Okay- so I am in the final stages of getting my bike back together. When I traded for it, a majority of the pieces were in boxes.  Today I am undergoing getting the carb cleaned up and ready.  I have the stock Mikuni carb and also found a Kehein PJK 38 in the boxes. Both were dirty on the exterior but looked pretty clean on the inside. I have decided to deep clean both of them and get them back together. Which is the better carb to use? I have a new top end that has been o-ringed, the correct squish set and what appears to be some minor porting by Jerry Hall. With this I need to use higher octane avgas or racing fuel. That is pretty much it for engine mods with the exception of new piston, rings, billet washers and appropriate roller bearing and boyesen reeds. Will the stock jetting work for either of these carbs? Where should I start? I live a couple hundred feet above sea level here in Washington State.  Thanks in advance

I prefer using the Mikuni carbs in all of the LT 500 applications.  The TM 38 carbs have 3 more tunable circuits than the PJ, PWK, PWM, or Air Striker Keihin carburetors. 

You cannot change the needle jet, float valve size or the air correction jet in the Keihin carbs. 

Ones understanding of what each circuit does and how they overlap is imperative for carb tuning.  Ones tuning ability also has to increase as the number of tunable circuits within the carb increases.  More tuning circuits makes it possible to fine-tune the engine to a higher level than carbs with less circuits.

If a manufacture made a carb that only had one tuning circuit and only made two jets to fit that one circuit, you would have two chances to make that carb work the best it could work.  In this example one of the jets would not necessarily make the engine run up to its full potential but one of the jets would make the carb run at its full potential.

My experience has shown that most professional tuners prefer carbs with more circuits.  Carbs with fewer circuits and fewer tunable circuits are preferred by those with less experience because there are fewer choices to make from a smaller selection of tuning parts.