Missing

Did you try it with the alternator belt removed ?

more than likely it does exclude the pump esp if the engine can proced to higher RPMS under load.

for the sake of clarity

does it do it while in nuetral if so with the engine stone cold (assuming it does it then too) pull the belt to the alternator and try again...

I have an alternative idea as to what it might be but the alternator check is easier to proform.

Thanks guys,

I will remove alternator belt tomorrow morning and see what happens.

George

OK, I disconnected the drive belt to the alternator and the miss at 2500 rpm has vanished!

Reconnected and the miss is there.

So, PhillipA might have hit it. A couple of years ago I adapted a 100A Bosch alternator to this 93RR 3.9. Come to think of it, I don't remember any problems prior to the Bosch which happened around the same time I installed Gas.

What now?

George

As I said in the earlier post. Change the position of the wire from the coil to the dizzy, away from the alternator, maybe behind the heat shield.
If you do not want to do that get a length of shielded wire from DSE. They sell some thick enough inner wire in a large figure 8 dual shielded for speakers. The OD is pretty big but split is about 5MM . Jaycar only have smaller sizes.
Earth the shielding at both ends.
That should fix it.
Regards Philip A

Thanks for that info Phillip,

but if its interference from the alternator, would it not affect both petrol and gas?

George

In that case yes it could be interference affecting the ECU.
Maybe you should try a capacitor on the ECU 12v Input. Jaycar sell big ones for the DOOF DOOF boys.

BUT interference with the ignition signal is by far the most common.
Maybe a bit of both.
Something like this will be trial and error. Nobody can give you a magic bullet.You will just have to try a number of things. I suggest the above as a starting point.
Regards Philip A

Just another thought. The ECU gets the RPM signal from the coil. With gas this does not happen.

So it is conceivable that hash from the alternator is affecting the RPM signal to the ECU.
So try the shielding first.
Regards Philip A