Looks good but is slow...

I'm pretty sure your engine is flooding, you say you get a some improvement at wide open throddle. Running a flooding engine at wot will clear the extra fuel unflooding the engine. The real sign that you have a flooding condition is that it clears up completly and runs great as its running out of fuel, once the fuel quits running in to the carb its left with just the fuel in the bowl. With no fuel entering the carb it can't flood itself out and runs like it should. Check the neddle an seat, float adjustment, also check for a leeking float. Turning the fuel off while the engine is running is how I test for a flooding condition, If the engine smooth's out and the rpm's pick up I know my engine is being flooded. Don't forget each time you let it run out of fuel it's also running out of oil!

Check the float height. You may want to try bending the little tabs in the floatbowl down a few mm. This will not allow the float bowl to fill quite as much and it might lean it out a little for you.

sounds like it's running rich. closing the petcock leans it out as it burns the fuel. where's your needle-clip set? try moving the clip up a notch and see what happens. if it goes faster, check your plug. if the plug is still black, oily, or dark brown, go up another notch. you want the plug cocoa colored. any lighter and you're too lean.

oh, and i forgot to tell you to check your plug before you move the clip. if it's already light tan, then you got a slow motor, and we'll have to figure something else out.

and, what mixture are you running? you should be at 32:1 after that many miles. if you're running it with more oil than that, it's too rich.