I have a Moki 2.10 that I got from a friend who was using it in a GP PW Extra 300.
This engine had been sitting on a shelf for over a year when I desided to bench run it. It ran very well with tons of power on 10% fuel. After testing it I put Hobbico after run oil in it and it sat for a few days. When I went back to it the piston was stuck and would not budge. After working with it and a lot of penetrating oil I finally got it free and it seemed to turn fine. A few hours later it was siezed up again.[:@]
Does anyone have a guess as to what is wrong with this engine?
Thanks.

The moki should be run on 5% nitro at most mine runs best on FAI fuel 80-20. if you are running 16% oil you may have done some damage. I bought an older engine I ran on a bench few days later wouldn't turn over. It had an accumalation of sludge in the rear bearing where it had been soaked in cleaner. I disassmbled ,cleaned and replaced bearings still running fine.
Hopefully it is tight in the bearings. you should be able to see any piston=liner issues by looking through exhaust port or removing head. Be gentle with it Moki parts are expensive.

Hammer,
I was using a 18% castor/syn. blend for oil.
I really think the problem is in the piston or the sleeve. When I took the back cover off the inside of the block looked clean and the bearings felt smooth once the piston was freed up. However, when I looked inside the exhaust port I saw what looked like small specs of carbon in the sleeve openings and on the top of the piston. I wonder if the after run oil is loosening some of this grit and lodging it on the piston.
So, do you think this is from too much nitro?
I appreciate your thoughts.

You may have galled the piston?

5% nitro is the right amount for the Moki. And you can use less than 16% oil if you want. I can't explain the "sieze up" but it is not due to the fuel you ran within the last few days. I suspect something is stuck inside the combustion chamber. remove the head and have a look around.

Ok, w8ye,
What exactly do you mean "galled the piston"?
I see some very, very fine lines on it but nothing that looks severe to me.

Right now, I have the engine freed up and turning smooth. But, as has happened earlier today, if I let it sit for a few hours it will sieze up again.
Thanks for everyone's input. I really appreciate it.

If the lines are fine you are probably OK

Galled is when some of the aluminum on the piston melts and comes out of those grooves and sticks in the high places on the piston.

If it frees and then sticks after setting a few hours it has some gummy substance in it

Go like the other guy said and pull the head and back plate and have a look around

Try flushing it with automotive carb cleaner.

Ok, I have the head off.
There appears to be a small amount of black pasty grit on the top of the piston and in the cavities of the sleeve.
Could this be the problem?
If so, where did it come from? Too much after run oil or...?

That's carbon and probably some varnish too...that's most likely what's gumming it up. I recently experienced something similar.
The carb cleaner (the "Jet Spray" ) will help to flush that out. Also a bit of careful scraping with a soft material (brass, alum. or hard plastic maybe ) scraper. You can also use a steel scraper if you are really careful not to gouge the top of the piston or the head.

Pull of the back plate and look around.

Make sure that grit is not burned plastic

Some rear bearings have a plastic type ball cage that may have separated and gone up through the ports and got on top of the piston?

If all the rear bearing balls get together on one side of the engine it will lock up

Thanks, proptop.
I'll get some carb cleaner tomorrow and try to clean this up a bit. It seems to come off easy.
Thanks everyone.

I have the back cover off. The lower crank case appears to be clean.
There is a slight ring of this "sludge" around the top of the cylinder wall where the piston stops.

The bottom end is most likely OK. Leave it open while you flush out your engine like the other guys have suggested.

You can use rubbing alcohol to flush it if you are anxious but you need to dry and oil it afterwards.