I use the VP-30 on an ASP Redhead 90. I used muffler pressure with no problems until it was pointed out on here that I misunderstood the instructions and was not supposed to use muffler pressure. I put a pin hole in the muffler pressure line and it still works fine. Only thing I can figure is that excessive muffler pressure must not be good for the diaphram. I wouldn't think a pitts muffler would be excessive though.

Be carefull about flooding the engine. If the engine doesn't start right away the fuel pressure will cause the carb to flood. I keep a hemostat handy and clamp the fuel line when this happens. I usually have time to unclamp the line before the engine quits.