I just turned the muffler pressure line into the vent line and routed it to a brass tube out the bottom of the cowl. you should'nt have a vent to begin with on the 75AX. If you look on the pump there is a little vent hole on the black part of the pump that needs to face down when installed, the instructions with the pump spell this out, you'll see when you get one.

Most people have plugged one outlet of the muffler to create more pressure before they tried the pump. This is usually the first remedy for losing pressure with a pitt's.

Sounds like your tuning method is real close, the idea is to just get the LS barely rich enough to idle as the pump is forcing so much fuel thru the carb. The LS needle controls over 75% of the throttle curve, it is more important than the HS needle for a nice running engine.

Run the needle in until the engine starts to slow down to record your top RPM, immediately richen when you hit that point. with a 17X4 your going to get up there pretty high. When you done turn the needle in to figure out how many turns out you are from closed, just for reference. Being from California, you should'nt have much change on the needle thru the seasons as we do out here in the boonies.

The LS needle setting was the number of turns in from the normal operating position without the pump, not from closed. Your's will vary due to fuel, temperature,altitude and prop.

My HS needle will be anywhere from 7/8th to 1-1/4 turn open with the pump, so much fuel is being sent in the needle needs to be closed that much. I also just centered the adjustment screw on the pump. When your done you should have a nice strong smoke trail from the engine during flight, much more than normal without the pump.

The AX line has had some people complaining about bearing noise which might be your raspy noise you talk about, I had the same noise on mine but after about 10 tanks with the pump it went away. I checked the bearings and they looked fine. There's been alot of discussion over in the glow engine section about this, no one really has an answer, I would'nt worry about it, just check the bearings when you have the back plate off and if there rusted give RC bearings a call for a new set. $10 You might need a set in the salt laden air out there.