The 240/275 doesn't use a horn. Bill Hempel ran a 240 forever on the Bolly 36X14 and got around 5200 if I remember correctly. More than likely you have a air leak or something else going on with the carb. I would send it to Gerhard at Aircraft International, he figured out why my 200 wouldn't run, the crank hub had slipped, so there is something else to look for, retarded timing.

Tom