Hi, I'm flying for about 2 years now and my engine is running a bit weird sometimes. The engine is a OS MAX .25 of about 20 years old. I could buy it from a friend for $30. He had never used the engine except for running-in. What should be the problem?
Any help is appreciated.

I'm not sure what you are asking. Do you own a weird-running OS 25 that is 20 yrs old & your friend also has a just-broken-in one that he will sell you for $30? Or does your friend simply have a 20 yr old just-broken-in OS 25 that he will sell you for $30?

In either case, I think that $30 is too much for a 20 yr old OS 25, just-broken-in or not (unless it has collector value).

I don't think that you will have any particular problems in getting it to start & run, but it is probably a cross-flow design that won't be as powerfull as current engines.

That's not the problem. The engine is running fine, but sometimes when it's hot it doesn't run regulary. Wat should be the problem then? It would probably need a refurbish to run smoothly again, but I'm not sure, that's why I'm asking.

Is it your engine, or your friend's? If it is yours, try running a slightly richer high-speed needle mixture. If it is your friend's, don't buy it.

It's my engine, but if I run the engine richer, the rpm goes down at full throttle. Or is that no problem?

Tweak it to max and back off about three to 5 clicks. I would also check the bleed air screw setting as this can change when it gets really warm or cold meaning it may need to be readjusted. A clogged bleed air hole can cause erratic running in these older engines

Thanks for the information, gonna try this tonight