L20b dying at random and detonating hard core

[Aulptraum]

I drive a 78' 620 and it likes to stall out at what seems like completely random, maybe with a slight correlation to stepping off the gas. I have to stay on the gas when I start it to keep it running, it'll usually die five or six times before It'll stay running for a while. It'll be chugging along and then just lose power, catch it back for a second, and either die, sputter a little and keep running, or give off a gnarly backfire that'll turn heads from a block away. Like I said, it seems to be more fitful when I let off the gas. I really don't know a lot about engines in general and even less about the L20, but I've done a little research and basically what I've brought out of it was that it may be a fuel filter (sediment in the lines) problem, cause the truck was sitting for a few months, maybe a vaccuum advance problem, or advanced timing? It seems to do better after it's warmed up a little bit, or if I drive it a little tame, which is what made me think vaccuum advance. But I don't know if that says anything. Should I just drive higher octane and call that good or what? I don't have a timing light at all and I don't know anyone who does, and even if I did I'm really not sure what exactly the problem is, so give me some pointers guys! Before I blow a piston ring or something!

[datzenmike]

Replace the fuel filter. This is cheap and easy and should ne done every 2 years... I do it every year.

 

Take the top of the air filter off on a dead cold motor and take a look. The choke plate should be part open.... pump the gas by hand and the choke plate should snap closed. Does it do this???? If not the choke needs to be adjusted. No choke on a cold motor and it can be almost undriveable. Add a partly clogged filter and you can get what you have described.

 

Your timing and vacuum advance won't change from sitting. ou could chech and set the timing though, this never hurts. 12 degrees advance before TDC

 

Higher octane is NOT required on your motor and even if it did help, is not fixing the root cause of the problem.