I'll have to disagree with bvmjethead. My BVM 91 came new with the carb insert already installed. The engine instructions said to remove it if you weren't going to use puffer pressure.

You will find that most DF fliers will swear by their system of choice. I prefer pipe pressure and have had that on all of my DF's. I have never experienced the problems during aerobatic maneuvers that some attribute to pipe pressure.

One plane that I bought already built had fan pressure. What I didn't like about it was it's narrow range of authority. I would have to turn the radio knob to max. lean, then adjust the needle to peak the engine. Once in the air I would end up turning the knob to max. rich and on some days it was not enough to get a good contrail. I ended up converting it to pipe pressure. With pipe pressure once it is set up I never have to touch the needle. It has sufficient range of authority to go from too lean to too rich using the radio knob. Once the engine is started I just have to peak it with the knob and take off.

Puffer pressure probably has slightly better range of authority than fan pressure. The reason I never tried it is because I preferred to stick with what worked well for me and I didn't want the few hundred rpm penalty due to the restrictor in the carburetor.

Joe