Well I had popping and bogging down issues upon start up. Installed new spark plugs, no change. I cleaned all the carbs but couldn't find any blockage. I did a preliminary synchronization of the carbs using one of the carb cleaning tool. I noticed then that the rear carb engagement had a delay. A lot of parts were missing when I bought this quad. I realized that a spring goes between the two levers. Now it runs great.