I have just completed the construction of a Coverite Gee Bee sporster model D kit. The engine sits inverted in the engine compartment and due to its design is a very tight fit . I tried to source a special muffler to fit in this cramped area but was unable to find one.It was then I decided to make my own.The problem I have now is the engine will run with the homemade muffler fitted but seems to be running very rich and at full power the engine surges very badly.Turning the needle valve almost all the way in makes very little difference.The only thing I can think of is there is too much back pressure from the muffler into the fuel tank causing the engine to receive to high a fuel pressure.Can anyone help please.

I read on another post about someone using a wheel coller on a retract air line to limit the air pressure. Would that work here? Slip the coller over the pressure line from your muffler then, through trial and error, tighten the set screw to gradually close the line. This may restrict the pressure to the tank enough to give you what you need.

Thanks mate thats a great idea . Just waiting till it warms up a bit outside this morning and I will go and try it. I'll let you know.

If your fuel tank is mounted to high above your carb can cause the same troubel.
did you try J'Tec they make a nice flat in cowl muffler they mount with a strap but I just drill holes and mount it to the motor.

I checked the tank level and the outlet of the tank is level with the needle valve.I replaced the carbie today with no change.The engine will accelerate normally up to almost full power then pushing it that little bit extra the rpm dies and then picks up or slowly degrades and stops.At first I thought it was running lean but even richening the mixture makes no difference.I'm going to take the engine out of the airframe and run it in a test bench and see what happens. I haven't seen JTEC mufflers in Australia for some years.Might do some more research and dig one up.

Putting the engine on the bench for testing and troubleshooting is an excellent idea.

My experience is when you can see and get to the engine to observe, make changes and control tank height problems are quickly found.

Bill

I think I may have found the answer to the problem. I reduced the size of the outlet of the muffler by 2.5 mm and now the engine runs to full power without any problems.Looks like it wasn't too much backpressure but not enough. I'll report further after a test flight.

Hi!
You must have missunderstood what the tank level to carb needle should be!
The rule is as follows!
"The center of the tank should be inline with the center of the carb when airplane sits horizontal"...that's the rule one has to obey if one wants to have a good engine behavior.
To say that the tank outlet is level with the needle valve, is a missunderstanding of the above rule...it doesn't matter where the outlet from the tank is as long as you follow the above "tank rule".
Regards!
Jan K