I have a 1977 Datsun 620 w/ a l20b and it is lacking power and doesnt accelerate like it should. It has always stumbled when give it gas, and recently I had an issue that required that the carburetor be replaced. It was the first time I had an issue where the truck didnt run and it had been sitting about 20 years before I bought it. The secondary on the carburetor was corroded shut and I was not able to fix it. I bought a weber conversion kit for it and the fuel system is not the cause for the current problem. I have also checked the spark plugs and the issue isn't there either. My dad thinks the problem might be that the timing is incorrect, could that be what is causing the problem?

it could be timing... try messing with it a bit on the distributor to see if that helps... but the big thing is making sure ALL of you vacuum lines are fresh... or even removed and capped off

I was checking everything over again and when I started the truck to see if I could get it to act right it started smoking and it had never done that before and a checked the plugs and found oil on the #3 cylinder and did a compression test and found that the problem is the piston rings have gone out. I am going to do one of the following options, put in a re-ring kit, do a complete rebuild, or put in a different engine. If you have any suggestions please post them.

well... its all relative for what you want to do, budget... power needs etc.

if you do it... do a full rebuild

if you want to swap... there's the KA or SR which are both decently reliable

the KA is easy as you can pull one out of an a D21 hardbody

the SR is a little harder because it will be out of a car