4dogspete

I'm wondering if I can change this thread just a bit.
I have a problem and would like some assistance. After a bit I'll post my rpm readings, but since I haven't tach'd my 91FX in quite a while, that'll have to wait.
Here's my problem.
I have a Midwest Little Cap 232 that I have my OS 91FX pulling it around. I built it several years ago and it came out a bit heavy - maybe 8.5#.
I started powering it with a TT91, but wasn't happy with it - kept overheating - or it seemed like that was the problem. So I swapped it out for the OS91FX.
Seems like I continue to have overheating problems, or at least it'll pull like crazy for about 5 minutes, then I'm into a dead stick.
I'm currently running Wildcat 18/20 fuel, propped with an APC 14x6, which broke on a dead stick landing - replaced it with a 14 x 7 - another dead stick. Replaced it with a 13x6 MAS and same result.
The engine is cowled in, and has an air dam in front to push incoming air to the head, and has a big outlet area for exit heat. The cowl has a large cutout where I previously had the 4-stroke engine in.
I'm wondering if that open area is just allowing too much air to escape from around the head. Or should I just be using cooler (lower nitro) fuel or maybe a hotter plug?
Any suggestions? Thoughts???
Thx all
Pete