Buy a fourstroke 

thanks for your insight Tim o. I am stuck with the merc

 Check your fuel line it might be sucking some air in.  The fact that it starts generally means your spark plugs are good and your fuel quality should be fine.

Throttle cable/rod could be loose and reving too low when you go to idle?

I would say that your main jets are blocked if it only runs on full choke.

Other thing to check is the fuel hose and connector being propperly on the engine connector.

if its not sucking air, its likely the idle jets not main jets blocked.

If it was the main jet as Glastro said it would idle fine when the choke is pushed in then die when throttle is applied.

   I have a 15 merc that has always done as you describe, I got it second hand 6 years ago.  Mine is the rebadged Tohatsu, and has way more power than the other three 15hp motors I use. the carby doesnt have a choke butterfly, and the choke knob opens a second circuit to enrich the mixture. Once started, it will run and start easy all day. I usually start it at home in a drum, allow it to warm up on choke for 10 minuits, but push the choke in , and it will run 5 seconds and stop, after 5 to 10 mins it is ok. I have always left the carby full, but am now going to try draining it out after use.  I have at times gluged up carbys by mixing diffrent brands of oil, not all oils can mix without polymising into a jell, forming small lumps of jelly in the float bowl and fueltank. The original owner didn't use it much as he wasn't mechanicaly minded, and didn't think it was reliable due to the cold start problems. It has become my most used toy at the moment. The o rings in the ends of the fuel lines have been my bigest headache over the pest few years, the later fuel lines are hard to replace, so I have been replacing the ends every year

Does it have an air filter?.. Sounds a lot like some of the symptoms my whipper snipper at work gets, runs ok on choke/half choke but bogs/stalls when fully open. Maybe the air filter could be blocked or fuel filter?. Generally from my experience with small plant 2-stroke engines are pretty bullet proof.

Thanks for the replys guys. I checked the fuel line and fitted new cable tyes to the ends and now it wont pump up the fuel filter bowl at all. I think its leeking at the bulb. I will buy a new line and bulb and try that. I noticed that the fuel looked a bit millky as though the oil hadnt mixed in properly.I will put in fresh fuel and try that as well. ill keep you posted

 Yep bulbs are prone to failure, when your boats sitting around for a week or two put some stabil in the fuel to preserve it, 

Also check the quality of the connector Piece onto your motor on Johnson/onc the ones most boat shops sell are crap even though they are onc brand.  Personally I would be using stainless pipe clamps instead of cable ties on your fuel line

 Yep bulbs are prone to failure, when your boats sitting around for a week or two put some stabil in the fuel to preserve it, 

Also check the quality of the connector Piece onto your motor on Johnson/onc the ones most boat shops sell are crap even though they are onc brand.  Personally I would be using stainless pipe clamps instead of cable ties on your fuel line

As suggested I bought a new fuel line and bulb and stainless pipe clamps. the bulb still never pumped up hard and it still never ran at idle! I then bought a new fuel tank and it still didnt pump up hard at the bowl or run at idle! I then removed the carby and cleaned all the parts and reassembled it ! it now runs fine and a litte fast at idle. I have slowed it down and can now use the idle contrloller on the motor which didnt seem to work before.Thanks for the tips and advice and now i can troubleshoot and strip a carby. I still have a spare quyicksilver fuel tank which i will buy a top fitting for and i can carry two tanks .they will have  two different hoses but will fit independantly.running flat out up the coast does chew a bit of fuel. I did my skippers ticket today and will be out at 6am weather permitting.Catch ya later

Just tried the motor again and playing up WTF?

 Boat = bring out another thousand

Seeing as you replaced the rest of the fuel line which is a good idea as preventative maintenance and not too expensive I would look st the fitting that clips on to the motor as they are a known failure point, otherwise it might be time to take it to a boat shop so you still have some hair on your head. Make sure it's someone who is a merc dealer might be a little more expensive in some cases but they should know their product

Describe what its doing now, is it still the same, or has the situation changed?

TnnnnTnnnnnTnnnnnTatTatTatTarrrrrrrrrr.....Two strokes 

The motor will start on choke and run  when the choke is off ,it runs for 5 seconds and cuts out . it was running ok for about 20 mins yesterday and would settle into idle on first pull. now it wont.the bulb still wont go hard and it seems to suck back air at the bottom but its not leaking  .the tank and bulb/hose are new only the top fitting is the original! ill try a new one tommorrow.

You say you cleaned the fuel filter? not replaced. So does your motor have one with a removable glass bulb. Can you explain.

Bite the bullet and take it to a workshop shouldn't cost that much just a matter of how quick they can get it done..better than be worrying whether it's going to play up out on the water ...must be doing your head in at this point

The fuel filter is a plastic round bowl which has a gauze cylinder debri filter inside it? its a cleanable type one. Ill try the new clip in fitting to motor and keep posting progress! cheers Darren

Fuel filter bowl, another source of a fuel leak. However you should be able to see any leak when you pump up the primer bulb. Another thing you could try is have a buddy maintain a gentle pressure on the primer bulb when you start it, and see if the problem of the engine dying when the choke is opened still happens, I'm thinking also of a fuel pump problem.

