Ok I have just bought a 1997 Mariner 15m
As far as I can tell it is very clean & runs smooth in a bin of water, the choke knob can be turned to alter the idle speed on this motor, but it makes no difference on mine at all, it seems that on slow idle there is slack in the long thin steel rod that connects the cam driven idle adjuster to the throttle arm, so when turning the idle knob to a faster setting all I achieve is taking up this slack rather than pulling the throttle to a faster speed.
Does anybody have a diagram or pdf manual showing the settings and layout of the mechanism and any adjustments you would be willing to e-mail me please?
I have trawled Google but this is the nearest I can find & I don't trust these downloads as I've been caught before Mercury 15 2t Timing
There is a pic of my motor in my profile album, I believe it's the same model as the Mercury.