I have had the same issues with Webra 50's. I didn't think that it was an air leak at first, especially after I changed all of the lines, resealed the back plate, checked the O-ring at the base of the carb, etc. I found once that it was the O-ring on my low speed needle by removing the carb, sealing it off, and pressurizing the carb (by mouth). Another time I found that the air was leaking between the piston sleeve and the casing (I have yet to fix this one. I now have 2 Webra 50s doing this)