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Have a 52 m37 that ran fine . With no blue smoke ,but idled rough . changed carb from VPW, now truck truck runs great , But now have blue smoke. any idea why THANKS.

Running rich maybe?

Blue smoke is oil. Rich would be black smoke. Start up smoke or warmed up at idle smoke? Had you got it up to operation temp. prior to the swap?

The sensible thing to do is run a compression test. Blue smoke is oil & most likely is happening because of piston ring/cylinder wall wear. A compression test will give solid answers. You will then know what is going on & whether you need to look at an overhaul to solve the problem or whether you have miles left to go. Post your PSI reading of each cylinder here after the test, will be glad to help you out. A general rule is this: 110+ readings with no more than a 5lb variation from 1 cylinder to another is good. 100 PSI isn't great, but should have some miles left to go. 90PSI is border line run, not run, can cause skipping of 1 or more cylinders. Anything below 90, start planning a rebuild, the party is over. Many say they have good running engines at 70 PSI, but you may as well know up front simply put, that ain't happening. I don't know if people who say that are in denial or what, but I do know what compression readings are on a good performing engine.