Short version of the long story is I have a tower .75 in my Bobcat and on the ground it reaches peak RPM but in the air it "acts" as though it has gone completely rich and won't reach peak RPM. Now I had a pet theory that positive air pressure on the exhaust is causing the muffler to overpressurize the fuel tank and you can read all about that here:

http://www.rcuniverse.com/forum/m_5103308/tm.htm

But Then I began to wonder about my prop selection. I have been running a 12x6P on this plane up till now. I did test run the 11x8P on the ground but it seemed to produce quite a bit less static thrust then the 12x6 so I used the 12x6.

The question is am I shooting myself in the foot with my prop selection. You guys have a great deal more hard earned experience than I have, please let fly your thoughts.

Is it possible that it is too lean and being mistaken for rich? Maybe a tank instalation problem is causing it to lean out in flight? Just a guess.

Nope lotsa smoke....lol

Could be a pressure difference on the carb since it's the opposite of ram air being a pusher.[X(]

I have a new Tower .75 on my Bobcat also. Using 11-8 Pusher and am very happy with performance. Running it slightly rich on the ground, level, its a pusher so you need to run it little richer than normal for cooling. Remember with the tank forward of the engine it should richen slightly in the climbs and lean out in the dive. Mine also has Robart spring-air retracts and flaps.
Hope this helps,
Matty

My buddy uses a 11x7, runs great, in a Sig Sundancer 50.

Matty,

Are you using an exhaust deflector?

I've got a .75 on a Balsa USA Enforcer pusher. Same kind of issues. I've had better luck setting the needle on the ground with a tractor prop in place. Set it fairly close to peak then put the pusher prop on. My sorta educated guess is that the pusher prop pulls the air off the carb, hence rich condition. It's really kind of a pain in the butt to get dialed in.

I have used the 12X6, 12X8,13X6 and 10X10. I Liked the 12X8 on mine but I think you are after more rpms and the 11X8 would be the best for you application.

Good info gus thanks a lot!

I would think the Bobcat would always be prone to running rich with the tank
mounted in front of the engine....in the same manner as an engine would be
prone to running lean with the tank mounted behind the engine. This is, of
course applied to upright mounted engines.

I would think the Tower engine would not need to be leaned more that 1 to 200
rpm static, to avoid over richness in the air. I was using a YS engine that has it's
own pump and pressure issues, so gravity and thrust were not a factor.

I will find out soon enough with my Tower 75 in the Bobcat.

FBD.

Promises promises promises Dave..........

Well be sure you put a Dubor exhaust deflector on that Tower .75, It fixed my problem.

Video:

http://www.rcuvideos.com/view_video....0e5a00c1e221eb

....you da' man !!!