I am writing to see if anyone knows of a common trait with this engine. Sometimes the engine only runs on the two left cylinders. The two right cylinders kick in sometimes for 5 minutes of riding time to 30 seconds of run time. There is fire to the plugs and gas in the carborator bowls when the cylinders are not working. What could be wrong that is o.k sometimes and goes bad other times? The boots on the carbarators are not cracked. What would maybe cause the vacuum from coming and then disappearing? Any ideas?