1995 90hp mercury (3cyl.) backfires at idle

[enginepower]

Only had this boat for 2 months now and already had to replace the switch box due to loss of spark on top cylinder. Overall, it runs great! Good acceleration and top end but the idle is weird. Mainly, when I put it back to neutral at idle, it will idle up some as normally then sound like it fired at wrong time and make a "pop" and idle down some then come back up and do it a couple of times. Has never died except when I first started it and it popped then died. DVA readings for all trigger, stator, and switch box to coil wires are in specs. Possible crack in a flywheel magnet maybe?

 

[enginepower]

bump....anybody hear of this before?

 

[racerone]

Open up low speed mixture needles 1/4 turn and see what happens.

 

[enginepower]

I guess I can give that a shot. Another thing I noticed is when in gear at idle, it will be running smoothly then get real rough and vibrate enough to where I think I hear the splines on the drive shaft rattling then it will quit then do it again. Seems to do this more when it warms up so maybe I need to heat it up and take some DVA readings. One of my trigger circuits ohms almost 300 lower than the other two. I'm thinking the trigger may have a partial internal short and gets worse when heated.