Having a look on the net at pictures of what I think is the fuel filter on your motor, it looks like the fuel line goes from the tank or primer bulb, to the fuel pump, then to the fuel filter. Is that correct?. That is a monster fuel filter for such a little motor. Tried to paste some pictures but dammed if I can.

You probably can run the motor for 10-15 minutes with only the fuel in the filter bowl. It would be interesting to see if, when you pump the primer bulb, the fuel filter bowl fills up. It will take some priming to fill it up, which may explain why you say the primer bulb does not go hard.

If you can see the fuel level in the filter bowl, it would be interesting to see if it is full, and if pumping the primer bulb does indeed fill it up. If you can start the motor, and the fuel level goes down ( it will take time), try pumping the bulb again to see if the level is restored. Your motor may keep going if this is the case.

Then I would say you have a faulty fuel pump. If like I said the fuel pump is up stream of that fuel filter. It may be as simple as a piece of tread tape in one of the valves.

If the fuel pump is down stream of the fuel pump, the results of doing the same thing above will be the same, anyway food for thought.

I have a problem at work with a Stihl blower, the pull cord won't retract. The pull cord looked worn so I replaced it, also replaced the clutch spring (damageda0 and retract spring. It worked OK for me, but out in the field, the pull cord buggered up again. Back in the workshop, i replaced the cord again, thinking I had used cord that was too big in dia, and the operators were reaching the end of the travel. Out it goes again only to come back again. Then the people that sold it to us turn up (hope your're not  reading this Paul) and I give it to them explaining what I had done, and what the problem was. Back comes the blower, out into the feild. I now have back in the workshop a blower with a pull cord that wont retract. So much for the so called Professionals. My advise, try and solve the problem yourself, how else are you going to learn. You took the inititive to service your motor,stick with it and learn more.

bought and fitted the final piece (top fitting to motor!) and still playing up.its now in the back of a mates van on the way to a merc dealer for a service/fix up. will keep you posted. maybe a diapham or fuel pump/suck issiue. I just want it done and go fishing ffs. tried me best.

 Hey mate how'd you go?

If you don't mind me asking who did you use and what did you think of their work/service?

Its a the local Merc dealer in wangara and will I hopefully get a call friday saying ITS FIXED COME GET IT ! then i pay the bill and ill let you know the outcome.

 Any update mate? How'd you go?

No up dates?

Gday guys .I got the motor back on thursday from hi tech in wangara. There was a problem with the timing and tuning which was sorted out for the minimum charge of $150. they striped and cleaned the carby and set it up. it now starts and runs as it should. Ryan was the guy I dealt with and he was awesome. he called me with updates and they will only work up to $150 without calling you for permission to spend more. he even waited till his head mechanic came back from leave to check the motor before releasing it to me.Very pleased with the service and they didnt try to make more money by replacing parts i already had done. highly recommended good dude. As my dad taught me 'advise is only free if you take it'. so thanks for the advice and input guys . catch ya later Darren

 Some times it needs the experts, at least you know your fuel system will last a while now lol.

And you have a decent mechanic for future servicing etc

Back in action ... good to hear ,  $150 is cheap piece of mind ...

 Sounds good, may get these guys to have a look at the carbs on my 40

Motor back in for fixing! will update when i get it back FIXED !Not happy Jan. Ill give them the benofit of the doubt till I get the bill. Cheers Darren

ended up up paying for the motor to be fixed again.When i took the motor in the first time they said sounds like the diaphams gone we will check it out. I was then told they had stripped the carby and was all good, it was timed and tuned an $150.The problem continued and i took it back. They then told me the diaphams had gone and they had none in stock.I had no boat for ozzy day weekend and missed out on a good weekend at moore river with mates. When I get a call to pick up the motor I am told that it was a separate issue. I told John what was said before and all knowledge about mentioing the diaphams and stripping the carby was denied by his workshop and counter staff. They all hid thier facing and left Me and John to argue the point. I said Ill pay the bill but wont be returning agian. John was stuck in the middle and to be fair he was willing to call them out to sort it. I declined as being called a lair would have set me off .Thats the story not happy and wont go back.Nothing personal just bad service and over charging. their moto is "do it once do it right",more like "do it once charge it twice".they also claim to carry all spares for mercury but didnt have a common part in stock.Make up your own minds!? Darren, ps it was $230 more

What a saga ..hope they explained exactly what the supposed issue was this time around and it stays resolved...get a few fish aboard , that'll get the smile back on the dial

 Cheers Chris the problem was the diaphragms all along ! The first time did not fix it. So they did it right the second time and it cost me twice ! Lessons learnt. John was helpful but I think there was some conflict between front and back of house which I got caught in ?! Anyway we will fish on. Darren

I'd be giving consumer affairs a call, conflict between the workshop and the front office is their problem and you don't have to pay twice when it should have been done correctly the first time.

couple hundred dollars?? fkn get real! diaphragms should have been done last 300 hrs service anyway so regardless you would have paid for them shortly anyway, aswell as checked/replaced with a carb clean, all come down to maintanence really, boats aren't cheap, good to see you on the water again