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By "belches" do you mean a very light backfire from the exhaust? My car will sometimes do that when I slow down with my foot off the gas.
Where is the spark lever set when this happens? With the stock head, the spark lever shouldn't be less than about half way down after the engine starts. I set my spark about half to two thirds of the way down for most all my driving. Once on the open road I have the spark about 3/4 of the way down, but never all the way down.
If you have a higher compression head, then you need less spark advance. Just make sure the spark isn't too advanced and causing knocking in the engine.