[twiddlebug222]

I have a 1998 JD 425 that had just started to run rough at all RPMs. To get the tractor to operate smoothly at any RPM, I adjust the amount of choke and the engine smooths out - loaded or unloaded. The amount of choke required depends on the engine rpm. This problem occurs when the engine is cold as well as warm, and with the air filter and gas cap removed.

The fuel filter stays full which would indicate the fuel pump and filter are OK. Since the engine runs OK with the proper amount of choke, the electrical system should be OK. No work has been completed on this engine. The engine starts right up after shutting it off

:4-dontknoWhat am I missing? The tractor has been running fine and just started doing this.

I am assuming the problem is in the carberator since the engine is being starved of gas.
http://www.techsupportforum.com/forums/report.php?p=1595825

 

[K-B]

Sounds like a plugged/partially plugged orifice or jet in the carburetor. A thorough cleaning or rebuild of the carburetor is probably in order.

 

[kbowley]

Try purchasing some Sea-Foam and mix it 8 OZ. to 2 gallons and pour in tank when close to empty. that should clear it up, if not you will need to remove the carb and spray carb cleaner through every orifice you can find, including the holes at the rear entrance area. Remove the bowl, brass jets, external adjustment screws and spray through all holes aggressively, compressed air would be a big help.

The reason it smooths out when you apply some choke is the mixture with the choke open is to lean. Applying the choke reduces the amount of air entering the carburetor and corrects the lean condition. ideally the mixture should be about 13 to 1 (parts air to parts gas). when a internal jet or orifice becomes restricted, the gas flow becomes restricted therefore changing the air/fuel ratio. by applying some choke you reduce the amount of air introduced to the air/fuel ratio correcting the lean condition and returning the mixture to ideal. This has an impact on performance because the combustion chamber gets less volume pulled in from the restricted choke therefore power is decreased even though the mixture is correct, there is not enough of it reaching the combustion chamber to produce full power. An analogy would be to put a carburetor from a 10 hp engine on a 14 hp engine. the mixture will be correct but since the carburetor is smaller in bore, less of the ideal mixture reaches the combustion chamber.

Ideally, with the throttle plate in the carburetor full open, vacuum should as close to 0 as possible, inferring that the valves/combustion chamber has no restrictions at the carburetor.

Hope this helps.
Kevin.