Small Engine Troubleshooting Problems? Ask a Mechanic Now.
Here is link to manual, showing the linkage. You say plug is showing black carbon? Too much fuel or too little air. Or the exhaust can be restricted. Check the exhaust system!
Remove the exhaust and make a test run, with exhaust off! I have to assume your choke is wide open. I really doubt it is the governor linkage. Your engine should be (late) model.
Choke flap would be open and not restricting air flow into the throat of the carb. Do you have choke cable or just lever? Choke being partially closed, restrict air, and will make mixture rich.
Test run with exhaust off, would show if exhaust is causing the low power.
If choke is not fully open, adjust the cable or lever till it is wide open
Just look at the choke while manually working it, it should go to the fully closed when pulled, fully open position, when released.
Is it fouling the spark plug? Will it run any better if you try to choke it when it dies down? Is fule ap open, and venting the fuel tank? Not causing the tank to draw a vacuum?
Fuel filler Cap! I thought it had a choke cable, sorry. Everything point to a lack of fuel as you drive .If I had it , I would try a pulse fuel pump in the line from tank to carburetor. Which is powered by the pulses of the piston moving in the crankcase. You would have to add a line from the engine to power it. From crankcase or rocker cover.