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Trolling with a 3 liter mercruiser

Old 06-09-2019, 06:48 AM
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Default Trolling with a 3 liter mercruiser

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 help and advice, COrnelis
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Old 06-09-2019, 07:43 AM
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What happens with the engine temp after prolonged trolling? Sound like your choke is partially closing?
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Old 06-11-2019, 06:35 PM
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The temperature is very steady ,but I checked the electric choke setting to make sure it does not close when the temperature goes down a bit.
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Old 06-11-2019, 06:55 PM
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How old are your exhaust manifold and riser? If your riser is dripping water into the manifold it can cause the exact symptoms you're describing.
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Old 06-11-2019, 09:39 PM
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How old are those AC Delco plugs? Gapped correctly? Maybe 1 heat range higher plugs could help if you mostly troll?
Certainly check timing against the specs for that year and model engine , keeping in mind the factory timing spec is likely for 87 octane ( perhaps advance a couple degrees BTDC for 91 octane if you need to use that?) and that higher octane is just for knock control if you experience potentially damaging knock with lower octane, ...and that higher octane-rated gas may have less BTU energy than 87 due to anti-knock additives with less energy.

Check to see if the coil is getting hot too. Any corrosion on dist cap electrodes?
​​​​​​Is engine compartment ventilation system adequate and still intact? Older ventilation flexible ductwork deteriorates pretty quickly and/or detaches from hull dorades.
Other than stabilizer in E10 gas, I tend to think of other additives as having net negative value since they aren't straight gas and dilute the fuel, but perhaps a little Techron `or Ring Free could help reduce the carbon buildup problem?

At minimum it sounds like your engine performance would benefit from shorter interval between higher RPM runs .
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Old 06-12-2019, 04:55 AM
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I have trolled for HUNDREDS of hours with 4-6-8 cylinder Mercruisers with no problems.
What do the plugs look like? Carboned up?
Check coil as stated.
What weight oil are you using?
In the distributor, check the mechanical advance to see that it moves freely, no sticking or rust, loose weights.
Possible "Professional rebuilt" carburetor jetted wrong. ??
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Old 06-12-2019, 05:48 AM
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Probably simple idle mixture adjustment on the carb...they do not come pre-adjusted.
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