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Do a couple things. First on top of the float caps is a brass air jet. Remove the jets. Don't put anything in their place. This richens the mixture at midrange which cools the engine. See if this doesn't cure the problem.
If you have a IR non contact thermometer, check the temperature at the top of the block. Does your engine have the tell tale fitting on top of the block?
When it goes to accelerate and falls flat, look into the carbs, are they dry or wet?
Try a quick application of the primer then accelerate. But remove and try the engine without the jets first.
Let me know how it does.