i consider myself a pretty sharp mechanic but this sled is driving me nuts. It came to me because it wouldn't idle. Simple enough, dirty carbs and plugged pilot jets .
I decided to keep the sled a few days in hopes for snow to test drive the sled to verify all is ok. glad i did because this thing goes into a fit at times like it's starving for fuel. has a spot in the throttle about 5500-5700 rpm where it feels and sounds like it's dropping a cyl. i have the airbox top off and when it acts up i open the hood and spray a little carb cleaner at the carbs. It seem to rev up when i spray at the center jug(like lean), and flutter when i spray at the outer 2 (like too rich).

I've check the fuel suppy.. hose and pickup in the tank clear, new goodyear, rubber fuel lines, rebuilt the fuel pump with genuine mikuni kit(old ones looked ok), one new needle and seat, all 3 viton tipped now. float bowls appear to be full when the bog happens.

tried switching carbs between the pto and center position tonight, no change, still seems to be the center hole thats missing. compression 120 w/ snap on gauge, leak down 5% or less, a local dealer put 3 new wiseco pistons in it not too long ago(don't know what happened then) . swapped coil wires from cyl/cyl-no change, coil primary resistances all the same, spark plug caps all 5k ohms, check timing on all 3 cyls with marks made on clutch at .173 btdc ( think this checks for crank twist ?)- ok and constant even during bogging.

raised needle clip to #4 in the center, still bogs. lots of wash on pistons.
I'm going friggin' nuts.
Has a brand new polaris primary clutch and the secondary buttons are fine. Don't think it's clutching at all cause you can here the engine drop a cyl.

Any ideas ? Can a stator or CDI do this ? i don't think so as it seems just one cyl effected . flywheel external trigger pads look fine.
Can anything in the bottom end of the motor cause poor suction through the carbs and a resulting miss ? . i know i'm stretching it now.

I got one for my wife that I bought off my parents, mint mint condition, thats why i bought it. It is the biggest mistake that I could make when it came to buying a sled . When I bought it it was boggy, after I rebuilt the complete motor it was boggy. I too consider miself a pretty nifety mechanic and this motor drives me nuts , it has to be the worst motor that was ever produced. I don't even want to talk about anymore, what I want to talk about is what i'm going to do. My wife is kind of timid on a snowmobile, she is quite the newby to the sport but loves it. So what I'm doing is an engine transplant, dropping in a 503 rotax fan cooled workhorse motor. This motor I have had experince with befors it only has 55 hp, thats alright I think the 600 triple has about 3hp. I learned on a 12.3 olympic for the first yeras when I was younger. she can learn on this, she drives so slow that engine in a liquid gets to hot. I quess what i'm trying to say is the time you are going to spend on tring to get that motor running good you can just swape that motor

Hole in the case from previously exploded piston parts? Your description of symptoms kind of fits an out of phase crank, or bottom end damage.

That's the problem I've had the crank out and retrued with new bearing and seals and the crank case was fine, Ive talked to alot of people some who are polaris trained mechanics, on their own and they told its nothing but trouble. apparently the 580 triple was bullet proof, but when they decided to make it bigger to a 597 with bigger carbs thats when it went down hill.

Well whoever told you that the 597's were no good is full of crap because there was small carbed(34's) 597's as well as big(38's).They had some crank problems but there is prolly more miles on xlt motors than any other.They sold piles of them so once in a while there is a bad one.Gen11 I would seriously check the crank for out of phase as you seem to have covered all other bases.Had a bad one back in the day and it was the PTO end had turned 10* outta phase
Steve