Fine tuning a 3 liter 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

 

[Bt Doctur]

sounds like your fouling the spark plugs from too rich an idle setting

 

[Bondo]

sounds like your fouling the spark plugs from too rich an idle setting

Ayuh,.... Agreed,.... Could be the float is atad high,....

 

[Rick Stephens]

Unfortunately, professionally rebuilt doesn't always means its a perfect rebuild. Devil is always in the details. Not trying to bad mouth anyone. Carb rebuilds can bite the most detail oriented mechanic. My bet is if said professional really is, they will redo it.

 

[rolmops]

Thank you all for your advice. I will check all those things

 

[havoc_squad]

I would think if you have float setting or too much fuel pressure causing too much gas, you should see some sporadic dripping from the venturies.

If you monitor the carb top in gear, at operating temp, and on the water with accurate tach running manual spec rpm and good compression over a 10 minute period, there should be no mist or fuel droplets on the butterflies.

If this is the case, I think it's time to do an idle mixture screw adjustment per service manual.

You should be able to just barely lean up that setting a quarter, half, or one turn because the overly rich is happening VERY slowly.

I would suggest 1/2 turn leaner and go from there following the service manual.

 

[Scott Danforth]

is this points? if so, verify they are set correctly, and your timing is spot on.

then verify your idle mix

I used to drop to a 15p to troll at slower speeds with the engine RPMs at just above idle.

 

[fishrdan]

Agree, sounds like the idle is adjusted a tad rich. Warm up engine completely on the water, then with the engine idling in forward gear adjust the: idle mixture / idle speed / idle mixture / idle speed, etc / etc, until the engine is idling as smoothly as possible at it's correct RPM (750?).

It's a chore to get the idle set perfect, but a must if trolling. I have a 140HP Mercruiser and can troll for hours on end without stumbling problems. If the carb is getting dirty and idling bad, I can usually tune the idle on the water again to make it through a fishing trip, but then I know it's time to rebuild the carb, again... I usually have to rebuild the carb every 2-3 years to keep it idling well, due to ethanol fouling up the works.

Tip: Take a picture of the idle mixture screw(s) where they are at now, then run them in all the way to lightly seat them, counting how many turns it took to seat the mixture screw(s). 1.5 turn, 2.25 turn, whatever. Then you know where you started from and can go back easily. I have a diagram of where my carb's idle mixture screws need to be set, and while rebuilding the carb I can set the mixture screws on the bench almost perfect (ff I rebuilt it well).

 

[rolmops]

I got rid of points a long time ago. It's way too humid around here for those.Timing if anything is a bit early but probably spot on. I checked the electric choke for possibly closing at low rpm. I even cleaned the flame arrestor. The plugs are brand new. This Friday I will go to a small local lake and make sure that the idle mixture is not too rich. I will bring a report once I have tried it out.

 

[Benny67]

Sounds like you're running rich. If your carb was just done by a so called Professional, you might check out your choke and the linkage involved.

It might be getting hokey after being ran for a few hours. ​​​​​​

 

[rolmops]

Just a little update. After digging out the service manual, I adjusted the idle mixture, took it to a canal, tied it to a dock, started it, put it in forward gear and made the final idle mix adjustement. Then I reset the idle speed and it seemed to be good afterwards. Today I took it out on Lake Ontario in what were supposed to be 2 footers, but were more like 4 footers and the engine behaved perfect at low and high idle. So your advice was spot on. Thank you all!

 

[Tycer]

Excellent. Thanks for the update. And the advice on this thread. Haven’t built a carb in 20 years. Still have a good can of chemdip and all my tools.