[hiccup]

Gasoline engines generate much more CO when they are running rich than when the air/fuel is dialed in. I would suspect something is up with your engine(s) that is causing it to run rich.

With that said, the fact you were working on the exhaust system this offseason is a red flag as well. I don't understand the logic that because you had it all disassembled that it can't be the problem.