Hi,

I have isolated the problem and I would like to thank everyone for their help. I'll first answer some of the questions asked, and then explain what I did to troubleshoot the problem.

The fuel I am using contains castor and synthetic oil, but I don't know what percentage. It is the Omega Fuel by Morgan and contains 15% nitro. Again, I have 2 GMS .47's with two different mufflers. The engine with the shiny center section muffler with no fins is the problem engine.

Last night I had sealed the carburettor throat leak with copper RTV. I tried the engine today at the flying field with the different suggested props, but I still was not satisfied with the engine performance. As Ed identified in a previous post, the muffler pressure was very low and you could not see any "bubbles" moving to the fuel tank, this means the tank was not being pressurized.

I suspect this is why the "nose up" test before flight caused the engine to go lean, even with the needle set rich.

I was lucky to have lots of interest at the field and one modeler, Merrick, came up with the idea to insert a baffle in the muffler to create a little back pressure. This was made by cutting a 1.25 (1 1/4) inch disk out of the bottom of a soda pop can, and then putting a small hole in the center for the long muffler screw to go through. The inside of the muffler diameter is 1.5 (1 1/4) inch and the exit hole for the exhaust is .25 (1/4) inch. We felt that the .125 (1/8) inch clearance around the baffle would be sufficient. We only used one baffle, and if you reference the picture below, it was the baffle closest to the front of the motor. (Note: In the final version, I chose to use two baffles).

The engine worked very well after the addition of the baffle, and you could see the "bubbles" in the exhaust pressure line moving towards the tank. All the props that were suggested, including my original prop worked well.

Another modeler, Al, suggested we go to his shop and make more professional aluminium baffles.

The suggestion was to make the baffels with 1/16th aluminum, 1.25 in in diameter, then use wheel collars, compressing "O" rings, on the long screw to hold the baffles in place.

Instead I opted to put the screw in the front of the muffler, slide on a brass tube spacer, a washer, an "O" ring, the first baffle, the next "O" ring, washer, the next brass tube space, washer, "O" ring, baffle, "O" ring, washer, and next brass spacer. The reasoning, I was concerned about the collars coming loose, and thought the brass tube spacers may work better.

A picture of the set up, with center section left off for clarity, is shown below.

Hope this helps others in the future with this enginr/muffler set up.

Again, thanks everyone for your help.

Fly4Fun,

Wayne Miller