To me in my experience with these damn things, it sounds like it may be over fuelling.
How did you test the ECU? This would be the second different thing in the whole system when swapping between the two fuels. They develop dry solder joints on the injector firing circuit, causing resistance and then slowly over time it just ends up opening the injectors wider and wider to the point you don't even need a fuel pump. If you have it apart, I'd check for these, get the ECU warm and visually check as well as with a multimeter.
Another silly culprit of issues seems to be the ECU earth point at the back of the LH head. There are two, one bunch from the ECU and another that grounds from there to the chassis.
Have you checked timing with vac advance connected and watched the timing move as you give it a rev? This could help point in the right direction.
For some reason I remember the CO% only supposed to be 0.5 or 1, it can be adjusted at the flapper under a red plug.
Have you adjusted the throttle sensor to within spec? And have you watched it over the range for dead spots?
I had an issue very similar to this over the course of 2018 with my 89, that in the end, after all the hair tearing, crying, yelling and parts cannon firing, ended up being the ECU. Two in fact! The original developed the issue over time and then the one that someone on here gave me ended up going that way while I was still testing things, but gave another set of similar problems.
I would also not be concentrating on the mechanical side, or any system that is common to both fuels. It clearly works on gas and properly. My bet would be the electronic side for the fuel system.
'15 Discovery 4 HSE- The family bus and the kids like it!
'89 RRC- My favorite of the bunch!
Ex '03 Commodore 'S' ute- 450hp of uncracked 5.7lt and 6 speed manual uteness - Still crying that its gone
Ex '06 GLXR Triton- *Gone and forgotten*
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