Just wondering is it possible to run 25% oil in a nitro engine.

Yes. Some control line fuel was 29% castor oil. Powermaster now sells G.M.A. blend available in 29% castor oil formula. Fox also sold a 29% oil formula called Duke's fuel.

I use 25% all castor in my ST G51 because it doesn't give the kind of run I need with less oil. The Fox 35 needs the 29% oil and other cast iron piston engines will be happy with that too even though they don't really need it. There's sure to be a maximum amount of oil that's useable in an engine although I've never experimented to find out what it is but I'd guess maybe up in the 40% area.

I'm pretty sure they won't run on 100% oil.

What about a 46 la will that work fine?

I burned a gallon of Duke's fuel, 5% Nitro, 29% oil, all castor in a OS .40 FP and had absolutely no problems. Good reliable idle and the power one would expect from 5% nitro at full throttle. The FP line of engines were very similar to the LA line of engines except they had the needle valve on the carburetor instead of at the rear of the engine. I also burned many gallons of ducted fan formula, 24% oil with no problems in the same engine. The biggest drawback to the high oil formulas is the fact that oil is expensive and raises the cost of fuel.

Will more oil help increase the compresion of ringless motors? The motor does not have any scoring on the piston liner it is just a tuoch low on compression.

Your LA will do fine with 25% oil and being a plain bearing it should have some castor as well. Extra castor can also help give a better compression seal but usually that's only with cast iron pistons. If your engine starts easily (especially by hand) and runs normally then compression is OK but check first that the head bolts are tight and that the plug isn't leaking. Could you explain why you think it's a bit low on compression?

It never hurts to try, maybe you can borrow a tank full from someone who flys ducted fan models or just buy a gallon of a high oil fuel. If it doesn't solve the problem, it's still perfectly good fuel. You can also buy some AA castor or Klotz oil and add it to your fuel.

I'm not really sure why it is low on compression, but I do know that it is as our club instructor had a look at it and he said that it was low on compression and compared it to one of his 46 size engines. When I turn the engine over by hand there is no force needed to turn it over. As I said I have checked the piston and the piston liner and I cannot see or feel any visible marks on them.

Fair enough. If this developed slowly then chances are the piston is worn out (that usually takes a LOT of running). If it happened all of a sudden then something's loose, either not sealing on the head gasket or the centre post of the plug has lost its seal. To check for a worn out piston, lay the engine on its side (without the muffler), close the exhaust port with the piston then fill the port with fuel. Turn the piston to the top of the stroke and look for a lot of bubbles through the fuel. There goes the compression. For leaking plugs or head gasket I use a little bottle of leak detector like they use when checking for gas leaks but a solution of detergent and water should work just as well. Run it around the head joint or a drop on top of the plug terminal and turn the piston to TDC again and look for bubbles. Oh yeah, and check the plug gasket at the same time.

I have done that test. The bubbles appear only minor though. Does this mean that I need a new set of piston and liners? Or will the extra oil help get a better seel.

Feeling a bit low on compression when you turn it over by hand doesn't necessarily mean the engine's worn out especially seeing you said the bubbles are only minor (through the exhaust port?). All engines will leak slightly at least until the piston wedges into the pinch at the top. If yours has lost the pinch then maybe that's what the instructor is feeling (or not feeling ). I haven't mentioned the dreaded nickel peeling yet either.....

But like I said earlier, generally if it starts easily, tunes as normal and runs well then keep on using it. A fuel with all castor (22% is good) will help give a better seal than all synthetic if you want to try that. You should be able to get Castrol M somewhere near you. I wonder how a blend of Castrol M and Bundy rum would go?