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Polaris Trailblazer 250 Piston Melt - Too Lean? Jet Size?

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I have a 2005 Trailblazer 250 that I purchased last year.

Configuration at time of purchase:

0.020 Namura cast piston

high stall clutch

hot seat exhaust

oil injection disconnected (running 32:1 pre-mix)

.140 main jet (installed by repair shop to correct "low bottom end power" complaint)

Drove for one season. At end of season, after a 2 hour ride, my son rode it hard for about 15 minutes at which time it stalled and wouldn't restart. Teardown inspection showed a melted piston. I've since bored it to .040 over and installed a new Wieseco forged piston. Crankcase air pressure leak tests were negative. I can't seem to find any information on what the factory jet size is. However, the previous owner did give me the jet that was in the carb prior to replacement (.150). Prior to and after repair, When cruising 20-25 MPH, slightly opening (tip-in) the throttle results in what sounds like pre-ignition (spark knock).

I relly hope to avoid rebuilding the top end again, so any answers to the following questions and/or other suggestions would be greatly appreciated:

Is the .150 (OE?) jet large enough for this configuration?

Should I install larger jet and/or raise the main jet needle?

Thanks!

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CARBURETION

Type . . . . . . . . . . . . . . . . VM30SS Mikuni

Main Jet . . . . . . . . . . . . 130

Pilot Jet . . . . . . . . . . . . . 40

Jet Needle . . . . . . . . . . . 5DP7-3

Needle Jet . . . . . . . . . . . 0-4(169)

Cutaway . . . . . . . . . . . . 2.0

Air Screw . . . . . . . . . . . 1 Turn

Valve Seat . . . . . . . . . . . 2.5

Float Height . . . . . . . . . Parallel to gasket

. . . . . . . . . . . . . . . . . . . . . surface 1mm

Fuel Octane (R+M/2) . 87 Non-Oxygenated or

89 Oxygenated

At .40 over I would move to a 150 main and a 45 pilot. The pilot is too small which is probably your problem with the off idle stumble. :thumbsup:

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Thanks Danco! I have a 150 main jet in there now. I'll verify the pilot size (is that jet marked - will I be able to tell)?

Is there any thing else I should check and/or verify to prevent another piston melt down?

Thanks again! :thumbsup:

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