Is the carb main jet adjustable from the outside, or do you have to swap jets? It sounds like it's running lean in the main power system, and needs a little more gas. To check the centrifugal advance, remove the distributor cap, and see if you can move the rotor a little against spring tension. If the rotor won't move a few degrees with a little effort, the centruifugal advance is probably rusted up and can't advance. Remove the distributor from the engine, and take the plate out of it where the points mount. You should find a couple of weights and springs. This assembly should move freely, and as the weights swing out, the cam that moves the points should rotate a little. Clean it up, check for wear, and repair or replace anything that's worn. Reassemble, and it should be OK. I'll bet it's fuel-related, rather than advance, though!
Jerry
