Vacuum Leak,

[oldtimer]

This sounds like I may be able to help. Let me get it right. On LPG all is OK. On fuel it won't idle properly without hitting the throttle continually. OK if you followed the good advice on how to check for vac leaks & found none, then you have none. So the problem is elswhere. You are right in thinking that carby may be at fault. It appears that you may be running lean. To check this you will need access to the carby throat. Get a little petrol in a seringe and squirt it down the primary throat of the carb or if you prefer disconnect the accelerator pump linkage & manually operate it to squirt fuel in, providing the accelerator pump actually works. Either or, if the engine speeds up momentarily the engine is running lean. There are several caused but the most common 2 are, idle solenoid is malfunctioning or there is a blockage in the idle circuit. How to tell which one? The idle solenoid first. When the ignition is switched on, the solenoid will click. If it doesn't check that power is getting to the solenoid. If there is power, the solenoid is cactus, if no power then trace the electrical fault. If there is power, remove the solenoid and cut or remove the plunger. I can't remember the actual design of how the valve works. It may be a straight plunger which can just be removed and the solenoid refitted or it could have the plunger inside what appears to be a jet. If this is the case twist out the jet & remove the plunger if possible or cut the stem of the plunger. refit the jet & re-install the solenoid. Start engine & see the result. Test the solenoid first once you remove it from the carb before you go snipping it. The purpose of the solenoid is to cut fuel to the idle circuit to avoid engine run-on when switched off. Now if the solenoid proves to be all OK, try removing the mixture screw and giving the idle circuit a shot of compressed air. This probably should be done in conjunction with the idle solenoid removed and hitting the circuit with air through the solenoid hole and the mixture screw hole. Refit everything and adjust the mixture screw for best idle. I have not mentioned a third option yet and that is since it is duel fuel the carby tends to dry out from lack of fuel usage. This can cause gaskets to shrink and form an internal vac leak in the carb. Always run on normal fuel regularly to avoid this. Hope I have helped.
Cheers
Mike