That's not the first time I've been wrong, but my diagnosis was for a sudden backfire through the carb after it heated up. I don't think you are going to fix the problem by changing the fuel line, but I would do it because that might be your next problem. Same way with the fuel shut off. Trash likes to settle there and you could very well have trash beginning to settle there waiting to cause your next problem. But be careful. The 2" nipple going into the fuel tank is easily broken and then you usually have to take the tank out to get the broken piece out. If a flat spot is all you are dealing with now, I would look at 2 things. Either your your carb has the float level set too low or the accelerator pump isn't working. Just because you have been in the carb doesn't doesn't mean you don't still have carb problems. Anyone who has rebuilt carbs has had times when they had to go into it several times before they got it right. The vacuum advance should have nothing to do with your problem. It advances when vacuum goes up. This is not the case during acceleration. However the centrifugal advance does advance the spark at higher rpm. A quick check to see if it is working at all is by removing the vacuum line to the vacuum advance to disable it. Put a timing light on it. Rev it up and see if it advances. If not, that will definitely cause a flat spot in acceleration. If it doesn't advance, take the breaker plate out and see if you have broken springs or things rusted together. I don't think you can get these parts at NAPA etc. Farmall/International tractors used the same advance and I usually get parts there.
