I'm having fuel delivery problems with my 85 hp J-4A. It allows only the cowl tank for takeoff, but has a Marvel Schebler carb to tolerate the low fuel head in accordance with the C-85 TCDS. I'm certain it is starving out when it stumbles badly (fades out for a couple of seconds - at least) shortly after liftoff with a half full tank after a few T&Gs, running on 2/3 MoGas and 1/3 100LL.

Might it be fuel line percolation/vapor lock? or is it due to the initial acceleration of the aircraft where the tank is behind the engine? Some have even said all Pipers do that (!).

I'm certain it isn't contamination, the fuel selector valve detent system, or the fuel venting, or even a partially blocked line. It seems to happen after everything warms up thoroughly and after a taxi-back. (I still am having high cylinder head temp (400 degF max) although the oil temp has come down to about 180 degF.)

There have been three such aborted takeoffs. Fortunately all have been on a long enough runway to set it back down. The cowl tank is about half full, and the aircraft is at gross weight. The fuel line has a 45 deg el coming out of the gascolator and a 90 deg el going into the carb. The fuel line system is only 3/8" nominally. Doing a full flow test, I get about 15 gph with an open line at the carb inlet level.

Next is to instrument the carb fuel inlet with a thermistor to see if there is a heat soak/hot fuel issue at the carb fuel inlet. From that, possibly adding insulation to all of the fuel system that gets exposed to under cowl temperatures.

Surely others have had a similar problem. Maybe some of you have done some instrumentation to find out what's goes on. I'd like to hear any similar experiences, and any observations on the effects of under cowl temperatures.