Hal,

The problem may well be that the engine is mounted too close to the jetpipe. The original jetpipe instructions show a 10mm gap, but this is a minimum and it will work fine with 20-30mm between the end of the exhaust and the bellmouth of the jetpipe.

If you have further problems, please email Wren directly, or phone and talk to one of the guys here.

Sara Parish
Wren Turbines Ltd