I don't know what type carberator you have but a lot of old tractors and combines used this engine. Most used an updraft carberator probably a Zenith or a Marvel. The majority of the larger engines like teh 6's or big 4's used a Zenith. These carborators had 2 neadle valves a main jet and an idle jet. The idle jet was on the top by the manifold and the main jet was generally on the bottom of the carb. if it is idling OK the main jet could be set to rich. To set open the governor to about normal speed and adjust the main jet turn it in (leaner) until the engine starts to labor then turn it out until you see black smoke or it sounds like it is starting to flood out (Make sure you keep track of how many turns you turned the jet between the two positions) set the main jet halfway between these two extreems. This should be a good start to setting the carborator. Some of these carborators were a little fussy about temperature changes and changes from summer to winter gas mixtures.

Kenneth L. Hollingsworth