The 3.1 has a fluid catch chamber in the base, so it is a little taller then kayfun lites or the r91's. They all perform the same, great when set up correct. I want to ask how the air adjustment screw is stripped.... But stuff happens i know. I really do not adjust the air screws on mine. The whistle would be caused from the 510 post not lined up with the air intake hole. Try to adjust the 510 from the bottom, put a paper clip into the air intake hole. GENTLY move the 510 screw till you feel the paper clip sliding in the air intake hole. Ignore the air adjustment screw now.... besides you say it is stripped. I hope that gets you back up with the atty.Good morning crss! Its supposed to be 68° here today. So..... In the process of trying to get rid of the whistle on my R91, I stripped the airflow screw completely and its not coming out. Darn. Oh well, I'm going to order another one! I think I'll go for a kayfun 3.1 ES. I reading reviews and its supposed to be really good.

