If the bypass is introducing bubbles in the fuel line, there must be a leak. The bypass is simply a way for the excess fuel to return to the main tank, and is pressurised one way, to the tank not from it, therefore the air must be entering somewhere. Did you make the bypass like in Kamps book? I used a piece of rubber from an inner tube first, NO GOOD! It deteriorated and failed quickly with contact with the kerosene. I later used a piece of cotton backed rubber of some type that I cut from a glove used to handle chemicals/fuel and that seemes to hold up for many many runs. One thing about a mechanical (spring) pressure limiter, once you set the spring pressure and lock the adjustment screw, there is no way for it to go off adjustment, or fail, or what have you.

AJC

PS what is the difference with the Fadec ECU I received with my 54' kit (from JDE back in APril 2002) and the current ECO unit , besides size I mean? Are they the same functions?