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Your float gap should be 9/32 " measured from the bowl gasket to the nearest edge of the float. Measure the float gap ~1/2 way down the side of the float, not up by the needle valve pivot. It's critical for a good running engine, you should check it. Sounds like you should consider pulling your gas tank and cleaning it out real good, and checking the shut-off valve, fuel line & elbow strainer in the carb. inlet for restrictions. It doesn't sound like your fuel flow is up to par. The engine idle is 400-500 rpm. The idle mix screw only affects running in that range, and with the throttle closed. When you get much above that, the mixture is being handled by the power mix screw. Use a portable tach to check your RPMs if possible. Adjust the idle stop screw (thats the stop for the throttle shaft, not the mix screw) first to get your idle speed in range, then adjust the idle mix screw for best(fastest) running. Might have to adjust idle speed & mix back & forth a couple times to get them both lined out.
Your mix adjust screws are set right. I recommend leaving them alone until you check your float gap & make sure your sand problem is flushed out.
Your poor running, while probably related to fuel delivery, don't sound that much different from poor spark, or a timing problem. Have you checked those? I have installed one of those new manifolds and agree they are darn difficult to tighten down a few of those nuts! You can check for gasket leaks by squirting some WD 40 around the gasket while running and listening for a temporary change in engine speed. Sorry for the long email, long distance troubleshooting is tough. George
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