If a rebored engine with new piston rings has a 15 pound variation on compression pressure somebody needs to turn in his "machinist" card for one that reads "Mr. Badwrench"! I'm also going to make a guess the valve guides are fitted WAY too tight and your backfire is being caused by intermittent valve sticking. Far too many so-called machinists fit 216/235 valves to "modern" clearances, and that just doesn't work on engines designed in the 1920's. Did you fit the Babbit rods properly, using Prussian Blue and a bearing scraper, and then set the oil clearance with shims?
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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