Re: Help Please on Lightwin Carb issue?

[Don]

Did the engine ever run well for you? Or, has it run this way ever since you have owned it. The compression seems OK, any signs of an internal water leak? You say that it ran pretty well with a different carb, why not just run it with that carb instead of getting another carb somewhere else?
The issue of RPM fluctuating up and down is peculiar, usually a messed up carb will just cause a lean/rich condition, not surging. The loose throttle shaft was a good thought though. The only other thing worth trying would be to set up an alternate fuel supply to eliminate the tank/filter/line/valve/tank vent as a cause.
Does it look like the intake manifold has been off before? If so, perhaps the gaskets are mixed up. Installing the outer gasket behind the reed plate will create a crankcase leak between the cylinders.
I can’t quite remember just where the bowl vent is but you can figure that out fairly easily. Remove the carb bowl, invert the carb body, then slowly inject gas/carb cleaner into the upper carb body cavity, the vent will make itself known quickly. Someone may have inadvertently plugged the carb vent with a lead shot/screw. Make sure there is nothing in the intake silencer or screen that might be blocking the vent hole as well.