D17 III runs for 1 hour before running poorly,dies

1961 Series 3 D17. Serial 42053. Shebler carb(TSX 771?). Point gap, timing correct. Runs strong full throttle for about an hour under a load, then starts to lightly miss. Gradually gets worse until engine dies and will not restart. Been doing this since I put another engine in it after spinning 2 mains in the factory motor. Same series block casting as original motor. 142 psi on all 4 cylinders. The tractor sat for several years outside. I put a kit in the carb, new sediment bowl assembly and flushed gas tank before the new sediment bowl. Still had too much crap in tank so I removed it, plugged the fuel outlet hole and cleaned the inside with muriatic acid. Flushed it and reinstalled tank with new web supports. I put a tube screen on the sediment bowl before re-installing it. Sediment bowl still gets a very small amount of rust particulate. Engine still would wind down and die and not restart. Let it sit over night and it restarted but ran poorly but long enough to get it back to my shop.  I have taken the carb off each time and cleaned it even though could not find any issue. I have taken out the main jet and checked while in the field. Fresh fuel in tank with a can of Sea Foam. Good fuel flow out of line at the carb each occurrence. Today I cleaned the float and then original float valve seat with muriatic acid and then cleaned all jet orifices with drill bits. Thought maybe the new needle valve and seat might be the issue so I cleaned and put the original back in. Set the float height at 1&3/16 above gasket which is parallel too. Put another new scree in the sediment bowl. Same result. Would not restart. Tiny amount of sediment removed from bowl, screen was clean.  Pulled/cleaned the load jet and checked the fuel line for flow. No issues found. Motor started and ran well enough to get back to the shop.  2 weeks ago I worked on the gas cap making sure the brass spring washer/seal was venting properly. Tomorrow I will drain all fuel and pull the sediment bowl to remove the tube screen. Think I will put a new set of points/condenser in. Cap looks great as does rotor. Put new wires on when replacing the engine, new brushes/bushings in starter & gen. Turned armatures. Radiator cleaned/sealed/painted. New regulator and water pump. I have owned this tractor for 30+ years. Frustrating after redoing nearly everything. May remove all screens and put one full flow filter inline. THOUGHTS? Thanks!

Since it runs an hour, then dies when HOT.. then restarts when COLD.. DR is right, the coil needs replaced..

I cut the metal line and put a small inline filter in before I cleaned the tank. Wishing I had waited to get a kit to seal the tank but needed it running asap. Took the filter out trying to increase fuel flow.

I agree with those who suggested a bad coil...

I knew my fire was weak from the color. I tend not to check fire at that distance with bare hands in the field. I have a coil on hand but it only has 1.3 ohms resistance. 12 volt positive ground tractor. I will pick up a new coil w/resistor this morning. If anyone needs AC Delco 45XL new plugs, I have a box of 8, never used. Happy to donate them for the help here. My new engine has an older head on it with 3/8" reach plugs. Ironically my engine had 3/4" reach plugs in it. Bought the 45XL's as I was going to rebuild my old motor but the damage was to severe to the block. Took them out to run a compression test before buying the engine. I noticed thread color changes mid-thread so I checked head threads depth. 3/8". 142 PSI all 4 cylinders. I adjusted the tappets at operating temp before sealing the valve cover. No smoke, no leaks. Motor was $700 from the retired dealer I bought the D17 from 30+ years ago.

Another thought to throw out there is how old is the distributor.   I've helped 2 different buddies with tractors of this vintage with similar problems that had some side to side play in the distributor shaft and points not opening correctly.  They had pretty much replaced everything.  Doesn't take much to lose your clearance with a worn out distributor shaft/bushings.  

Fuel never made sense as choking as it started to run poorly had zero effect. I checked fuel flow and it is getting more than the rated 4.5 gal. per hour fuel consumption at full load. I ran out 1+ gallon thru the carb drain in 12 minutes with 3 screens in place. Itwould take 13.33 minutes required for 1 gallon to match 4.5 GPH. Tank was below 1/4  so it had minimum pressure head. 1 psi for every 2.33 ft of elevation. I put a new coil on. 12 volt, internal resister. Ready to go mow and see if it is fixed.

I believe the new coil fixed the problem. Just mowed for 3 1/2 hours. 6 ft Bush Hog 206 in mix of mostly giant blue stem. 3rd gear low range wide open. Heavy load. Never missed a lick, ran strong. Would have mowed longer but got a late start. The orange paint on my new coil was slow to dry. Thank you to all for your help/advice. I have 8 AC 45XL new spark plugs I would give away if someone would use them. They are old but never used, new in the 8 pack box. My preferred plug for 30 years but my replacement motor has a series II head with 3/8" reach plugs.