2.5HP Mercury Mariner Outboard erratic running

[kas_1611]

I have a 5yr old Mercury Mariner outboard that has been trouble free until last year. It is the 2.5HP 4 stroke version.

Last year it started having problems which may have been due to absentmindedly leaving some unleaded in the system for a few months. Basically I forgot to drain the tank and fuel line. When I came back to use it the engine just wouldn't start. Got the marina mechanic to look at it and he cleaned it all out, blowing through the carb with compressed air and it fired up fine.

Out on the water I tried using it and it took several pulls to fire then would not idle. But give it the beans and it revved fine. Got to and from the shore using the choke and fast idle. As we were not going to be using the outboard for a few days I decided to just let the engine run until the fuel ran out to see if the idle problem would fix itself. A few minutes of idle and it died then pi55ed petrol out of the overflow lines.

Got back to home marina and tried cleaning the system again and checked everything over. Float was freely moving, jet was clear and no signs of anything sticking, jamming or otherwise not working. Fired it up again and it started first pull and idled like a dream. But give it the gas and it would not rev at all. First little movement of the throttle and it would die but restart first pull.

It has been like this since then and I can't figure out why. Some days it fires first pull, others it takes 3 or 4. Sometimes it will idle but not rev others it revs but won't idle. There are days when I check the insurance with a mind to "accidentally" dropping the damn thing off the back

Any ideas what could be the problem and what I can do to rectify it?

If it helps the numbers embossed in the carb are 3DVCVGF or could be t

Cheers

Keiron

[mike66]

I believe that motor is a Tohatsu. Their carbs are notorious for getting clogged. You can try disassembly and soaking in a can of carb cleaner (20 bucks) for four hours, or buy a new carb (100 bucks). I would just buy the new carb.

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[bvander]

Carbs not designed well, but break down quite easily. I have it down to 20 min from start to finish. 3 places they clog: float valve can stick to seat, hole in base of run jet channel can clog, and really finicky is the idle jet, with super fine holes. I use a can of carb and choke cleaner to blow out the galleries and jets, q tip for the float valve seat, and a fine piece of copper wire for the holes in the idle jet.
Put back together and should be good to go. If gentle wont need new gaskets. Should get an carb kit regardless, sometimes need to replace the gaskets, good to have handy.
Can remove the carb with motor mounted but highly recommend you have a secure steady place for disassembly, bunch of small parts.
Carb and choke cleaner melts most finishes, especially varnish on cockpit table, so recommend a layer or two of plastic between work area and table.

[kas_1611]

Thanks guys for the info. I thought I had given the carb and good clean out but hadn't considered the idle jet. Am back on the boat next week so will pull the carb off the motor, sit on the dock and break it down completely to give it a thorough clean, let it soak in some fresh unleaded for a few hours and try it again.

Failing that then it is a new carb at £160+VAT

[mike66]

If you can get a can of Gunk carb cleaner at a reasonable cost, it would be your next step if you can't get it clean. Comes with a parts basket inside. Dismantle the carb as much as you can and then soak four hours (no more, it can damage the castings). Won't damage the hard plastic stuff, but don't soak the gaskets. After the soak, blow out the jets and passages with whatever aerosol you have on hand (WD-40, carb/brake cleaner, compressed air, etc) and you should be back in business. I now have two carbs, one clean and ready to swap on my 6hp Tohatsu. Good luck!

[Boatguy30]

The tohatsu dealer I know just replaces the carb. Cost around $100. He's a good mechanic also just says those carbs are to fiddely and not worth messing with.

[Boatguy30]

$100 in the U.S. That was a few years ago.