I have a super z 60" mower with the 28hp Kohler engine. Gas smelled bad so I decided to drain tanks this spring. Gas did not look good at all so I flushed the tanks out. I do use a gas stabilizer in the fall. I also took off carb disassembled it and clean what I could. I put carb on engine, new gas filter and fresh nonethanol gas in one of the tanks. With the muffler off the engine started fine but I noticed a blue flame coming out of the exhaust manifold. The more I revved the engine the flame became more red. Is this normal? It seems too rich to me or a valve is not seating well enough. Like I said it started easily after not running for months. Last year it had gotten harder to start and would backfire if I did not let the engine idle down before shutting it off. There is no mixture adjust on the carb just the main jet in the bottom of the carb.