OK, I flew in a contest last weekend and had some problems with my second flight engine run. The plane is my 54oz, 620 square inch Slider. The engine is an OS40VF. .265 venturi, PA needlevalve. Fuel is Wildcat 10/22 half caster/half synthetic. Brand new gallon. The prop is a Bolly 11.3 x 4.5 depitched to 4.25. Pipe length is 18.5 inches on a Randy Smith "51" sized 3 chamber pipe. Ground rpm was 10,800

The first flight the run, from my neophyte perspective (I've only run this engine since late last fall) was near perfect. Nice and steady. The weather was in the mid-50s and overcast. Humidity was probably around 60% with probably an 8 mile an hour wind. I thought it was a bit fast at maybe 5.0 to 5.1 seconds a lap. But the engine ran near perfect, as I said.

The second flight, it had warmed up to about mid-60s and the humidity had dropped quite a lot. Just a little breeze . No change to the setup at all. Same ground rpm, but the fight was considerably faster. Maybe 4.9 second laps and it seemed I had lost that nice deceleration coming toward the ground.

So, here's the question. It was suggested to me that I should have either pulled the pipe out a 1/4 to 1/2 inch or reduced the needle setting to less rpm. Observers said that the engine was too leaned out. I haven't run this engine enough to tell by ear if it's too near the end of the power band. I know that you have to adjust the setup (pipe length, prop and such) depending on the conditions, but generally, as it gets warmer, do you have to either increase pipe length and/or reduce the rpm? Is there any sort of rule of thumb when it comes to changing setting for conditions with these engines? Would a bigger pipe, say a 60 sized pipe help (this was suggested by one expert flier I know).?

Thanks for any input. I'm just trying to learn this setup and engine.

I've got two tye wraps at each joint. I'm pretty sure there are no leaks in the exhaust. Now I'm checking the NVA and fuel like. I did have one questionable fuel line connection and changed out the tubing. I've never had a problem with Crap Traps.

Let me be clear that that this was more a question of what to do with this specific engine as conditions change and a more general quesiton: as the weather gets colder (or hotter) do you generally go to a longer pipe? More or less pitch? Different plug? etc. The first run was great. But between the time of the first and second run, the weather cooled off considerably and the humidity picked up. The second run was set the same on the ground, but leaned out quite a bit in flight.