50hp mariner95 4stroke IDLING ISSUE

[KVTBouy]

New to forum, my first thread!
Bought this motor not long ago. Owner said it runs fine, but my first outing proved otherwise. It started well but died when reved up. Idling was rough with black smoke. Back home, did carb service, new fuel, new fuel line, new plugs, seem to run ok at home.Took it out for test run-does the same thing again. I have also noticed smoke stain inside cowling.
It not all bad news though as the motor runs fine at high rev, has plenty of power to push my 15ft fiberglass.
Also its RPM reads at 5500 when in gear.
Please help as i have run out of ideas.Is there anyone with suggestions?

 

[CharlieB]

Re: 50hp mariner95 4stroke IDLING ISSUE

The clue is the smoke stain inside the cowling.

Try running without the cowling, I bet it runs fine.

Find where the exhaust leak is and replace the gasket(s).

 

[KVTBouy]

Re: 50hp mariner95 4stroke IDLING ISSUE

Thanks CharlieB,
Exactly, thats whats happening. When i start it at home the cowling is always off. At test runs its on.
Was wondering why it runs ok at home but not on water.
So i take it that at idle the smoke gets into inlet and suffocates the motor, right!
But wonder why it runs fine at higher RPM.
When i get the motor into gear(after several attempts) it wont miss a beat....runs like dream. Just wondering why "exhaust leak" wont affect its performance when its up and running.
Also the motor is very cheap to run. If I could fix the idle it would be a great motor to keep.

 

[CharlieB]

Re: 50hp mariner95 4stroke IDLING ISSUE

Once the boat is up to speed it may not be recirculating the exhaust fumes inside the cowling so much.

Loook very closely at the engine base gasket, between the motor crankcase block and the mid-leg.

Check the Merc Parts site, have your model and serial handy, look at the parts and see how the exhaust gaskets and gasket to the mid, these are the common leakage points.

http://www.mercruiserparts.com/mercruiser_parts.asp

 

[KVTBouy]

Re: 50hp mariner95 4stroke IDLING ISSUE

Got home early from work yesterday. Took off the bottom housing cover and there you go.....just what you said CharlieB!
Exhaust leak! Corrosion has eaten around the gasket and made two holes about 10 mm each.

I guess when motor is stationary at idle the water pressure pushes up on exhaust , hence leaks more.....
but when in gear exhaust passes freely through prop.

Anyways problem found but solution may not be as simple.
Would appreciate any possible suggestions. Anyone with similar experience!
Do I need to get new housing?
Will try to post some pix.
Thanks CharlierB, you were spot on.

 

[CharlieB]

Re: 50hp mariner95 4stroke IDLING ISSUE

Sand or file the corroded surfaces clean then build them up with JB Weld, after it has cured file the surfaces smooth so they can seal against a new gasket.

Or have a competent welder weld them up. File as needed to smooth surfaces.

 

[KVTBouy]

Re: 50hp mariner95 4stroke IDLING ISSUE

I had cleaned used muffler putty. Hope that holds like my friend told me. If not then will use jb weld. Didn't know about but after you memntioned I read good

 

[KVTBouy]

Re: 50hp mariner95 4stroke IDLING ISSUE

Started motor in drum today. Runs fine. Im so happy now the problem is fixed and the putty has held well.
I know now what to do next if this wont work. Thanks Charlier You been great help. Was hard to diagnose
but easy fix.

 

[KVTBouy]

Re: 50hp mariner95 4stroke IDLING ISSUE

Finally decided to do permanent repair before putting boat in water, for peace of mind! Was not an easy fix as I though
Think JB weld is a great product, worked so well. The bolts holdin the lower case had come lil lose- probaby the reason
for the leak, letting exhaust seep out causing holes in the housing.
Now I can go fishing as we coming into summer here in NZ.

 

[KVTBouy]

Re: 50hp mariner95 4stroke IDLING ISSUE

Thought of changing all carburetor gaskets and o-rings so pulled out carburetor yet again
All went well but now the motor wont idle.It runs smooth, starts easy but idles at 1200 then
RPM shoots up to 2500. I even tried turning idle screw all the way back and mixture screw
all the way in.
I wonder where is the fuel entering the inlet when throttle is closed and screws are all tight(idle jet is close)

Is the choke closed or electro-thermal ramp paying up??????

Will appreciate any suggestions