Ignition malfunctioning

I have an old Yamaha 175 three wheeler - it has been rebuilt and was running fine. It recently began to yank the pull start from my hand when trying to start it. It acts like it is trying to fire the plug during the pull start motion, which it never used to do. I checked the coils and the resistance was within spec per the manual and a swap of CDI pack provided the same response. The only thing I could not get to read right was the coil, the resistance across that was not what was expected. The keyway has not slipped and the timimg seems about right.
The plug fires when tested and I don't believe I have internal problem like piston hole - no compression test yet but it has compression.
Anyone have any idea's or ever had this type of problem.
I don't want to just start buying parts and change things out.
Thanks