I have same issue. When I started working on this it would start up and not rev. More I did worse it got. Now I have new points condenser new coil. I got propane coming out of the vaporizer going to the carb. I can see spark is weak. I voltage drop to the positive side of coil get 1 volt so I run. a straight wire to positive side of coil from positive side of battery. No better spark. Voltage drop ground it's all good. Battery is 12.7. It's been. running perfect for a year sense I did a head gasket. New fuel system carb vaporizer and lock off. New alternator and water pump.. Flushed radiator. I noticed lots of back firing then it got weak running. Now nothing. Compression is 125 I read in here coil case has to be grounded properly? Cleaned my spark plugs that didn't look that bad. Good blue spark. Nothing. Going after the carburetor tomorrow I hear the spring loaded slide has to move freely or no fuel goes into the cylinders and it won't start. Turned out to be the vaporizer. Too much propane. Pulled the hose off going into carb and it started. Feed less propane into hose while it's running and it runs perfect.