so you still are having problems - when you convert your car to gas when hot - you say it jerks and what not - meaning you are having gas and petrol both being fed at the same time or not at all on either.. If I were you - I would isolate the manifold first of any vacuum leaks - (an internally leaking brake booster can cause jerks and hesitation too - resulting in the vacuum signal not reaching the carb for fuel intake)
btw carburettors are not exactly rocket science - and you must look elsewhere to see where your problem lies. When it dies off - converting from gas to petrol or vice versa - remove the brake booster vacuum pipe and see if it comes to life again, also pour a little petrol (2cc) into the throat of the carb to prime the engine - it should start on that amount of fuel - if it stars then dies - and you are sure the carb is full of fuel - check the fuel spray from the accelerator plunger pump. You have a big vacuum leak somewhere causing the engine not to suck from the carb - fix it
If it still does not start - listen to the engine for free spinning when starting - meaning you have lost compression by the hydraulic lifters hanging open (thick oil etc).
To double check fuel flow insert a small piece of clear tube in the fuel feedpipe or a glass pipette like thing - to see that you actually cut off fuel when switching to gas and allow fuel when switching off from gas. One more thing to check is the electric cut off solenoid on the carb - if its acting up you will have no fuel flowing to the bowl when it cuts out.. Seen it fail once or twice too - giving the same stupid symptoms like you mention
