Rough running 4.3 MerCruiser at idle

[Misty IV]

This past weekend while fishing the LSR Challenge, my main engine started running rough while trolling. Under power above 1000rpm, it seems to run fine, but anything below that, it simply will not idle and sounds like it's not running on all cylinders, almost like a misfire. Two weekends ago when I was up, I had a similar issue and ended up changing the water/fuel filter and the idle problem disappeared. I find it hard to believe that over the course of sitting for two weeks, the problem could be back that fast. I'm thinking ignition problem. The engine was replaced in 2005 by the original owner with a Jasper remanufactured unit a few months before I bought the boat. Engine has a little over 200hrs on it. Spark plugs, wires, rotor, cap are on my list. Any other thoughts on what may be causing this idling issue? Thanks in advance for all replies!

[Chas0218]

Do those first but you might need to rebuild your points. Also if it sat for 2 weeks and you started it you could have water in the gas seeing how it settles to the bottom.

Did you run any dry gas in it to remove what water could be in there?

[Misty IV]

The last time I was up, I ran the tank down to almost 1/8 full. I didn't have an opportunity to fill it back up before I left to come home. I filled it up Friday afternoon with fresh gas at the R&R One Stop in Port Ontario. I did add Marine Sta-Bil to the tank before refilling the tank.

When I started it initially at the launch Saturday morning, it ran fine, idled perfectly. It was after a few hours on the water that the idle problem appeared. I thought vapor lock may be a cause, but I don't think that would account for the missing and surging RPM's at times. Maybe I got a bad tank of gas from there?

[seasquirrel]

Poor gas or clogged filters would be the first place I would look. Distributor cap and rotor would be a good place to look also if it looks crusty underneath on the contact areas. Is this a carburated engine or fuel injected? Air mixture screw on carb? Sometimes carbon build up on the inside flap of the throttle body can mess with the idle too on that type of setup

[Misty IV]

2 bbl carburetor model

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[lakebound88]

honestly, sounds like an ignition problem. I wouldn't pass up the water in the filter, however, if it were water it would die down. It sounds like you are not burning hot enough at idle, remove the cap, check to see if its "green" on the contact points. Common issue and an easy fix..

Check those minor things first.. I am leaning away from the filter right now, more towards cap and rotor..

[IA bowhunter]

I agree with LB88, I have a 350 merceuiser and last trip out it was idling rough changed the plugs and still rough, so today I changed cap & rotor and started up and very smooth. Might want to also check that rpm's at idle are in the range.

Just my .02 cents

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[Misty IV]

Thanks for the replies. Hoping it is something simple, like cap & rotor.

Lakebound, the last time I was up when the problem started initially, yes, engine simply died. Found water in the filter after changing it. Ran beautifully after the new filter was put on. This ethanol fuel is drving me up the wall.

Bowhunter, I was thinking the same thing, as well, with the idle setting. When it is running fine, it idles great, right around 650rpm. When this issue starts up, it'll drop to 500 and will surge up to 1000 then back down to 650, then starts misfiring, etc. Real PITA when you're trying to maintain a steady trolling speed...

[IA bowhunter]

For my 5.7 the idle should be 650-700 in gear which it sound like u might have a fuel/carb issue.

Like everyone else says on here, might be worth calling Hank.

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[bottom-feeder]

get an electronic conversion kit from hot-spark

get rid of the points and condenser

find them on e-bay

best 70.00 and 10 mins you can invest in your boat

bottle of k100 in fuel tank will do wonders to get rid of water too

[Fish Hunter]

A few years ago, my 4.3 in my Islander, would not pull any rpms.

I found that I had plugged jets from ethanol fuel, once I pulled the carb.

Went to a local auto parts store and asked the kid behind the counter for a re-build kit for a 2 bbl Rochester.

He needed to know make/model/year.

I told him it was a 1990 4.3L I/O boat engine, and that is when the tailspin started.

I finally left, and carefully re-used the gaskets, with sealer, and it's been fine.

Have used Startron ever since, w/o issues.

Misty's does sound ignition/electrical related.

[lakebound88]

I would say it is electrical. Water in the fuel can be ok at idle, may cause a surge, but you really notice it on the top end side of things, you can't get there!!

I just replaced my ignition system in my boat, ALL of it. I went with the Mallory YLM breakless ignition. BUrned up 3 coils before mallory finally told me I needed the coil pack assembly versus the one that comes in their kit.. Go figure right?

But now my boat runs bettter, starts up without pumping the throttle, idles smoother and faster holeshots.. I have about 700 into this new system, but the piece of mind is worth it.

To me , it still sounds as if your contact points are "green".. Check the inside of the cap. Maybe you even have wires jumping.. start up and grab some wires!! Kidding, dont do that, but if you take a spray bottle, and mist some water over the wires, and hear baccon cooking, replace your wires:)..

Racing guy taught me that trick..

Good luck, Nick

[Misty IV]

I won't be back up to the boat for another two weeks, so having my brother who is up there this week on vacation check things out for me.

Is there a particular brand of parts I should avoid or stay away from? I've looked online at the various parts and there are so many manufacturers of marine caps, rotors, etc. Should I stick to genuine Quicksilver/Mercury Marine/MerCruiser parts or am I safe with the others, like Sierra? Some of the names I've seen around a lot are made in China and I'm very uneasy about using that stuff.

[Gill-T]

Check the fuel/air mixture screws on the carb as these can back out over time.