2001 3.0 Mercruiser won't idle

[1ARUNEM]

3 weeks ago I de-winterized my 2001 Maxum which has the 3.0 Mercruiser. I cranked it up and ran it (idle) for about 15 - 20 minutes to warm the oil. Performed an oil change, checked over fluid levels, etc.

I then cranked it again and ran it for about 5 minutes with the new oil. I turned it off then restarted a time or two with no issue.

I covered it back up and it has sat until Saturday when I hauled it to the lake (about one hour drive) put it in the water and here is where the trouble starts. I pumped throttle twice it starts then dies. I pump throttle twice more, same thing. Pump two more times and set throttle a little higher and she fires up and runs at high rpm but will not stay running at an idle.

I was able to put it in gear and get up on plane and it runs seemingly fine, however coming off plane at anything under say 2500 rpm it wants to shudder, run rough and die..

Thoughts?

I am going to go ahead and pull plugs (and change since its been a few years) and inspect cap, rotor, etc.
Possible carb issue? Just odd that it ran/idled GREAT three weeks ago and only difference was pulling to the lake.

 

[Roadblock007]

Your engine will always run differently on muffs vs in the water. There's more drag on everything. If it were me I would reset the carb idle and mixture screw to base as set out in the manual. Then use a portable tach to set them live on the water. It's free to do if you already have the portable tach and might solve your issue.

Is there any black smoke or any smell of fuel when it idles rough and dies?

 

[Scott Danforth]

how old is the fuel and when was the last time you serviced the carb?

 

[1ARUNEM]

Boat had probably 10 gals of 6 month old gas. On the way to the lake I topped off about 15 gals of fresh fuel. Old fuel had Stabil in it for what it's worth.

No excessive fuel smell or smoke to speak of.

This is my third year with the boat and I have not serviced the carb as of yet.

 

[Bondo]

This is my third year with the boat and I have not serviced the carb as of yet.

Ayuh,..... Welcome Aboard,..... You said you serviced the motor,....

Did you change the fuel filters,..??
It sounds like some crud slipped through, 'n is cloggin' the idle circuit in the carb,.....
I'd start with the filters, 'n if that don't cure it, Rebuild the carb,....

 

[1ARUNEM]

Besides the ignition parts I will be changing just for good measure, I was figuring carb service would be on the list.

 

[Rick Stephens]

No reason to change ignition parts. If it runs fine at high idle, it ain't spark. You should be able to get decades of service from plugs and wires and the like.

Fuel problem, not electrical problem. Idle circuit gummed up. Does the boat have a decent canister water separator? If not, driving down the road with a half tank will stir things up nicely and send it to the carb.

 

[1ARUNEM]

I am going to inspect the cap, rotor, plugs, etc. for good measure and change as needed.

There is not a canister separator, only a filter at the carb.

 

[Bondo]

There is not a canister separator, only a filter at the carb.

Ayuh,..... Then you should add one,.....

There should also be a filter inside the fuel pump,.....

 

[havoc_squad]

Ayuh,..... Then you should add one,.....

There should also be a filter inside the fuel pump,.....

Especially if the OP uses gas with Ethanol in it.

It doesn't matter how fresh gas is with Ethanol, soon as it is exposed to high moisture in air, it starts trapping water in with the fuel.

How long and how often the gas is exposed to high moisture determines how much water contamination would be in the tank now.

Then there is the normal breakdown of fuel octane over time.

So in short, two seperate problems using gas with Ethanol: Water absorption and fuel octane breakdown over time.

 

[Rick Stephens]

Ditto the above. Clean out the tank. Add a water separator. Do a carb rebuild. Having a break down and walking back is no fun.