Faulty 8HP two stroke

[ostra4]

My trusty yamaha 8 HP two stroke has been playing up ,always started first pull often after long winter break but recently although starting first pull and running faultlessly for an hour or so it cuts out a low speed .It won't restart , I stripped carb and main jet was clear , all back together ,started first pull , half an hours fast planning , throttle back to low speed and conks ,won't restart, no matter what throttle or choke settings. .Carb off again ,same procedure and same result,runs brilliantly then cuts out at low speed although sometimes will run perfectly well at low speeds .Im down to 15 mins now for carb off and back on - Any thoughts. Please .The file is fresh this past week or so and using quicksilver two steokeoil at 100 to 1 .

 

[claymore]

Ebay the sodding thing.

 

[ex-Gladys]

My trusty yamaha 8 HP two stroke has been playing up ,always started first pull often after long winter break but recently although starting first pull and running faultlessly for an hour or so it cuts out a low speed .It won't restart , I stripped carb and main jet was clear , all back together ,started first pull , half an hours fast planning , throttle back to low speed and conks ,won't restart, no matter what throttle or choke settings. .Carb off again ,same procedure and same result,runs brilliantly then cuts out at low speed although sometimes will run perfectly well at low speeds .Im down to 15 mins now for carb off and back on - Any thoughts. Please .The file is fresh this past week or so and using quicksilver two steokeoil at 100 to 1 .

You don't mention cleaning the slow running jet... Old motorcycle rule of thumb on what is controlling things, 0-1/4 throttle =slow running jet, 1/2-3/4 position of needle in main jet, 3/4-WFO - size of main jet

 

[ostra4]

I love my engine claymore, been through a few four strokes and back with two stroke -

 

[ostra4]

To be honest I'm not sure where the slow running jet is located but after removing carb ect and replacing ticks over perfectly at idle .

 

[NickRobinson]

Not seizing then freeing off during the 15 minute carb strip? (half serious, but I'd try 50:1)
More seriously, have you eliminated the ignition? If not, next time it won't start, spin it over, plugs out and earthed to see.
Will it start with neat fuel or Easystart down the inlet?
Checked the fuel flow/pump/lines/filter/tank vent?
Let us know how it goes, or doesn't...

 

[AngusMcDoon]

Could also be fuel pump which works at high throttle but not at low. Easy diagnosis for this one...if after it conks it restarts easily after squeezing the priming bulb a few times, it's fuel pump.

 

[anoccasionalyachtsman]

Dunno how old it is but as NickRobinson suggests, 100:1 might be a bit thin. I remember a Johnson 4 wanting 25:1. Having said that, the only time that I've seized a two stroke was at full throttle, not at idle so I'm not convinced that it's seizing either.

Which brings me back to the fact that you clean the carb and it starts again, taking at least fifteen minutes, so either the carb is getting dirty again (in which case clean the tank, change the filter and don't ever buy dirty fuel again) or it's cooling down right proper and didn't need cleaning. So try not cleaning it for fifteen minutes and see what happens.

 

[macd]

Which brings me back to the fact that you clean the carb and it starts again, taking at least fifteen minutes, so either the carb is getting dirty again (in which case clean the tank, change the filter and don't ever buy dirty fuel again) or it's cooling down right proper and didn't need cleaning. So try not cleaning it for fifteen minutes and see what happens.

+1 to all that.

 

[claymore]

I love my engine claymore, been through a few four strokes and back with two stroke -

Oh - I love Yamaha 2 strokes and have a couple of Maltas but you must never let them believe they are indispensible.

 

[Spirit of Glenans]

Check the jubilee clips on both ends of the fuel line and each end of the bulb, could be sucking air. Don't ask me how I know this one.

 

[ostra4]

Okay that's a few things to go on , Iv tried it this evening,one pull from cold and away it went .The faulty fuel pump had crossed my mind and I tried priming after the engine cut out at low revs,I gave it some wellie and squeezed extra hard but wouldn't start . If the fuel pump was faulty does anyone know if its possible tp perform perfectly at high revs then die at tuck over after prolonged running ?The fuel was bought in last couple of weeks but I suppose it could have been suspect even so .Thanks for the knowledgable replies . Will let you know as things develop what transpires -

 

[anoccasionalyachtsman]

So had it cut out, and this evening restarted easily? I've got to say that I'd be looking at the coil which can fail in such a way that heat makes it unreliable.

 

[AngusMcDoon]

If the fuel pump was faulty does anyone know if its possible tp perform perfectly at high revs then die at tuck over after prolonged running ?

Yes, it can do that. A slight split in the diaphragm or an air leak means that when the pump is operated at high speed it still has sufficient capability to overcome the fault, but not at low speed.

