86 70hp merc running rough under load

[jv123]

I have a 70hp 1986 merc. that is acting as if it is starving for gas when in the water and in gear. In nuetral at all RPM's it runs fine. Compression is good I changed the plugs, gas, hose and bulb, and flushed both carbs with carb. cleaner still no change. This motor has 3Cyl. but only 2 Carbs. which seems odd. How do they function? Someone suggested adjusting the float needles with a nut on the bottom of the bowls. How?

I would greatly appreciate any helpful advise I'm at ends with this thing.

Thanks