Oh yeah, I've taken out and replaced everything in there as well as cleaning the hell out of it.
The idle jet, main throttle needle and seat, new float needle and seat, new mixture screw.....the whole nine. I downloaded mikuni's generic service manual so that I was sure to get every passageway cleaned.

I'm convinced at this point that its somthing other than the carb. I've seen it suggested that bad crankshaft seals could also cause this problem, that's what I'll check next.

 

***UPDATE***
I adjusted the float arms from 20mm to 18mm and it idles without the choke now!! However it still doesnt want to start without the choke and if you take it to full throttle it bogs out.

Any suggestions?

 

What did you get for compression?

 

The proper float adjustment on your carb per the service manual is typical for this model Mikuni. With the carburator inverted and the float bowl removed the float arms should be parallel with the float bowl mating surface on the main body when viewed from the side. It has no mm spec. As for your now high speed problem, Assuming your not jetted too lean, a compression check should be done to eliminate that as suggested and it could be timing also so I would check that. Carb s/b a VM30SS-Factory 155 main jet- Timing 25 degrees +-1@3000 rpm COLD-- a warm engine will retard 2 degrees on this model.

 

Sure a bad crank seal will cause a lean condition. Remove the clutch cover and spray carb and choke cleaner behind the clutch while it's running if the rpm's change there you have a bad crank seal pto side. While the cover is off wiggle the clutch up and down, excessive movement would indicate the crank bearing is out. That would in turn take out the seal.

 

Also check to see if the exhaust is free flowing.

 

SUCCESS!!
In the end it turned out to be.....and your gonna love this one....that the head was LOOSE! Loose enough that I barely needed a wrench to get the nuts off. Compression is in the high 80's now, up from 40 lol, I know not great but it runs.

Just to finish off this thread heres what it took:
A good cleaning of the carb
Carb rebuild kit (jets, gaskets, needles, etc)
A new carb boot or intake manifold (the rubber thing that bolts onto the cylinder and holds the carb)
New spark plug
New gas and oil
A whole HELL of alot of patience and help from you great folks

Many thanks to all who contributed,
Mike

 

glad you got it solved. wouldnt hurt in the future to do a topp job on it when you can. thats kind of low compression but hey if it works right.

 

If the compression is that low i would recomend you do a top end job. Its very simple on two strokes and you will have a much beter longer lasting atv. It is also pretty cheep to do a top end it's like $100 from dennis kirk for a WSM rebuild kit.

 

Are we good or what????????By the way the 1988 Trail Boss had two different models. The 85-early/mid88 was a gen I model. Late 88 was a Gen II. While motor components were the same for the most part the front suspension was changed completely in the strut/spindle/hub area. Make sure you know your model no when ordering the manuals on cd or otherwise. Now go forth and ride.