John Deere 3020 gas runs rough under load

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I have a John Deere 3020 gas that idles well but when under load pulling or running up hill it runs rough.
I checked all four cylinders / plugs and they all have good spark.
I played with adjusting the governor but made no difference.
I also tried adjusting the carburetor but also no change.
I checked the air intake but very clean with no obstructions.
I also checked the intake manifolds for cracks or leaks but all looks good there too.
Can anyone please give me some advice with which way to move forward?
Carburetor replacement?
Thanks in advance for any help you can give.

 

[ET]

It's most likely an ignition problem.I would start with fresh plugs. The ignition system works harder under load.

 

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The previous owner installed a Pretonix coil.
Could that be the problem?

 

[used red MN]

The previous owner installed a Pretonix coil.
Could that be the problem?

Very slim change on the Pertronix especially if it ran well at some point while it has been installed. First replace the spark plugs as suggested and then see what happens. People tend to ignore spark plugs because there so used to modern vehicles with the precise fuel control and powerful spark that just seem to go forever without a problem. The reliable firing of plugs in carbureted tractors like this is at a far lower level. Try new plugs then we’ll talk more. Also a governor’s operation is often misunderstood, it has zero effect on how “rough” an engine runs. It is the equivalent of your foot on the accelerator in a car. It’s just changing the opening of the carburetor throttle valve as speed needs adjustment or load changes are varied.

 

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Thank you ET & Used red MN. It was as simple as the plugs.
I thought the worst and relieved it was something so simple.
Thanks again guys.

 

[drummer481]

Thank you ET & Used red MN. It was as simple as the plugs.
I thought the worst and relieved it was something so simple.
Thanks again guys.

Just out of curiosity what plugs did you put in? Mine is kind of doing the same thing while I have been bush hogging, but it is 95 degrees here in Indiana right now. I kind of just blamed it on vapor lock since it runs ok after cooling off.........

 

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NGK AB-6