You might not believe this,but on my Minarelli that did a similar thing,it was the points set too wide,.024",AND it wasn't getting enough air.Seems like she'd go up to about 32mph and then die back to 27 or so.NOW I remember that I fixed the main problem with the points setting and it ran 100% better,and then I found out it needed a couple more holes in the airbox.On the other hand,I just added power today to my Motomarina with Franco-Morini engine by filing and opening the points to .018" Also,is your exhaust clear?