90 Horse merc with issues

[MKHC130]

I'm having issues. When i first start and run the motor it runs fine, but after about a hour or so trolling and go to another spot it will only get 2000 RPM. I checked the plugs and they look fouled, but not really sure. Clean the plugs and run the motor and it runs fine until i troll for about an hour and the same occurs. I've changed the plugs, filter and tried carb cleaner, but it still happens. Any assistance will be great.

 

[JKP]

Re: 90 Horse merc with issues

I have a 96 Merc 90HP....serno 0G255002.
Had similar issues....would start up just fine, and initially after start-up, I could get the engine to develop pretty near full power,....then after just a few minutes, the engine would start to lose rpm, and would only run at low rpm....2-3 K. I've had a complete engine tune-up, including carb rebuild (all 3 carbs)....engine would run well out of the water at high rpms, but under load, still would not perform. On further troubleshooting we have found that the stator is putting out anomalous power readings, (there was a Merc technical bulletin out on this problem back in 98 I think to replace the stator assembly with an improved one) but my engine still had the original stator. Also found that one of the magnets on the flywheel had cracked and was loose. So I have now had the stator replaced, and I am on the hunt for a flywheel.
Hope this helps.

 

[MKHC130]

Re: 90 Horse merc with issues

Thanks for the info. I will try anything to get this fixed. I've tried disconnecting the spark plug while it was running to see if one of the cylinders was bad, but nothing.

 

[CharlieB]

Re: 90 Horse merc with issues

Rule # 1

NEVER , Never, EVER, start replacing electrical parts without PROVING it bad.

They aren't cheap.

And there is NO RETURN on electrical parts. Period.



Rule # 2

Clean your carburators.

Set the float height as shown in your FACTORY SERVICE MANUAL that you have ordered.

The majority of outboard motor problems are caused by a lack of use, the carbs gum up and either stick a float, flood, or restrict a passage and won't flow enough fuel.

NEVER ASSUME that your problem is anything and start replacing parts.

 

[MKHC130]

Re: 90 Horse merc with issues

Thanks for the help. I have all intentions of cleaning the carbs this weekend to see if that helps. I have a question though, after the first start of the day it runs fine, i can get 4000 RPM out of it, but after i troll for an hour or so it won't go over 2000. Wouldn't the carbs dirty cause this all the time. I don't know that's why i'm here asking all the questions hoping to get some fresh ideas.

 

[CharlieB]

Re: 90 Horse merc with issues

It is entirely possible that you may have two, even three, separate problems with your motor.

First, idle mixture may be too rich, allowing extended idling to foul you spark plugs. Initial idle mixturfe screws are set at 1 and 1/2 turns out from 'lightly' seated. Final adjustment is made running in forward gear while on-the-water, the motor must be under load and not running on the hose or the idle mixture will be too lean. Slowly turn each mixture screw out to achieve the best idle speed from the motor. give a minute after each movement for any excess fuel to be burned off and the motor time to 'normalize'. After setting 'best idle' in gear, then further open each screw just the width of the screwdriver blade slot on the screw for even more fuel for accelerating the motor. Test acceleration by rapidly opening the throttle to WOT in gear, the motor should accelerate smartly. If there is ANY hesitation, bog, or lean pop/cough, then further open the mixture screws another blade width. Repeat as needed to perfection.


Second, stator voltage can limit WOT RPM. Test stator voltage at 2500 RPM to confirm good/bad voltage from the high speed windings.


Third, WOT RPM should be in the area of 4800 to 5600, preferably near the higher side. However, before condemning your prop pitch you should ensure that engine compression is still acceptable, carbs clean and adjusted, and stator voltage is adequate, as any of these can limit engine power.

 

[MKHC130]

Re: 90 Horse merc with issues

Sorry it took so long to thank you for your inputs, btu we were at the lake this past couple of weeks. Thanks for the input. I cleaned the carbs and adjusted idle on the carb but that did not help. I am almost at the end of my rope. I have someone checking the compression this week, so i will know it that is a problem later this week.