KartDen

Having a weird LO206 issue and could use some diagnostic help before I start throwing parts at it. Symptoms in short: it fires and runs fine on the stand and for the first couple laps, then starts going flat. Full throttle on the straights and it will not climb the way it should, top end keeps dropping off, and it stutters/hesitates coming out of slower corners. Feels like it is starving under load more than a chassis bind or clutch slip. Idle is also a pain. It will settle, then randomly surge a few hundred rpm and come back down. I have chased idle air bleed and needle clip a bit and it still does it. Plug looks roughly normal, nothing crazy lean or oiled up. I have already gone through the obvious stuff. Throttle cable free and fully opening, no pinched fuel line, filter not visibly clogged, clutch not slipping, brakes not dragging, chain and sprocket fine. Head bolts and valve lash are in spec, header gasket sealed, intake/carb gaskets checked. Catch can setup is dual-sided so I do not think crankcase vent backpressure is the issue. One thing I do see is bubbles in the fuel line. Not sure if that is normal on these or a sign the pickup/pump/filter side is sucking air. I tend to run the tank on the lower side for weight, same as I always have, and the stumble still shows up even when the kart is sitting still in the pits after a session, so pure corner slosh does not fully explain it. Weather and heat seem tied in somehow. Cooler shorter runs have been closer to normal, longer hotter ones are when it falls on its face a few laps in. That points me at fuel delivery or carb side rather than a hard mechanical failure, but the intermittency is what is driving me nuts. Also noticed the worst hesitation seems to show up when I get hard on the throttle after a couple longer flat-out sections, not just any random exit. Where would you start? Replace pump, pickup, and filter as a set to rule out air ingestion? Try a known-good carb so jetting and emulsion tube are out of the equation? Or is this a classic heat-soak / float level / idle circuit thing I am misreading? Lower end is older with plenty of carbon on the piston when the head was off last, so I am also wondering how far down the refresh path people would go before just swapping the motor. Open to any checklist items I am missing.