I have this os max 35 that has gotten this cracking sound when it's running. It causes the a vibration in the plane and a loss of power. I took it out of the plane and took the back plate and head off . I didn't see anything that look bad. The head bolts were not as tight as they could have been. So I put it back together and ran it on a test stand. That didn't change anything. When it ran good I got 12.5k rpm . Now I only can get 10.5k rpm. It idles good and starts ok. Tried different glow plug and prop. To me it sounds like the fuel is not igniteing at the right time. Nothing I have tried has helped. I'm lost . Has anyone seen a problem like this or have any ideas for a solution.

Preignition? Could need a shim under the head to lower the compression, or a cooler plug or smaller prop.....

OK Thanks. But why has it ran good for 2 to 3 yrs. and now it has problems.

Different fuel with more nitro in it? Something changed with the engine to make it run differently.

Carbon buildup, plug? praying to the wrong gods?

I'll take it all apart , clean it all up and give it another try.

Hi!
You should run 5-15% nitro.
OS 8 or ENYA 3 glow plug and a 10x6-11x6 prop.
Are you sure the engine is clean inside and not has lot of carbon build up on the piston?

I took the engine apart and cleaned . I found that the end of the rod that connects to ristpin was warn and had play in it.

So now you need a new rod I guess. Either that or drill it out and machine a bushing. Do they even sell parts for those?

I don't have the equipment to make a bushing and do the repair. I did see there are some rods floating around in the UK for 10 to $15. Probably save the $15 and put it on a new engine when I need one.

Right now I have put an os max 40 in the plane . I did really like the 35 in the plane it had the right power for the way I like to fly.