1995 90hp Mercury 2-stroke rough idle

[lucasvdv]

Hi all,

My 1995 90hp Mercury 3 cilinder has a rough idle. It runs really good at higher speeds but not at idle.
I therefore am trying to adjust the idle mixture screws to a proper setting. My service manual instructs me to initially set these screws at the following setting. Top carb 1/2 turn out, bottom carb 1-1/4 turn out and bottom carb also 1-1/4 turn out. The motor will start at this setting but with these initial setting. The manual says that you have to screw in the screws until the outboard stalls/surges and then back out 1/4 of a turn. But if the top screw is only a 1/2 turn out there isn’t much room for adjustment. So my question is, should I leave the top carburetor at the 1/2 turn out and adjust the other two, or maybe start with a different initial setting?

All advice is welcome, because this is the first carburetor idle adjustment for me.
Also I included some snapshots of my service manual.

Thanks in advance!

 

[Texasmark]

From OEM Merc. manual 90-8M0110566 Dated November 2015......(note this a revision of the earlier manuals and has some differences...I know I had one with my 90 when I had it), covering 1994 thru about 2007 year models...upper end only lists by serial number but it's about 2007 models:

Carburetor, Specifications section, Page 1A-5, initial setting for all yearmodel, Model 65 with jet propulsion (rated 65, not 90) thru 90 HP engines, from a soft seating....use 2 fingers to gently screw it all the way in....back out 1 1/4 turn +/- 1/4 turn all 3 carbs identical.

Idle RPM in the water, in F gear, boat moving is 675 +/-25 RPM....set by adjusting the timing on the knurled knob on the spark advance lever....timing sets idle RPMs.

Adjustments: Page 3B-25: In the water in F gear, after a few minutes warmup, with engine vertical, (you can start with the top one and go down the line 123) turn in the mixture screw till the engine responds and back out till it smoothes out...a little rich preferred...per the manual. Idle is reset after adjusting for specified value as stated.

My addition: With all 3 adjusted, get out in open water, flat preferred and 2 people, with engine still vertical. Firewall the throttle and the boat should positively respond with no stuttering or stammering. If not adjust outward, like a 1/16 of a turn, same for each carb, till it will take throttle coming out of the "hole". Done deal.

The writers of the manual, I am sure, expect the engine to be in satisfactory running order and all the other things that could cause a poor hole shot, wrong prop, improperly loaded, dirty hull, inadequate HP for the load to be pushed and all and anything else that would cause a problem not carb related.

HTH.