Yamaha 4AC 6EO high idle to slowly dies.

[Robbie260]

As per title have a small outboard engine that I'm attempting to tune up. When started it seems to have a extremely high idle then slowly drops until it cuts out, unless reved up. It is a single adjustment screw carb. Any ideas as to why I have this problem. I have cleaned out the jets and stopped a fuel leak that the carb was spraying everywhere. Getting to the limits of my knowledge of carbs and thier adjustment at this point unfortunately though. Any advice is welcome.

 

[Arc Tourist]

Have you opened, or checked the vent on the fuel tank for an obstruction ?

 

[Robbie260]

Have you opened, or checked the vent on the fuel tank for an obstruction ?

No I have opened the vent but haven't actually checked there is air flow through it. Thanks will give that a check. The fuel comes through the line from the tank tap fine though when you take the hose off and drain it into a seperate tank. Yeah the engine seems to want to run away with its self when starting initially then slowly gets slower and slower until it just stops.

 

[the snooper]

Check the needle valve and float to make sure petrol can flow ok, also if metal float make sure it's set to correct height

 

[Cobbler]

I had a Yam 28 horse that had tickover problems & found there is an idle drilling into the airflow by the edge of the butterfly. I don’t know if they all use the same system, but worth a look. On mine if you looked down the carb with the butterfly held open & shone a torch down you could see a very small drilling in the part where the edge of the butterfly rests. This supplies the idle fuel. I hope this makes sense.

 

[dobbslc]

Could be an air leak in the intake somewhere, spray some wd40 onto it when it's running to see if the engine speed changes. It could also be the cranks seals pulling air in, has it been sitting unused for long?

 

[Bullet2012]

Check the needle valve and float to make sure petrol can flow ok, also if metal float make sure it's set to correct height

My first thoughts as well, also check the fuel pipes internally as modern fuels can break down the ”rubber” internally

 

[Mr Fro]

Could be an air leak in the intake somewhere, spray some wd40 onto it when it's running to see if the engine speed changes. It could also be the cranks seals pulling air in, has it been sitting unused for long?

My thoughts too.

 

[slim_boy_fat]

No cr*p in fuel tank? It could allow fuel flow initially but get drawn to the outlet, finally block off the flow.

 

[Robbie260]

Right so here gose:

Fuel tank vent ok,
Fuel tank filter cleaned,
Fuel tank clean,
Fuel shut off valve OK,
Small air leak on carb found and fixed,
Rubber hoses all in good nick,
Needle Vv OK,
Passage ways for idle air and Fuel clear & flushed with wd 40,
All passage ways flushed and checked clear,
Engine revs fine and be revd to max rpm and dosent die or cutout,
Engine won't idle cuts out,

Wondering if it's the diaphragm. Not sure what else it could be.

 

[dobbslc]

If the diaphragm was knackered I wouldn't have thought it would rev up?
Did you Spray around the carb / rubber manifold with wd40 or carb cleaner?

 

[Robbie260]

If the diaphragm was knackered I wouldn't have thought it would rev up?
Did you Spray around the carb / rubber manifold with wd40 or carb cleaner?

Yes find a small leak made a new gasket, and replaced it. It hasn't improved the idle situation though.

 

[Rannsachair]

I have really struggled with outboard carbs gumming up orifices etc when left with modern fuels - ended up buying an ultrasonic cleaner to do a decent job.

 

[KemppiFrog]

Wot he said^, about an ultrasonic cleaner. Does sound like the bleed for the idle is restricted.
How old is this engine? I have a 3.5 AC and it dates from '80. Aquired it as non worker and after cleaning the fuel system (lodsa crud) it has worked fine ever since, about 20 odd years. Noisey tho..

 

[Robbie260]

Just thought I better update this thread, Chinese copy carb was sourced off ebay for £27 and fitted with a new gasket to the engine intake. Now runs like a dream.