Yamahah 5hp 2 stroke - Idles ok, struggles with throttle?

[Tim Good]

I've looks at lot of previous posts of similar issues but non exactly the same so I thought perhaps I'd post, especially since I'm new to 2 strokes.

So its Yahama 5hp . Was working fine yesterday and went on a long trip planing up the River Yealm. I refilled it with fuel today and now suddenly a bit dodgy. Same fuel as the day before I might add.

It starts and idles normally. But with a little throttle it stutters and struggles. I find that I can get it working out if I apply 60% choke... which would normally cause it problems once running. Once I get up to speed with some choke and I remove the choke it get a burst of power and then it tries to cut out and I have to take all throttle off to to prevent it from stalling.

I've never stripped down this carb and thought best i post before doing so.

Thoughts?

 

[KREW2]

A bit long winded but very informative.

 

[Tim Good]

A bit long winded but very informative.

This is bloody marvelous!

 

[Bobc]

I've looks at lot of previous posts of similar issues but non exactly the same so I thought perhaps I'd post, especially since I'm new to 2 strokes.

So its Yahama 5hp . Was working fine yesterday and went on a long trip planing up the River Yealm. I refilled it with fuel today and now suddenly a bit dodgy. Same fuel as the day before I might add.

It starts and idles normally. But with a little throttle it stutters and struggles. I find that I can get it working out if I apply 60% choke... which would normally cause it problems once running. Once I get up to speed with some choke and I remove the choke it get a burst of power and then it tries to cut out and I have to take all throttle off to to prevent it from stalling.

I've never stripped down this carb and thought best i post before doing so.

Thoughts?

Main jet blocked or float needle stuck.

 

[Boathook]

Anything like my Malta it will be the main jet. Doesn't take much and at first glance the jet may look OK.

 

[pyrojames]

I have had the same with my4hp yami. Basically same engine. It frequently happens at the beginning of each year if I have left fuel in it over the winter. The old fuel gums, but then breaks up with fresh fuel, and then blocks the jets/filter/needle. Carb needs stripping carefully and cleaning.

 

[Tim Good]

I have had the same with my4hp yami. Basically same engine. It frequently happens at the beginning of each year if I have left fuel in it over the winter. The old fuel gums, but then breaks up with fresh fuel, and then blocks the jets/filter/needle. Carb needs stripping carefully and cleaning.

Hmmm so you clear out the old fuel first or add some kind of additive?

 

[pyrojames]

Hmmm so you clear out the old fuel first or add some kind of additive?

Best off draining for the winter, or running regularly!

 

[masterofnone]

Always close fuel tap and run til stall to clean fuel from carb

 

[Tim Good]

So as a follow up I empty to fuel take and found the strainer, which is installed inside the tank via a brass coupling, was in poor condition and had a hole in it. The tank was also full of grit and crap.

I installed an in line fuel filter and then removed the carb to find lots of horrible crap in that also.

Cleaned and inspected what i could ensuring the float needle and various jets were clear.

Got it back in the water and started very easily but the same problem persist. I didn’t have any specialist carb cleaner and just sprayed it through with degreaser instead. Not totally sure what else I can try at this stage.

Thoughts?

 

[Boathook]

I would still think that the main jet is a culprit. It only takes a small speck to cause problems and is very difficult to see that the jet isn't 100% round.

 

[CFG98]

View attachment 113414

So as a follow up I empty to fuel take and found the strainer, which is installed inside the tank via a brass coupling, was in poor condition and had a hole in it. The tank was also full of grit and crap.

I installed an in line fuel filter and then removed the carb to find lots of horrible crap in that also.

Cleaned and inspected what i could ensuring the float needle and various jets were clear.

Got it back in the water and started very easily but the same problem persist. I didn’t have any specialist carb cleaner and just sprayed it through with degreaser instead. Not totally sure what else I can try at this stage.

Thoughts?

Carb cleaner is cheap and a lot stronger than brake cleaner. Does what it says on the tin.

