I have a 92 mach 1 w/ a 617 type 643. My problem is that I foul the left plug out every twenty miles. I have cleaned both carbs which were very clean. I took the machine to our local dealer and they resunk the carbs after I did it w/ my vacuum guages (they were near perfect). I have 135 lbs in both cylinders. I had to install a new coil since the previous one was not within spec. If anyone can help I would greatly appreciate it.