1995 3.0LX High Idle, Stalls Below 1000

[Takedown]

Hi, new to the forum. I have a 1995 Starcraft 1710 I/O with the 3.0lx mercruiser in it. Got the boat basically for free and have been fighting an idle issue for a couple weeks now. I've replaced the fuel filter, water separator screen on the carb, cap, rotor, wires, spark plugs, did a carb rebuild and set the ign timing. Still having the same problem as I did in the beginning. Boat will start just fine and run great at any rpm over 1200 but anything below that benchmark the engine runs but shakes like mad. Doesn't appear to be misfiring or have a dead cyl either. It appears as if the electric choke is working as it should and I did put new gas in it recently to dilute the 1/8th tank of old gas. Still didn't seem to help the problem. I have messed with the idle screw and the idle mixture and it still doesn't seem to want to idle properly at around 750-800. I haven't done a compression test but I plan on it tonight but I just wanted some input from you guys.

Could this be a sign of a weak coil or cracked distributor? Ideas????

Here is a video of it idling below a thousand rpms. Its one of the very few times that Ive been able to get it to atleast.
https://www.youtube.com/watch?v=z1YVb4ucY7A

 

[Scott Danforth]

welcome aboard

the most expensive boats are the free ones because they will nickle and dime you to death

your carb still needs work as you never cleaned out your tank and keep pumping crud into your carb.

the inlet screen on the carb is just to keep the rocks out. it is not a water separator. a water separator filter is the size of the oil filter.

clean out all the old fuel, clean your tank

go thru your carb again. you have dirt in the idle circuits. make sure to pass a piece of fishing line thru every passage. if you cant, that passage is plugged.

 

[Takedown]

The inside of the bowl was pretty clean upon tear down. I cleaned the carb inside and out with carb cleaner, plastic brush, a pic and alot of patience. I did remove the two jets and the other brass insert infront of them. All of them were free and clear of debris. Now by idle circuit what exactly are you referring to?

 

[Takedown]

I also replaced the idle set screw and shot cleaner inside that passageway and made sure it was free.

 

[Scott Danforth]

the idle circuit. its the circuit that provides fuel, thru the mix screws.

the jets provide fuel when the throttle is opened

 

[jrs_diesel]

The idle circuit is not part of the jets or venturi cluster. I assume you are working on a 2 bbl Mercarb. If you are, if you look down the throat of the carb (choke plate wide open), you will see a slot cut into the throat right where the throttle plate sits when it is closed, on the bowl/float side. That is where fuel is drawn in at idle with the throttle closed, the mixture screw under the bowl will adjust how much fuel is drawn in. If that passage is dirty, plugged, or the screw not adjusted right, then you will have a crappy idle, if any idle at all.

Simply spraying carb cleaner into the mixture screw hole into the passage will not clean it out all the way. That is only half of the idle circuit. The other half is the passage that connects the bowl to the mixture screw. You'll have to pull the carb and open it back up to clean that one.

Coils either work or they don't. You can pull the distributor cap and check for carbon tracking on the inside, and or corrosion. Same thing with the rotor, you want nice clean contacts.

It sounds to me like your ignition is in decent shape though. I would definitely go through the idle circuit in the carb again.

 

[harleyman1975]

Does the carb top have 2 tubes with little holes in them? They all have to have wire run through them to make sure they are open. I use a twist tie with the paper stripped off. agree the carb needs more work.

 

[Takedown]

Thanks guys! Ill dig more into it tonight

 

[nateo]

A few years ago i spun a engine coupler at wot and over reved the engine. I replaced coupler, then replaced bent valve. Howver i still had the exact same symptoms you just described. I did everything you just listed and more. Still wouldn't idle below 750 , my 5.0 would idle at 650 before all this happend. I spent a whole summer on it, eventually coming to the conclusion that it was something internal in the engine such as a rod bearing or something. I sold it with full disclosure. I got $800 out of it I bought a new boat. Only regret is that I wasted so much time trying to fix it.

The shaking may be a rod flopping around or something, mine did that too.

Good luck, hope you're back on the water soon