Ragged out idle 90hp 2 stroke

[Eagleseye pop]

Have a 2002 90hp 2stroke merc. Came on boat I just purchased that had been sitting a couple years. Would start but rough and die often and wasn’t charging. went through and changed oil in lower unit ( old looked good) replaced plugs, two new batteries, wires , coils , stator and trigger. Timing is spot on. Set carbs to factory setting (and have tweeked them some) compression 115 all three cylinders . Drained cleaned and added new gas to tank. Boat runs out pretty good but idles very rough! Charges now and will idle but very rough at idle and low forward no wake idle speed. When I come out of the hole it planes out and seems fine…….only thing I know left is carbs or reed valves?.. any ideas on how to troubleshoot before just continuing to replace parts……

 

[Texasmark]

Idle RPM is set to 675 +/- 50 RPM in the water in F gear, boat moving. With spark advance knob on throttle arm going from control cables to stator up under the flywheel that advances the timing, per the serv. manual.

115 compression isn't too bad but the manual says anything below 120 to expect problems. I had that engine for 10+ years and was a good runner.

On the link and sync setting of the linkage between the carbs and stator, I assume you did that per the manual. Seems the low speed carb needles don't need to be adjusted since you can hammer down on the throttle and get on plane easily. If that's not the case, and you have hesitation, open the low speed jets, just a tad (flat blade screwdriver width) at a time and do the "hole shot" till it clears up then reset your idle
RPM.

 

[Eagleseye pop]

I’ve went through the manual and set up cabs and linkage as described. Planning to remove carb cover today and check running to see if I have any blow back ( reed valve possibility?)
Starts quickly and stays running just shakes until you give it a little gas/throttle then sounds pretty good. Pretty convinced it’s a fuel issue and maybe the lower compression is the culprit just don’t know so I keep trying to eliminate any other possibilities. May ultimately just be the way it is but surly a diagnosis can be determined. Starting to head scratch as I’m running outa ideas

 

[Eagleseye pop]

PS…. Could all check out within specs as well

 

[Eagleseye pop]

Coils

 

[racerone]

What does " factory setting " on the carburetors mean to you ?-----To me it is the setting to allow motor to start and keep running.-----It must be fine tuned on the water and in gear.-----Done to correct for local atmospheric conditions and condition of fuel.-----Thermostat in place and working ?---Enricher valve leaking perhaps ?---Check valves ( if equipped ) on recirc system checked ?

 

[Eagleseye pop]

checked linkage to insure they were synced, set adjustment screws all three at 1.5 turns. Started and tweaked 1/8 turn each to get best idle I could gel. Didn’t change miss but keeps running at idle and again when I have in lake and ran it this morning seems to run fine accelerating, planing out and crushing. Just idles like crap when in or out of gear until I accelerate. Doesn’t change the Ben after warmed up. Not familiar on recirculate valves?

 

[Faztbullet]

The 3 cylinder engine are not known for smooth idle and a loose/worn steering cable will aggravate the shaking. I have seen them on aluminum boats cause a harmonic vibration that rattles all the loose items in hulls.

 

[Eagleseye pop]

Yeah I just replaced steering cable and box. It’s not a Normal shake, it’s definitely missing, loading up etc. to irrational to be “normal”
Thanks, every bit of feedback helps!

 

[Faztbullet]

Make sure it has correct plugs, reicirc system is working

 

[Eagleseye pop]

Correct plugs for sure. Not familiar with recirc system on this motor? How would that effect idle only issues
?

 

[Texasmark]

Next time you are out tilt the rear of the engine up maybe 10 degrees while its vibrating and see if it stops vibrating. I don't have any actual deliverable proof that fuel is puddling in the rear of those cylinders, but back in like 1960 time line, Mercury fishing motors were designed with the driveline tilted forward, raising the rear of the crankcase a bit. Merc. as I recall said that it prevented fuel puddling at low RPMs....I think the Mercury Mark 20 was one of those engines....white in color, before Merc. converted to Phantom Black Lacquer.

I had the 90 triple 2002 model for 10 years but I don't remember if I had a vibration problem and don't recall if I tilted that engine.....the wake is pictured in my avatar....so it would run at WOT.....but you can try it.....nothing tried, nothing gained....or lost.

I currently have a 115 2+2 and when running at low RPMs I tilt the rear up slightly and no brag, just fact, the engine runs smoother and when hitting the throttle and getting on plane it comes up faster and smoother. Course the 2+2 is only running on the top 2 cylinders and the fuel mix. in 3 and 4 is just enough to lubricate, not ignite producing power so I can understand pooling. I think that engine was produced for 20 years with the same design. I also think that it was made for things like Great Lakes trolling, offering greater efficiency since those fishermen may troll all day. Just a guess.

 

[racerone]

Part # for check valves is 21-42657A3

 

[Eagleseye pop]

I will definitely try that. Been reading up on that system and it makes sense that if it’s not working it could be puddling fuel causing it to run rich at idle so it’s worth a try for sure . I have an appointment next Monday at a well known local shop to get some assistance. I’ve done everything else I know to do. I am confident it’s fuel related and reed valves are the only other suspect but seems not as likely as carbs all three are drawing in and no spitting or pushing out air so who knows. Because everything else checks out I’m sure it’s something simple as it runs great otherwise and compression is considered good for the engine age. Thanks for all the help, it’s appreciated greatly!