I've been helping a new club member with an evolution 46 engine in an Avistar trainer. The engine ran -OK- in the summer, but won't run at high RPM in temperatures less than 20 degrees F.

I know it sounds crazy, but what we've discovered is the oil from the muffler appears to be plugging the pressure line.

Symptoms:
1) Engine initially tuned rich--leans itself to max rpm and then quits (in about 10-20 seconds)
2) Fuel quickly pulls back from the carburator as engine quits (indicating possible tank vacuum)
3) Disconnecting the lines from the carb and the muffler and blowing revealed a definite "plug" in the system.
--Blowing into the carb line (back into tank) would produce audible bubbles and an inflate fuel tank--(with fuel squirting out of the fuel (carb) line when mouth is removed)
--Blowing into the pressure line (from the muffler back into the tank) allows no air to pass
4) Blowing hard enough, you can clear the lines again. Nothing is visible except oil in the pressure line from the muffler.
5) The problem gets worse as the ambient temperature gets lower

Things we've checked:
--We've had the fuel tank apart at least three times. Replaced "smaller" ID aluminum tubes with "standard" brass tubing for the fuel and pressure lines going through the tank stopper and into the tank. Eveything else deemed OK by three experienced modelers.
--No obstructions in the tank--blowing hard enough will push the oil through the pressure line and allow smooth flow
--Reconnecting and running the engine redevelops the plugged condition again
--Ran three different fuels with the same problem (2xWildcat 15% from different batches and 1 Omega 15%)
--Had muffler apart--didn't see anything unusual
--Flushed the muffler and pressure & carb lines with fresh fuel

Has anyone else ever experienced this before? Could others with undiagnosed Evolution engine problems (in the cold) check and see if this might be happening?

Any advice is most welcome!

Thanks,
Doug