. WHAT I MISSED WAS ONE SHORT SENTENCE WHICH SAID SOMETHING LIKE "..... WHEN YOU HAVE FINISHED..... .ADVANCE THE TIMING TO 25 DEGREES BTDC. A 100 hp 4020 WILL NOT RUN @ TDC, but if you give her some spark advance she runs sweet. Have had no more problems since and probably had no problems with the fuel system; as I said, it ran good enough for me to want to buy it. Since I don't know what else you have done to your tractor, I offer this as a possible solution in case you are coming out of an overhaul or have just acquired this machine. Good hunting. Mark (PS) I did notice that if you shut off the 4020 engine when it has been running, by turning the key off, the 12 v fuel inlet solenoid closes and shuts off the tank (and of course the distributor looses 12 v), but there is still tons of fuel in the system. If you try to restart prior to the engine cooling, forget it. What works is shutting off the manual fuel valve to the tank and let the engine stop as a result of fuel starvation. Then restart it any time you want just like starting a cold engine (throttle to full open and a second or two of full choke then no choke - with a good hot battery and 15w 40 oil) and she kicks right off, while reducing the throttle as she comes up to 1000 rpm (redline is 1800)puring like a kitten.
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