15hp 4 stroke stalling issues

[drbinsc]

Hi All,

Brand new to the forum. Bought the motor from a guy that says the 2005 motor has less than 10 hours, he bought it for a rib boat to run out to his sailboat and then sold everything but the motor. The buyer apparently already had a motor and didn't want to spend the extra cash. Believe the hours to be very low as the prop still has all the paint and inside the cowl is clean.

It's been sitting in his shed ever since, and claims to run it several times a year to keep it going. Supposedly Seafoam or Stabil in the gas since new.

Problem is runs fine at 1/4 to WOT, but soon as I slow down it stalls. Cannot get it to idle. I bought it because my little 4hp 2 stroke is showing age. Any ideas on what is the issue?

Any suggestions are appreciated.

[Ski]

I would look at the carb, and give serious consideration to just replacing it with new.

[JanS48]

"Carefully" drill out the brass idle plug

1st clean the carb, then "Carefully" drill out the brass idle plug - you will see an idle adjustment behind it. Again - carefully turn it in clockwise noting how many or fractional turns it takes to bottom - write this measurement down.
From what I can tell this is an air-bleed adjustment which means turning it in makes it a bit richer. I found that 1 turn out from bottom works best.
I have an 03 15 HP 4 stroke. The idle on these motors is less than great, mine varies quite a bit with what seems to be the engine heat, after a very long run at high speed it idles too fast - adjusting to idle well in this state results in poor idle when cooler - its tough to get it just right. And this is after talking with a Mercury trained mechanic. Good luck.
Jan

[Bladerunner]

Hello All,

I found similar problems like this recently on these models up to 25hp. most are tiller models on tenders and work boats!, I work mostly on Diesel engines in trawlers etc. and I get asked to look at them because they are temperamental especially at idle.

The recent fix was on a 2008 Mercury 15 hp. 4 Stroke, I found that this was a rebadged Tohatsu 4 stroke!, after many hours looking at filters, fuel lines, connectors and dirty fuel and carby's I found what I think is the problem, the Carburetor top Cover gasket on especially the 15 hp 4 stroke!, is "too" thin and it allows air to leak past the "Labyrinth" of fuel and air chanels it is supposed to seal!, see exploded view below gasket ref:4, I replaced the original gasket when I first tried to solve the Idle Mixture Screw problem - no response lean or rich etc. with a new one from the Carby overhaul kit, this did not work also when installed, much to my disgust especially after all that time etc. so I made my own gasket from slightly thicker gasket material I had, 2 mm Nitrile, I did not cut out all the openings as seen in the original Merc. gasket which was 1.5 mm thick, I left it plain except for the breather hole only, I reassembled the carby and used 316 S/Steel Allen head screws - which were easier to install under the flywheel which you cannot replace without removing the carby and success!, I finally had Idle Mixture Screw response, easy start and full control at Idle!.

So that was my experience after scratching my head etc. over these smaller Merc's temperamental idle problems!, I hope this helps you?.

Regards

John Hayes
Bladerunner Boats
Home Page

[Ski]

We just throw the motor in the test tank and try it out. If it can be kept running with starter fluid then the job stops and a new carb is ordered. Total labor to the customer is a hour, and they get a motor that starts on the second pull.

I have spent many countless UN-billable hours messing around with small Hp carbs with mediocre results, and what does get billed out usually exceeds what a new carb would have cost. It's a hard pill to swallow when the customer gets a $450.00 bill for a motor he just spent $500 on so I tell them upfront what my intentions are and the fix is correct the first time.

[Bladerunner]

Hello Ski, even cheaper fix than replacing the Carb, is replace the cover gasket with a thicker 2mm Nitrile gasket, worked every time on the last 3 15hp. 4 Stokes, customers very happy.