Serial OA969038

Engine has been in storage a long time, just bought it used a couple months ago.

I've replaced the water pump impeller, rebuilt the fuel pump, replaced all fuel lines and filter, rebuilt all three carbs - including new jets, new floats, new needle/seats, and replaced the plugs with NGKs
(same number as the ones that were in it when I got it - which still had the black paint overspray on them from the factory)

The engine starts easy and runs well in neutral, not velvet smooth at all rpms, but I've not even adjusted the idle needles yet (2 turns out from bottom to start).

In gear - it bogs, stumbles, surges, and just won't GO, it won't stall, but it just won't accelerate.

I have a feeling it's ignition system related - but I'm not sure how to diagnose it. Plugs have been replaced more than once before rebuilding the carbs (found one cracked float and had some debris behind the seat on one carb)

Overall the engine looks new under the cowling, wiring is in good shape, no burn marks evident in the coil packs, etc. Spark plug wires are in excellent shape.

If this was a car (which I'm much more familiar with) - I'd be thinking the mechanical advance mechanism inside the distributor was stuck, keeping the advance at the idle spec....

Pleas enlighten this car guy on the ways of the merc tower of power 2 stroke outboard ignition advance system!