Fine tuning a mercruiser for trolling.

[rolmops]

I have a 19 foot Islander (1984) with a 4 cylinder 3 liter engine with good compression across the board and the points replaced by electronic ignition.  When I troll at roughly between 750 and 950 rpm everything goes fine for the first hour or two , but after that, the engine starts running rougher and sometimes it just quits on me. Than I have to restart it in neutral and when it starts it leaves a bunch of brownish to black carbon floating on the water. Than it will run again for another hour or so. When I pack up to go back to port the engine has a hard time to pick up rpm and sometimes just stalls. I have to very gradually increase rpm and after a few minutes it runs just fine. Why is there carbon buildup? I replaced all my fuel so it is 91 octane ethanol free and fresh. The spark plugs are ACDelco43LTS. My carburetor was professionally rebuilt. Is my timing a bit off? What else should I look for so she runs smoothly at low rpms without carbon buildup?

Thank you for any advice and help, Cornelis

Edited June 12, 2019 by rolmops

[Earl28]

What your neutral idle.

[tech2526]

When your carb was rebuilt, did you just put it back on. Or did you set idle mixture and speed? Sounds like you idle mixture is to rich. What do you plugs look like? If they are black. Its too rich. Your timing could also be off.

 

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[sherman brown]

unless you moved the distributor I wouldnt think its off by much. but if the electrical ignition was installed recently it could be off it isn't a big job to get the timing right. I would guess its in the carb. was the air screws adjusted with the motor running? if not its easy to get close. the way I always set mine back when I was drag racing. turn the screws all the way in, now turn then out 1 1/2 turns, go by the tach with the motor warmed up and running. and turn them out 1/2 turn each and see if the rpm goes up. continue this until you get max rpm's. then turn them back in 1 turn. this will get you very close. the high octane gas doesn't really help unless the motor knocks. but if you want gas without alcohol then it isn't going to hurt anything.

 

another thing that might help. go to a auto parts and ask if they have a hotter plug that replaces the plugs you have now.

[guffins fisherman]

I went through those same symptoms with my Merc 5.7, electronic ignition. With your carb being rebuilt recently, I would think the mixture screws are off.  Don´t forget to check your cap and rotor.  If they are not connecting well, you will have crappy ignition, leading to the loading up of the heads you are getting.  Napa carries marine application ignition parts, which by the way are a must.  Be very attentive to the position of the plug wires if you do decide to put on a new cap.  Switching them would be very bad for your motor.

 

Good luck

Matt

[rolmops]

Thank you all for your advice. I will check the fuel /air mixture and the timing. I replaced the plugs although they were not very black. They probably cleared up after running a few miles at WOT.

[Jakob5pennies]

I have a 4 cyl Merc 1995 had problems for 3 years rebilt carb change plugs Everytime out change to A warmer thermostat finally took to a Marin 3 times for a fix this spring made them place in lake and drive the spark advance was not working properly readjust carb on water in gear ltrooling now all seems fine crazy about $2000.00 later 

[Double Dipper]

If it’s a merc carb I would replace the mixture needle and mount a vacuum gage off the intake. Highest stable vacuum best mixture. Sometimes I lean it out a 1/12 of a turn and I adjust it several times a year especially from early April to late June


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[rolmops]

After a proper idle mixture adjustment all problems cleared up. It has never run so smooth