48 A will not start or run rite

40JDG

Member
I have been working on a 48 A and to make it start it has to be idled all the way back and starts really hard. The tractor will miss when on a load. The carb has been rebuilt, and i have tried a different carb off a good running tractor. Tried a couple different mags. New autolite plugs. Valves are set rite. It will idle and run perfect once running but not under a load. Timing is good. I am not sure what else to try, i had the crankcase cover off to clean the sludge in the bottom and noticed it has aluminum pistons. It is almost impossible to turn over by hand, do the high compression pistons need a lot hotter spark to make it run?
 
Anytime it idles well and misfires under load, ignition is suspect. I see you tried a couple of mags but did u use different wires? You need good solid core, non resistor wires.
 
Hard starting tractors almost always have a weak spark. What type of ignition system is on this tractor? If you happen to have one of those Wico distributors that Is shaped like a magneto its in your best interest to throw that thing in the bottom of your favorite scrap metal barrel.

If you have a delco distributor I'd suggest checking coil polarity and voltage at the coil while the engine is cranking over. Metal core coil and plug wires are a must.

If you have a true magneto send it to Mike Shelby at popinjohn.com and reinstall with metal core wires and non-resistor plugs.
 
You might consider removing the oil cup and checking for an air restriction.

If your tractor runs better under a load without the oil cup, then you might consider cleaning the air stack.

We have had problems with dirt/mud daubers building nests in the air stacks.

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Hope this helps.
 
Had jd 60 foul plugs under load several years ago. Had sat for 6 months. Finally pops put tractor in 6th gear locked up brakes and slipped clutch loading tractor. Found out governor was sticking and opening up all at once dumping lots of fuel in cylinders all at once. Sprayed penetrating fluid around shaft going down in governor and worked back and forth to free up. Ran fine after that. Just an experience we had and may be something you may want to check. Good luck.
 
Try a known good set of plug wires and unhook the air intake elbow at the carb. to eliminate plugged up intake and wires problems. If you still have weird problems then I'd sure look at the camshaft bearings. A models can have a lot of problems with these. I have cured several by just replacing the left hand one that is easy to get to. That side seems to get the most wear too.
 
Worked on a modified G for a friend. Aluminum high compression pistons & a C series mag that made his 37 A run like a top! G was very hard to crank because of the high compression but it started well and ran fine..... until it was under load! Using the dyno we found at about 2/3 throttle it would die like you'd turned off a key switch. Trying to choke it just made it harder to re-start. The only way it would recover was to remove the load AND pull the throttle back to dead idle position. There it often would recover and sit there idling smoothly as though nothing happend. You had to sneak the engine speed up gently to get away from idle or again it would (sometimes) fall on its face. A Delco electric distributor fixed it.

Had to work on the timing a bit to keep it from knocking harshly. Sticks in my mind it had to be almost 10° ATDC static to keep it from knocking at full load. The Delco just had too much advance at full speed for the G with that much compression. There was no pre ignition "ping" to it. That was purely one nasty harsh KNOCK!

It was a hard pullin, easy starting well behaved beast after that!

Maybe yer A is on this same track?
 

IF the Fire goes OUT when load is applied, it is either weak Spark (at the plugs) or the engine going LEAN..

I DO prefer to use Hi-Performance "Spiral-Wound" Low-Resistence Plug wires..they carry a REALLY GOOD Spark..

Auto-Zone carries them for V-8 engines and so will do 4 2-cylinder JD's...!!!

Ron..
 
Thanks for all the replies, I went to napa and got the hottest autolite plug they had and it made a huge difference. Starts a lot better and is really snappy. I will get it out of the shop tomorrow to see how it acts under a load.
 

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