1975 Merc 650 running issues

[ShoestringMariner]

I ran my motor a fair bit this week. Runs well to great most of the time. Startup is a bit rough (low throttle nresponse, lean sneezing, sudden dead stops, generally rough running) once warm it usually runs pretty good. This week, top end power felt off. It would run strong, then 95 or 90% then 100% and repeat erratically. Fresh gas, Amsoil syn. Fuel stabilization and a gulf of sea foam. new tank all new lines. I did not touch carbs or fuel pump this year.

Plugs look good good and consistent.

the only thing I found is the timing belt looks sloppy. I get 1/2” deflection in belt tension, maybe more. What’s the norm?

i sense this is an electrical issue. Could this be a trigger issue!7
thanks in advance
Scotty