Tony,
My 52 dumper ran popped back through the carb and had similar running characteristics until I spent a weekend going through it. Biggest problem was an intake valve hung open, but the timing was way too retarded as well. Also was running lean which didn't help any. Have they checked to make sure the intake manifold gasket isn't leaking?
Mine still runs lean, but it is a carb problem that is easily fixed by leaving the choke on a little till I have a spare afternoon to rip into it again.
