I have used a lot of the .40 and .46 LA series engines, not the .65 very often but the one thing they all had in common was the screws would come loose. Whenever I had a new one the first thing I did was pulled the screws one at a time and put a dab of locktite on the ends and then tightened them back down, let the engine sit over night so the thread locker was dried. Loose screws allow air leaks then create operating problems, I think Mr. Grant has the right of it with a loose back plate. With a new engine the glow plug can also get fowled and create a problem.