I have a 87 LT230s quad runner, I had to bore it one over because of a tear in the cylinder wall and worn piston head and rings. I also added a DG pipe and a K&N filter. I valved lashed it 2 months ago and still had a lean mixture. So a cleaning of the carb and readjusting the air screw, but still nothing. So I got bigger main and pilot jets (one size up), because I wasn't getting mid to WOT power. Now my mixture is rich blacken my spark plug and making popping high pressure air coming out of the exhaust and wont hold idle longer than 5 mins. I moved the needle jet up to lean it but still haven't had luck getting power after 1/2 throttle and idle wont hold after riding it for awhile. When riding it 0 to 1/2 throttle position the quad has power and revs, but after half way the engine drops off and power is killed. I'm not sure if im not adjusting it correctly or something is misplaced or missing. The carb is stock Minuki Vm26, Jet needle is stock and needle jet. Please give advice I'm out of ideas.

 

Might be out of my area,(worked on a few Lt230s)but I'd check the valve clearance along with doing a compression check,possibly a leak down also. What piston to cylinder clearance did you have on the new piston? OPT

 

If you moved the needle up then that is making the mix richer. Lowering the clip on the needle raises it raising the clip will lean it