My EF fairmont is currently running on duel fuel, but when i run it on gas and go to take off the car shudders, kinda like its miss firing but it has no loss of power. Once i change it back to petrol the car runs fine again. It only shudders on gas.
anyone know how to fix this? i have talked to my dad (mechanic) and he seems to think that its running on both fuels. but im not sure.
the car only seems to do it some times though, like ill take it for a drive and will be fine then all of a sudden it will go s**t again and start to shudder etc. the car is an auto so it isnt my driving.

Try checking plug leads, coil packs even wiring to anything to do with your ignition.
Running on gas tends to find little electrical problems that don't show up when running petrol.
I have a van running straight gas and all it took a few weeks back was a loose coil lead that would cause stuttering only when I was trying to take off from a dead stop and every now and then when overtaking.........damn near drove me crazy tracking it down.