Re: Help Please on Lightwin Carb issue?

[Don]

OK, well a water leak is surely a possibility, considering one broken exhaust cover screw, depending on which screw is broken. The outer screws would cause an external water/exhaust leak, but having one of the inner screws broken might let water leak from the cooling passage into the exhaust cavity. Water leaking into a cylinder(s) would surely cause an RPM variation though. There may not be enough water to drop a cylinder completely like you might expect though. You might try running the engine at WOT for several minutes, then shut it down quickly. Pull the plugs and look for water droplets. These exhaust covers get pitted from salt water/exhaust as well, which might create a path for water leakage.
The good news here is that you say the engine idles well, is that correct? An engine that idles well usually has a powerhead that is in good shape, with no crankcase leaks, which makes my intake gasket suggestion a waste of time.
A restricted/plugged up fuel supply would surely cause the engine to hunt at high speeds though. I can’t remember if your engine has a copper or rubber fuel line. It would be fairly simple to make an alternate fuel supply if your engine has a rubber fuel line. Just rig up another fuel line connected to a little lawnmower tank (using premix not straight fuel from the lawnmower!). These engines are known for having plugged up filters, fuel valves, fuel cap air vents, and crud in the metal fuel lines, so it is worth ruling all this stuff out. You mention having other lightwins, you might just use a tank/valve/line/cap from one of your other engines instead of the lawnmower tank rig up I mentioned.