Ok so the theory. These things don't really work that well on the bigger saws. Most times they close up as you are pulling and not when the engine fires. My 660 project was an example of this. I wasn't all that concerned at first because I never use decomps. But after having now had problems with starter components, coupled with the Highway popup piston increasing compression, I wanted to use the decomp more regularly.
But it was just garbage. It would close as soon as I pulled on the starter, and invariably that would cause the handle to be ripped from my hand. I'm sure the timing advance was also contributing!
What this mod does is to give the internal bearings a groove to sit in when the button is open. That groove is just enough to add some resistance to the button closing, of which there was basically none beforehand. So far it has worked very well for me. I did try drilling the holes larger, but this just increased the leakage from the button when closed. In fact I have now started plugging one of the holes (the cheapies have 2) with a pop rivet which gets filed flat. This limits the leakage even more.
I know it seems like a lot of work to go to for a decomp but they are expensive here, AND I just like to find ways of making things work better.
Let me know what solutions you have found to also work.