This "lean run" condition is especially true if your engine has a Nikasil cylinder as opposed to an industrial hard chromed one. I must admit that it is neat seeing an overlean engine destruction process when the flyer will not listen to reason from someone who cares.[>:]

AMEN!!!!!!!!!!

Exactly I do agree, a Chome bore will take a lot more abuse. A LOT MORE. Nickel gives up pretty fast. Not sure why they do not all use chrome anymore.

My Experience is opposite. When racing Kreidler 50cc engines, we were more than happy to replace the chrome lining with NIKASIL after a porting job. The latter had a much better bond to the high silicon aluminum cylinder, and had micropores that would keep an oil film better. For that reason, the piston had an easier life. The standard Kreidler chrome plating was pitted for oil pockets, and was not as durable as NIKASIL.

In one of the engines I build I can get the cyl in either chrome or nikasel. I have found the chrome to resist scratching to a far greater degree. The chrome plated ones rarely develop scratches from the dust we tend to suck in flying, but the nikasel picked up many scratches very fast. Also I have had a few people score the pistons and if it was in a chrome bore the cyl was undamaged unless it went too far, were the nikasel was damaged right away. Just my experience with airplane engines. In other applications it may be different. I have never had either let loose from the aluminum.

There seems to be a lot variance in Nikasil, depending on who applies the process, and how he does it.
Mahle in Germany coated ours, and needed diamond hones for the finishing to final size. Normal carbide hone stones were not hard enough. For us that was a major investment in the tuning process, even more expensive than the special forged pistons we needed for the 16,000+ rpm of the engines.

Some also use a strait nickel correct, not the nickel silicon?

I think the best way to ensure the engine is not running lean is watching the spark plug color if is gray is lean if is brown is ok.

Sorry. I got so caught up in everything else no one has really helped you at all with your question. Does it restart again? On that engine with the stock EI they were known for being timed way too advanced at times. I have seen them as high as 45deg. At that advance they will create a ton of heat. Also check for any air leaks. One thing with that engine is, it will not run the best until there is some time on it. That was not true for all of them, but many of them. I have reworked 50-60 of them and I saw many different issues with them. Most were not bad. The stock EI can be a little troublesome, but they may have fixed that in the later models. SO I guess I would check the timing, check for any leaks, and then the carb is the next suspect. Again I do not know what carb they are using now, but the one on them a year ago was worthless.

Gspar,
On my forum, read tuning the Walbro carb. I gave made pictures of my spark plug readings, and cylinder head temperatures that went with the readings. Also I managed to get a good indication of the difference between a cold plug, and rich settings.
http://www.prme.nl/forum/viewtopic.php?t=10

I hope this helps in determining your problem, and get your engine tuned well.

In gas engines? Not that I know of. Straight (electroless) nickel is used in plating parts prone to wear, like cams and model engine (glow) cylinders. In these applications, Hard chrome is not always better, but in model engines it is.

As i said before the problem was the hi needle was set 1/4 turn too lean, after opening this 1/4 turn the engine runs just fine.
This engine is a stock Spe 26 and turns a menz prop 17x6 at 8000 rpm. The carb is a walbro wt-501 (9.57 mm venturi). I´m going to test a wt-542 carb ( 13.4 mm venturi) to check if i can get more rpm.
Thank you

13.5 venturi is too large for a 26cc engine. The original carb should provide ample power, and a wide power range, unless you want to race with the engine and can live with seizing pistons a finnicky setups. For racers, that is what they all take in their strides.

I just need around 300 rpm more to have some extra power when hovering . I don't want to push the engine beyond that.
Other people in this forum reported good results for this engine replacing carbs with larger venturi .