I have a Picco P-0 .085ci engine that starts fine, idles fine when cold, revs-up fine, but once the engine gets hot, towards the second half of the fuel tank, the idle starts to fluctuate up and down every couple seconds. It is also somewhat more likely to lean-out on speed-runs once the engine is fully heated, but it only happens maybe once every 5 speed runs. The crankcase and crankshaft are relatively new, so an air leak through the front seems unlikely; what else might be causing this behavior?

For the record, I'm running it on 30% nitro, with sufficient extra shimming to compensate for the higher-powered fuel, and a hot glowplug because the engine is so tiny a medium glowplug has a tendency to stall the engine when it idles too long.

Is your engine 2nd hand?

the tune is going lean once it warms up, fatten the needles and the issues should go away......Treat the LSN as a idle mixture control only

If its running well off idle, but smoke to a half second ok! The LSN is set too lean for it to fluctuate. P-0 .08 small engine, adjustment on the engine are really minute(small adjustment). If its warmup turn your LSN CW(leaning out) if the idle speeds up and dies, its too lean, or turn down the idle speed. Richen the LSN(CCW) for it to drop down the idle speed( By the LSN), Like I said b4 increments are small 1/16 or less.

P-0 I had since it came out, installed on a Xray NT18. Using 30% fuel. Still have the model, haven't ran it for years. Better than the Toki 07 engine. if you can adjust you air gap with photo attached you'll be happy by the way it runs. Takes patience.

No, I've had it since new, and I've rebuilt it enough times that it's basically a different engine now.

For some reason I keep blowing the rubber seals off the front crank bearings; not sure if they're overheating and deforming, or what the deal is.

I do treat the LSN as an idle mixture control only, because that's all it really does.

My best hypothesis at this point is that the carburetor is getting just a teensy bit too hot towards the end of the run, and that's causing the problem. I have no proof though. Or maybe there's an air leak that only opens up enough to cause trouble once the engine is at full operating temperature, I dunno.

Enriching the fuel mixture seems like the obvious solution, but the problem when I do that is it tends to bog-down on acceleration, and has trouble reaching proper operating temperature at all. And I'm not talking about loosening the HSN by a half-turn, or a quarter-turn, or even an eighth-turn; I'm talking about loosening the HSN by the smallest amount I can turn it without the sealing O-rings pulling the needle back to its previous position.

It is amazing how small the NT18 is. That Picco P-0 engine is tiny, but it looks big on that chassis. I almost got an NT18MT, but I opted for the Turnigy Nitro Buggy instead because it has room for a pullstart.

if the idle fluctuates then richen the LSN, leave the HSN alone

Even if it only fluctuates when the engine is hot?

Leak in the gas tank maybe? You said you were blowing the seals off the front bearing? Is any part of the flywheel rubbing or very close to touching?

Stormer! I need to know, when you turn the LSN CW, what does it do? What your Air gap set at?

The flywheel is tapered on the back so it comes nowhere close to touching the front seal. My idle gap looks to be about 0.75mm -- a little wider than Geezatec's, but the lowest I can adjust it without the engine stalling every time it drops to idle.

I ran the car again today, and it behaved fine. I loosened the LSN a teensy bit, and I tightened the HSN fairly substantially because for some reason it was running rich enough to bog-down when I revved it. I haven't run it since spring, so it's entirely possible the weather has changed enough that a re-tune was necessary anyway.

The interesting part is, even though I had to lean-out the HSN quite a bit (over a full turn), it never exhibited the fluctuating idle that I'd had trouble with before. It could be loosening the LSN a teensy bit did the trick, but I don't think that's the whole story, because I had to fight with the engine to get it up above 200°F -- something I've never had to do before. Usually that engine is hard to keep below 240°F.

Aside from the cooler weather, there is one other change since spring that has been proving to be significant for all of my vehicles -- I spiked my 30% nitro with an extra 1% pure castor oil. All of my engines are running better because of it -- I can push them harder and they stay cooler, presumably because of the better lubrication, and perhaps slightly better sealing caused by the slightest hint of varnish on the parts. (my Byrons Gen2 fuel uses a synthetic/castor mix, but it's degummed castor so it doesn't leave varnish behind -- I'm starting to think that's actually a bug instead of a feature.)

So in short, it seems to be working better and I have no idea why -- only guesses, and tenuous guesses at that.

Castor oil will increase temp, not run cooler

What radio system are you using? Agree with Malaysia castor does raise the temp.

I run Spektrum radios.

My experience has been the exact opposite, with all of my engines.

Keep in mind I only added a small amount, just 1% extra oil. It works out to about 43mL of castor oil per gallon of fuel.