Hi,

I have an OS .46 FX engine that has served me flawlessly for many months in a Rascal 40. Recently I bought a Kyosho Pitts S2 and put this engine in the nose with a Bisson Pitts style muffler.

The engine still runs great and provides ample power for this plane, but I have to keep landing every 2-4 minutes to richen the mixture for vertical performance as it has a tendancy to start running lean in the 2-4 minute timeframe. I have never had to do this with this engine before. The fuel tank is well padded and isolated from the airframe, so I don't think it is foaming/bubbling.

Now I am aware that 2 stroke engines need the correct size expansion chamber and exhaust outlet to run at their best, which brings me to the Bisson Pitts muffler. The total exhaust outlet size is roughly 2 times the outlet size on the stock muffler, while having an obviously smaller expansion volume. As mentioned previously the engine has plenty of power.

My thoughts are that the Bisson muffler is not providing enough back pressure to properly pressurize the fuel tank, leading to lean running conditions as the fuel level decreases.

I'm wondering if anyone else has had a similar problem with the Bisson muffler. Any thoughts?

Thanks,

Dave