I recently started having misfire issues on my 80 series. At first it was a light stumble whilst cruising on the freeway. After some investigation i found that my no.1 spark plug tube was full of oil. So i proceeded to renew all the spark plugs, spark plug gaskets and the cylinder head cover gasket.
After i completed the above my misfire became even worst, i could feel it all the time now, not just on the freeway
I pulled the plugs and found all the plugs to be fine bar the no.3 plug which was completely black, meaning it was running way too rich.
Anyway, some research on ih8mud revealed that this was a fairly common fault with a possible cause being chafing or melting of the insulation on the injector wiring loome. I also have LPG and found that where the wires to the injectors had been cut into for the LPG wiring, the insulation put around the joints was just electrical tape and flexible conduit. Because of the heat in this area it was all extremely brittle when i started inspecting the loome. I subsequently found five injector wires melted together where the loom contacts the EGR pipe. See pics below.
Loome melted against the EGR pipe
Wire melted together
The bottom orange wire is the one that runs to injector no.3 - the one that is stuck on. You'll also notice that there are two other wires showing bare copper, it's possible that more than one injector would open too often.
In my case i think that me working around that area has disturbed the the wires where the insulation had melted and made the short to ground even worst. I know on the 80's if you experience a short to ground on an injector it will open, possibly flooding the cylinder and cause a misfire. In this case the injector would still be open whilst running on LPG and in turn cause a missfire on both fuels.
Have you pulled your plugs to see if any of them are fouled?
Not sure if this is of help, but it would be worth checking over the injector loom for any possible shorts, in particular around the intake manifold and also where the loom runs through the fire wall, i have read that they can be prone to chafing there.
Cheers
Ryan