bit of a mix-and-match system on this one.
Italian tanks, OMVL90E vap, bigas switchover unit and CO.RE.GAS closed loop controller and, I assume, matching stepper motor.
The petrol system is not closed loop. It's had a single O2 sensor fitted on the exhaust. I tracked down some instructions for tuning the closed loop thing, and the diagnostic lights appear to behave as they should - richen the basic adjustement and it goes red, lean it down and it goes green, get it in the middle and it switches from red to green, which is what the destructions say it should do.
Gas system is fine up to about 3000 rpm on more or less full boot, then it shows signs increasing signs of not-enough-gas - exactly like they do when the gas is running low Back off and it goes back to running right. Below that it pulls well enough, more or less indistinguishable from petrol, above
3000 on petrol it continues to pull, but on gas it doesn't.
I'm going to look for obvious faults in a bit, but I'd welcome thoughts about what might cause it.
Had similar sypmtoms once before from a dodgy mixer - the inner part of which was misaligned, making the gas feed into the mixer have a permament restriction. That's one thing I'll be checking...