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LO206 idle surge and flat revs a few laps in
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.
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Before you start throwing pumps and carbs at it, go back to the vacuum and fuel path basics that still catch people on these engines.
First thing is the catch can. Make sure the gasket between the two sides is actually sealing so you are not getting crossover from one side to the other. Same pass, get the engine idling and spray carb cleaner or brake clean around the intake manifold gaskets and o-rings. If the idle picks up, you have an air leak that only shows once things are warm and under load.
Bubbles in the line plus the flat top end after a few laps often point to intermittent fuel pickup trouble. Debris or contamination in the tank can partially block the pickup and starve it once heat and hard throttle come into play, even if the filter looks fine at a glance. Pull the pickup and filter and inspect them carefully.
With it running, gently flex and press on the carb and watch the idle. A cracked phenolic spacer or a hairline crack in the plenum can stay quiet on the stand and then open up under track vibration, leaning it out exactly when you need clean mixture..
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From what I understand the float height and drop on these LO206 carbs can mess with idle stability and how well it feeds under load once things heat up. If the level is a bit off it can still look fine on the stand but start starving or surging a few laps in, especially when the engine bay is hot. Honestly I am still learning how sensitive that setup really is though. Have you checked the float height lately, or is that something most people leave alone unless the carb is already apart?
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