I went out today and ran two different .61 engines that I have today and I got what I feel are very different results from two engines that I feel should have very similar performance. I first ran my Thunder Tiger Pro .61 that has a strap on Tatone pitts style muffler and used a master airscrew 3 blade 12x6 prop and I got a reading of 12,000rpm at full throttle and idle at 1,800rpm. I then wrote down this info and proceded to check out my other engine. The other engine is an OS 61FX with a slimline pitts muffler swinging a master airscrew 2 blade 12x6 prop. I got a reading of 11,400rpm at full throtle and 2,000rpm idle. I used Omega 15% fuel and both planes have an os #8 glow plug. A little histroy on the engines: The TT Pro .61 is well broke in but still in excellant condition and the OS .61 FX has had approximately 1 gallon of fuel ran through it. I honestly expected the .61FX to outperform the Pro.61 but that dont appear to be the case. I double checked my readings on both planes and yes I made sure that I clicked the 3 blade switch while taching the Pro.61 and the 2 blade switch on the .61FX on my tach. I even checked the calibration of my tach and all was fine. So any explinations? Also I checked the manufactures website for both engines and it says that the OS.61 Fx should produce 1.9HP @16,000rpm and that the TT pro .61 should produce 1.8 HP @13,000rpm. Also it appears that the pro .61 is performing great and that the OS 61FX is not according to that data? So any suggestions or ideas?

Thanks,
FUZZ

Fuzz,

When we run the O.S. Max .61 FX with an APC 12x6 prop, we expect to see it turn in the mid-12,000's. If your engine is running slower, it's either damaged from something, or was bad out of the gate.

The usual cause of damage is an accidental overheat, caused by an unitentional lean run. You'd need a new piston/cylinder assembly to repair the engine.

Bill,

Thanks for the info, I bought this engine new from Tower Hobbies and it is still under warranty. I have made a point to always run it a few clicks rich to avoid overheating. I followed the standard break in procedure, but I have never been able to turn good RPM #s with this engine. My next question is: will the OS warranty cover this problem, because I'm pretty sure that overheating is not the cause of the problem?


Thanks,
FUZZ
Johnny Millwood

Johnny,

If the engine's not right, the warranty should cover it. Send it in to our address below.

I have found the source of the problem. It is the slimline pitts muffler. I put the Tatone muffler on my .61FX and the engine really came alive and I got 12,700rpm with the Master Airscrew 12x6. I then put the slimline muffler on my Thunder Tiger Pro .61 and it too lost appx 1,200 - 1,400rpm with the slimline muffler. I dont understand why the slimline has done this but in any respect I wont be using that muffler. I have tried to contact slimline about the muffler but their email is not working, so I assume I will have to phone them. Thanks again for all of your help. I am not an engineer but to me it just seems like the chamber on the slimline is just to small. I have enclosed pics of the muffler. The slimline is on the right.

Johnny Millwood