Hello,
I have the same issue with a 1966 Merc110, so just slightly older than your ’69.
Mine clearly runs on one cylinder in forward but “seems” to run fine in reverse. The thing to remember is at high reverse speed there’s a lot of air entering the blades due to the exhaust gasses. In forward gear these gasses exit the propeller but in reverse they wind up caught in the blades. The motor revs well because there’s almost no load with all those air bubbles disrupting the water.
That said, your (and mine) motor is very likely running on just one cylinder when there’s a decent load on the propeller.
My advice is that you check your points and try to get a stronger spark, install new spark plugs, check compression on both cylinders, and otherwise pursue the notion that one of your cylinders isn’t firing well when your motor is under load.
Jim
