'89 100hp Merc 4cyl oil injection-runs on 2 cylinders

beckoning

Petty Officer 2nd Class
Joined
Dec 15, 2010
Messages
160
This motor is a wonderful, wonderful motor that has given me years and years of superb service. Now, she sputters and refuses to run on 4 cylinders.

She starts and will keep running but at 2000rpm, but at 2000rpm+, cylinders #3 and #4 don't kick in as they should. She runs very rough. She always runs somewhat rough on 2 cylinders below 2000rpm, but smooth as glass when all 4 kick in. In gear, she will run up to 2000rpm, but instead of going to 4cyl operation above 2000rpm, she continues running on 2-3 cylinders. In gear, she will run up to 3000rpm on 2-3 cylinders at full throttle, then after 10 minutes will start to sputter. Once in a great while, all 4 cylinders will kick in for a few seconds, then it's back to 2-3. I cleaned the plugs, ran SeaFoam straight into the carbs and let it sit overnight (hoping to clean the jets). While it is running at 3000rpm on 2 cylinders, my mate takes the helm at full throttle (3000 rpm, 2 cylinders) and I either press the gas bulb to see if it is starved for gas, or activate the electric choke. Neither has any effect, or it causes more sputtering. It seems fuel related. I'm thinking it could be one of a few things: a) linkage is out of kilter so the other 2 cylinders aren't being activated, or b) one or two floats are stuck open or closed, or c) some jets are clogged (seems unlikely since I had SeaFoam in it overnight), or d) the fuel pump isn't working correctly, or e) one or two coils are going bad.

Any ideas will be accepted. My next stop is the Mercury dealer ($$$$) to try to get her running right.
 

brodmann

Chief Petty Officer
Joined
Jun 17, 2008
Messages
426
Go to a national auto parts store and get a $10 spark gap tester and set it at 1/2". Test each plug wire to make sure spark is sufficient to jump the 1/2". That will eliminate the ignition system. Next, while it's running on less than all cylinders remove the carburetor cover and spray some starting fluid (or carb cleaner, whichever you might have on hand) directly into the mouth of the carbs and see if RPM's pick up. That will let you know if you have a fuel delivery problem. Report back and let us know what your found.
 
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