20mph! No way. I hit 18 mph on mine down a slight hill and I felt it was going to fall apart. The other one I have with the wide open motor hits a solid 12 on flats. 20mph is a lot for a solex. If your looking for more speed get that one running and sell it. Buy a Puch or TOMos.
If it is not staying running, the carb could be clogged with its tiny passages or the fuel pump might need a new diaphragm. If those are clean it my need the points cleaned and re-gapped and new condenser.
But if it is running pull out the old phone and use a gps app. You should be between 12-14 mph or slightly higher of your lucky.