120 merc I/o won't idle when warm

Jbpope

Cadet
Joined
Aug 21, 2016
Messages
9
Just got this boat it hasn't ran in a year. The guy started with the muffs on started great ran great. Got it home, put it in the lake started fine idle fine warmed it up for about 10 min. Went for a spin ran great at 3500 rpm. Shut it down for 30 min started fine went for another spin brought it back to the dock and shut it down for 45 min started fine but wouldn't idle and couldn't get it into gear with out it stalling, started and tried to get it to idle bunch of times each time having to give it more gas till it wouldn't start at all.

Any thoughts ?
 

trobin0406

Seaman Apprentice
Joined
May 31, 2016
Messages
33
I am thinking to start with the basic.. Was the old gas completely drained and new put in?? Did you change any of the fuel filters? If all that checks out it could be junk in the carb that was stirred up from your drive clogging the jets.
 

Jbpope

Cadet
Joined
Aug 21, 2016
Messages
9
There was very little old gas in the tank, my hope was that if I filled the tank it would dilute the old gas to the point that it wouldn't cause a problem. I did clean the filter at the carb and there was stuff in it, but at that point I already (I think) flooded it. Ah learning curves. Any other thoughts?
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Pull the carb fuel filter again and check to see if it's dirty, again. Also pull (replace) the fuel pump's filter and see how much junk is in it. I'd recommend getting a water separating fuel filter since the filter is much bigger (stops more junk), and it can retain water. If you install a water separating fuel filter, you can omit the fuel pump's filter.

What is the idle RPM when cold and when warmed up? Have you tried adjusting the idle mixture and idle speed screws? You can ball-park it in the drive way and I would shoot for 750RPM, but you need to do the final idle adjustments with the boat; warmed up, in gear, in the lake and the boat floating free (not tied to dock). Depending on year, the idle RPM should be somewhere between 650-800 RPM. The carb may need to be rebuilt, but I wouldn't dive into it quite yet...

Oh yeah, what is the engine serial number and year of the 120HP
 

Jbpope

Cadet
Joined
Aug 21, 2016
Messages
9
Ok I will check the filters and the year and numbers . I'm assuming the fule pump filter is on top of the fule pump? Where would be the water separating filter be if it had one? What will it look like? I have a good understanding of engines but not marine engines, l have a 68 gmc 4x4 that I work on.
Thanks
Brett
 

Bondo

Moderator
Staff member
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Apr 17, 2002
Messages
70,465
. I'm assuming the fule pump filter is on top of the fule pump? Where would be the water separating filter be if it had one? What will it look like?

Ayuh,... Welcome Aboard,..... Yer motor probably has the filter in the fuel pump, 'n a filter in the carb's inlet fittin',.....

It didn't come with a water sep filter, but if added, it'll look like an oil filter, 'bout the size of a common Chevy oil filter,...

Btw,.... dilutin' the rotten bad gas, probably wasn't a good idea, it's still in there,....
 

Jbpope

Cadet
Joined
Aug 21, 2016
Messages
9
OK, I pulled the gas tank out and empty it the gas was kind of reddish maybe cause of the fule stabilizer not sure, blew out the gas line. The gas line had a crack or cut at the hose clamp (I cut it out ) the gas filter was very dirty ( new filter) the spark plugs blackish but not burnt out (new plugs) and the terminals to the batteries were shot ( new terminals) and I'm going to put in new gas and use the old gas for the lawn mower (good idea?) I'm going to town tomorrow to get the parts. Oh not sure what you call it but cam that the idle screw hits was pointing the wrong way so I flipped so that the idle screw hits it .

So should I try all this and take her out
or should I wait and get the tune up done?

Thanks, Brett
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Was the fuel line split through, to where it could be sucking air? That could have been your issue if it just broke through when the problem started.

I would get it running 100%, or at least until it idles well and run on the muffs for 10-15 minutes without dying. I wouldn't go over 2500rpm or so on the muffs, but work the throttle a bit and make sure it can return to idle without dying, and idle without issue.

If you haul it to the lake and run into problems, you're just going to be hauling it back home again....
 

stonyloam

Vice Admiral
Joined
Mar 13, 2009
Messages
5,827
If it has stabilizer in it that could be the red color as you suspected, pretty sure Sta-Bul is red. Make sure that the cam the throttle stop sits on is not loose, if it is the screw can slip off and drop the idle too much, happened to me.
 
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