That's why giving a bit of manual assistance at low speed is a good test, but if doesn't sound like that is the problem if you've tried squeezing the bulb. If the pump is faulty when you squeeze it after a cut out you can feel & hear the fuel going in.

 

[ostra4]

Thanks for that Angus , I m going to remove carb and get it blown out and cleaned as I don't compressed air and check fuel pump for splits .Iv noticed after the engine has cut the primer bulb feels soft as though no fuel which made me think about fuel pump but couldn't understand why it would start and run perfectly after removing carb but not after engine cut out no matter how hard I git the bulb .The engine is from about 2003 so its not one of the really ancient ones and Id hate to see it go .

 

[ostra4]

Iv been removing carb after first time it stopped and wouldn't restart but having done this several times after which the engine always starts then I need to look elsewhere , I'm going to put a new pump diaphragm in , what happens when I remove pump cover...is it just the diaphragm in there or a spring as well does someone know, it's around a 2003 engine ?

 

[VicS]

Iv been removing carb after first time it stopped and wouldn't restart but having done this several times after which the engine always starts then I need to look elsewhere , I'm going to put a new pump diaphragm in , what happens when I remove pump cover...is it just the diaphragm in there or a spring as well does someone know, it's around a 2003 engine ?

I am not sure why you have diagnosed the pump diaphragm as the culprit.

The primer bulb will not stay hard once the engine has started and the pump is drawing fuel from the tank .
If however it is tending to be sucked flat then you need to look at the fuel system from tank to primer for possible restriction , and of course to double check the tank vent is open and clear.

However the diagram at http://www.boats.net/parts/search/Yamaha/Outboard/2003/8MSHB/CARBURETOR/parts.html shows the carb and fuel pump components .

If you dismantle the fuel pump note very carefully how everything is assembled as you take it apart.

 

[ostra4]

I'm probably grasping at straws Vic but when it cuts out the primer bulb is soft as though no fuel is comming through. If a coil became faulty at high temps then the engine should start when cold maybe ?

 

[VicS]

I'm probably grasping at straws Vic but when it cuts out the primer bulb is soft as though no fuel is comming through. If a coil became faulty at high temps then the engine should start when cold maybe ?

When you initially prime the carb with the bulb you should be able to sense that fuel is being pumped through by the bulb.
When the carb bowl is full the bulb will become firm and resist any further pumping although if there is any air trapped in it it will feel spongy ( Pointing the bulb skywards while priming should normally ensure any air is displaced)
Once the engine is running and the fuel pump is drawing fuel from the tank the bulb will loose its firmness

If the fuel pump is not functioning correctly it should be possible to keep the engine running by gently but repeatedly squeezing the priming bulb. If the engine stops due to lack of fuel then it should be possible to re-prime the carb and start the engine immediately. Similarly if there is an air in-leakage into the fuel line or connector. You should be able to sense that fuel is being pumped by the bulb while you reprime

If the fuel line between the tank and bulb is restricted, or if the tank vent is closed or blocked, the fuel pump will tend to suck the bulb flatish ( like you can suck your cheeks in)

The fact that you can only restart the engine after stripping / cleaning/ rebuilding the carb leads me in the direction of believing that for some reason the carb is repeatedly becoming blocked.

I would do a thorough carb clean, perhaps installing the components of a "carb kit".

I would ensure that the little clip between the float needle valve and the float is in place. Soft tip needles can stick on their seats if it is not fitted.

I would check/ clean/replace the in line fuel filter and i would thoroughly clean/ flush the tank and fuel line to ensure that there is no water or debris anywhere causing the problem.

However before getting too deeply into it I would check the compressions . The spec figure is 153 psi but I would expect it to just run at 60 psi and reasonably well at 80 psi or more They state that there should be no more than 30 psi difference between the cylinders. Personally I would be looking for them to be within 10% of each other if it is to run smoothly.

I would check the sparks, expecting to get two strong sparks able to jump around 1 cm with ease.

I would also try new plugs, even though there is no indication that they are faulty.


FWIW I would not be running it on as little oil as a 100:1 fuel mix. I would try 50:1 but if it's not happy with that use 75:1.

 

[ostra4]

Than you for that very detailed reply Vic .I removed the fuel line just before pump and a healthy squirt of fuel came though after the little filter,I fitted new plugs the other day , stripped carb and started first pull, swift run out and back to boat then it cut out .Carb off and refitted and for the first time it won't run, there is a good spark at the plugs .
I don't have compressed air so later today I'm taking engine in to my trusted local mechanic and take it from there.I didn't know the primer bulb became soft after the engine is running but I suppose Iv never needed to squeeze bulb after engine stopped before . The engine is recommended at 100/1 fuel mix although I usually do add a little extra especially if the engine is going to be thrashed when my grandson is around !it will be a few days in the workshop but I will report the outcome when hopefully its sorted , many thanks again for all the helpful advice .