[Brett]

No idea if it's supposed to have a flat spot.  :-/

But as long as the fit is snug in the slot, I'd leave it alone.
Problems maintaining idle usually indicate dirt in the carb on my outboards.
Check to see if the plugs are showing black and wet.
Indicates too much fuel entering the firing chamber at low rpms.
If dry and tan then fuel starvation is taking place at low rpms.

 

[Brett]

I think your outboards just in need of a tune up.
If you're sure the carbs clean, then I'd be checking my plugs
and checking my carb linkage and throttle synchronization.
Verify idle rpms in water with engine in forward gear.
Not on hose flush, important to set idle rpms to compensate for
back pressure created by water level in exhaust housing.

 

[Brett]

Many of the older carbed 2 strokes didn't have the intake baffle.
I think it was meant to be a venturi noise suppressor.
Kept intake whistle down to a whisper.