I have an OS 61 fx in a revolver and it doesnt seem to be getting the rpm out of it that it should. Im running a 13x6 prop. I have also tried a 12x8. The plane flies slow. I would have thought it would scream with a 61 in it.
My main concern is that there is no difference in rpm between 3/4 and full throttle. Why is that?? Im watching the carb barrel move and not going by the movement on my radio. The HS needle is out about 1 1/2 turns and the glow plug looks good. I have about 1 1/2 gallons through this engine. I fly at about 4500 ASL, so I usually go up in engine size for all of my planes. Im just suprised its as slow as it is. My mustang PTS is just about as fast with the evo and a 11x6.
Any thoughts or input would be appreciated. The plane does fly good, but I expected more from it with that size engine.

I would think you need to open your high needle some more. It's true you can fly an OS out of the box, but contrary to what many people say, they take some time to break in. I've got about 4 gallons through my 1.20 and it's finally starting to make the power it should.

What does the manual say for recommended high needle setting? I would think closer to 2 or 2.25 turns open.

manual says 1 1/2 to 2 turns out.   Its not lean by any means.   It is a little on the rich side, but not spitting all over.   Im also running a pitts muffler on it and not the stock exhaust.   I know how to tune a 2 stroke.   Im just not sure why its dead from 3/4 to full throttle.   I also pulled the HS needle out to make sure that it was clean and there was no blockage.  

I use to have an os 61 fx with pitts muffler on my extra 300,it's not a screaming engine smaller engines use to fly by me.</p>

It's quite a heavy engine as well ,used 12x6 on mine seemed to work better,i also own a revolver ,using an os 55ax and i tell you it's fast!!!!! the 55 is a better choice for a faster combination.</p>

You are using too much prop

Most people use a 12 X 6 or 11 1/2 X 7

Agreed...a bit too much prop load...try an 11x7 or 11x8 or maybe a 10x8 or 9 and let it rev more.</p>

Iagree. An APC 11x7 will be just right.

Jim

the fx 61 is a nice reliable engine, but no where near a powerhouse, the 55ax is considerably lighter and can be just as powerful. 11x7apc for stock/pitts echaust.... 12x6 apc tuned pipe

Thanks for the input guys.  Seems like a small prop for a larger engine, but I will take your word for it.   I did some experimenting with props and did find that a MAS K series 13x6 did let it rev alot higher than an APC did.   The APC did however pull harder vertical.  I will try some smaller ones next time I hit the field.   Thanks again.

I have several OS Max 61's on various models. I find that the APC 12 X 6 delivers the best performance.

What you are experiencing isn't that wierd. There simply isn't that much difference between 3/4 throttle and full throttle on that engine. There should be some difference, mostly noticable in the vertiacal. But at cruise, you can't tell the difference.

Since you are higher up, you might try higher nitro which not only includes an oxidizer, but also the oxygen itself.