After replacing the pump diaphragm due to a small hole in it the engine suffered from a flat spot and hesitation at about 4000rpm. Turning the pump in about 1/4 turn seemed to remove that problem but now the engine surges at around 8000 rpm and refuses to rev any higher. 17x12, cut down 17x12 (about 1/8 off the tips) and even an 18x11 carbon prop that revs to 8600 on other engines still peak at around 8000 and surge. The revs we can live with but not the surging.

The main needle doesn't appear to make any difference, turning in or out just slows the engine so that seems to about right. It's currently around 1 and 3/4 turns out, the same as the other engines.

Is this a consequence of fitting a new diaphragm and if so does anyone know the cure? Does it have to be "run in" to soften it up a bit? Is there a trick to installing it that we don't know about or is there another fault turned up at exactly the same time? The pump has been stripped and cleaned carefully again but no improvement.

The first comp is getting very close so panic is setting in!

Anyone come across this and found the solution?

Mel