9N running rough at idle fine under load

I've fixed alot in the 9n but it's running rough when at high throttle idle. Sounds like when you are going down a steep hill and the engine is holding you back. When I put the plow in the ground and work it it sounds great. After a lot of work it does get rough and will start to backfire some. Not a lot but I know something isn't right. New wiring harness, new plugs and points. Everything is gapped to spec. What else can it be? I've checked the carb, I have great fuel flow with fresh gas, the tractor to sat for about 2 years. Does the carb need rebuilt? Thanks as always. PS the seals aren't leaking anymore to those that followed that saga.
 
Fullers,If it is kinda popin out the exhaust when hot after a good work out,you might want to check the valve stem clearance they may be to tight.Intakes cold .010-.012 exhausts cold .014-.016
 
A compression test is easy to do & will answer a number of questions about your problem. You could have a stuck valve.

Most auto stores will loan them out if you don't own one.

Minimum acceptable pressure is 90 lbs w/ the lowest pressure reading w/in 75% of the highest reading. Run the engine to operating temp, turn it off, remove all 4 plugs, remove the breather hose & make sure the choke and throttle are both open. Crank it at least 5 compression strokes or until the gauge stops moving. Write down the first compression reading (that is the valves seating) then write down the reading after 5 strokes or when it stops increasing. You should have two numbers for each cylinder. Then, add a tablespoon of oil to each cylinder & repeat the process, but you only need the final reading for each cylinder. Write down the results & post back for help figuring out what it all means.
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Hi Bruce, from radiator back I have a first reading of 60-60-35-60. After five compression cranks I have straight 90-95 across all 4 cylinder. I didn't do the oil test but wanted to see what you think this means. I can run the oil test this weekend and report back if you think it'll help.
 

Is it popping at idle after a work out or while it still has a load on it? I once had a problem with my 2N's valves sticking open caused by oil coking in the guides after being heated by working the tractor hard. Usually only one would stick causing a miss and sometimes a backfire. The final straw was when all four intakes stuck open creating a no-start. After $700 in parts and machine work all has remained well for the last 22 years.
 
You need a valve job. Regular compression is fine. Wet test will tell you about rings. But I think you found the cause of your problem.
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(quoted from post at 22:53:44 04/27/18) You need a valve job. Regular compression is fine. Wet test will tell you about rings. But I think you found the cause of your problem.
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What are the chances the valves have too much carbon on them to seal properly?
 
What are the chances the valves have too much carbon on them to seal properly?

It is possible but not likely as the valve stem and seating surfaces are moving continuously during operation.

Carbon normally builds up beneath the valve head, and ports.
 

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