 

[Adios]

View attachment 113414

So as a follow up I empty to fuel take and found the strainer, which is installed inside the tank via a brass coupling, was in poor condition and had a hole in it. The tank was also full of grit and crap.

I installed an in line fuel filter and then removed the carb to find lots of horrible crap in that also.

Cleaned and inspected what i could ensuring the float needle and various jets were clear.

Got it back in the water and started very easily but the same problem persist. I didn’t have any specialist carb cleaner and just sprayed it through with degreaser instead. Not totally sure what else I can try at this stage.

Thoughts?

What do you mean by "Cleaned and inspected what i could"? Did you dismantle it more than whats in the photo? There is more you can do if not. At least get the float valve out and have a look, probably need to push out the pin holding the hinge of the float, if its corroded at all its worth getting a new one.

Carb cleaner isn't the same as a degreaser, it's specifically good at dissolving the gummed petrol bits which end up like sandy grains ideal for getting everywhere. The spray dissolves it so it will then flush out from where its stuck

 

[gus]

An excellent presentation and detailed explanation but one vital detail missed out and I am very surprised it was not mentioned. A lot of reference to the vacuum in the crankcase and the resulting pulse to make the carburettor function. What was omitted was a failing of the vacuum/pulse from the crankcase caused by worn crankcase seals. The most obvious symptom is an engine that will start when cold but then not start when warmed up. I have had various 2 stroke engines in the past where everything checked out okay in respect of the fuel supply etc. but still gave trouble. Replacing the crankcase shaft seals would resolve the problem as it would then allow the crankcase vacuum/pulse to operate as it should.
Correction! Found an explanation which includes the seals.

 

[Adios]

Always close fuel tap and run til stall to clean fuel from carb

I do that myself but it occurs to me with no air able to replace the fuel in the carb doesn't the engine stop due to the vacuum before its drained all the fuel? My latest little engine has a fuel drain screw in the bottom of the float bowl so i'll test that out next time and see whats left.

 

[Tim Good]

What do you mean by "Cleaned and inspected what i could"? Did you dismantle it more than whats in the photo? There is more you can do if not. At least get the float valve out and have a look, probably need to push out the pin holding the hinge of the float, if its corroded at all its worth getting a new one

I removed and cleaned the main jet and nozzle. Also the Float and needle valve. The idel jet and the pilot screw (ensuring it went back at the same setting). Also the capillaries on the top which is covered by a small plate and gasket.

I'm wondering now if i should remove the fuel pump assmebly and see if any crud is lying around in there. I may damage the gaskets in doing so though.

 

[Adios]

I removed and cleaned the main jet and nozzle. Also the Float and needle valve. The idel jet and the pilot screw (ensuring it went back at the same setting). Also the capillaries on the top which is covered by a small plate and gasket.

I'm wondering now if i should remove the fuel pump assmebly and see if any crud is lying around in there. I may damage the gaskets in doing so though.

Try lots of carb cleaner sprayed everywhere you can first but otherwise think of it as a service, few quid for some new gaskets and clean the whole system out with carb cleaner and then it really should be ok, quite simple system so not much can be wrong with it

 

[duncan99210]

I have the same engine, similar vintage. Whilst my other, smaller, engines responded well to the strip the carb and clean it, I had the same problem as Northcave. The engine didn’t respond to this treatment, nor did giving the pump mechanism a clean. So I took the easy way out and took it to the local outboard man, who waved his magic wand and sorted the problem for £30. that was 2 years ago and no problem since. Best £30 I ever spent.

 

[Adios]

I have the same engine, similar vintage. Whilst my other, smaller, engines responded well to the strip the carb and clean it, I had the same problem as Northcave. The engine didn’t respond to this treatment, nor did giving the pump mechanism a clean. So I took the easy way out and took it to the local outboard man, who waved his magic wand and sorted the problem for £30. that was 2 years ago and no problem since. Best £30 I ever spent.

did he tell you what he did for £30 or are they like the magic circle?

 

[duncan99210]

Didn’t bother to ask: at that price he could have sprinkled with fairy dust for all I know or care...