my engine (petrol/lpg) was running just fine a couple of days ago, i left it ticking over on the drive but when i got back it had stopped and wouldnt start. i think it had run out of petrol, put a couple of gallons in but it will not start. it tries by firing on a couple of cylinders but will not fire up. i have taken the fuel feed pipe of and checked that plenty of fuel is coming in, any ideas please.