Yamaha 4hp struggling for power

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afterpegassus

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I have a 1998 Yamaha 4 hp and it's served me faithfully since I got it.
Recently it's started to struggle for power in gear, but will rev its head off in neutral.
I stripped the carb and gave it an ultrasonic bath, replaced the gaskets and refitted.
Engine still showing same symptoms.
Any suggestions on resolving the issue?
 
afterpegassus said:
I have a 1998 Yamaha 4 hp and it's served me faithfully since I got it.
Recently it's started to struggle for power in gear, but will rev its head off in neutral.
I stripped the carb and gave it an ultrasonic bath, replaced the gaskets and refitted.
Engine still showing same symptoms.
Any suggestions on resolving the issue?
Click to expand...

Poked a soft copper wire through all the orifices ?

Checked the fuel filter and the fuel flow?

Tried a new plug?

Checked that the spark will jump a 7/16" air gap ?

Checked the compression?

Are you using an external tank or just the internal one ?
 
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Not poked with a wire but could see light through the jets.
Plenty fuel.
New plug with 0.65 gap and engine fires no bother.
Have not checked compression.

The engine will sometimes pick up speed but will fall off again at a whim and is why I'm guessing a carb problem.
I'm not sure if this would be true of a compression issue but I'm no mechanic.
 
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afterpegassus said:
Not poked with a wire but could see light through the jets.
Plenty fuel.
New plug with 0.65 gap and engine fires no bother.
Have not checked compression.
Click to expand...

Clean the jets by poking a wire through them. They need to allow the right amount of fuel through, not just light.

Check spark will jump 7/16"
 
Riddling with a wire may well scratch or deform the jet sufficiently to ensure it never works properly again.

Try something a little less barbaric like a splinter of a toothpick or an air jet.

Modern sparks jump millimetres or thou, not fractions. Come to think of it, I've never seen a 7/16 feeler gauge... ;)

I has a similar problem that turned out to be sludge on the pipe walls between tank and carb that intermittently blocked the main jet. Tiny grains of it.
No amount of cleaning the pipe (x5) cleared it, it required a new pipe. (and an inline filter immediately before the carb)
 
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Old Bumbulum said:
Riddling with a wire may well scratch or deform the jet sufficiently to ensure it never works properly again.

Try something a little less barbaric like a splinter of a toothpick or an air jet.
Click to expand...

That is why I always specify a soft copper wire .

Modern sparks jump millimetres or thou, not fractions. Come to think of it, I've never seen a 7/16 feeler gauge... ;)
Click to expand...

You dont need a feeler gauge to measure 7/16" you can use a ruler....Adjustable spark testers usually have a scale marked on them but you dont need anything fancy . A piece of dry wood with two nails through it, points bent toward each other like Lakesailer posted years ago will suffice.
 
rszemeti said:
Sounds like time for a new set of rings.
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Oh .... You have done a compression test on it for the OP while he was not looking have you ?


Its possible though.... I expect the poor thing has been run on 100:1 fuel mix all its life
 
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VicS said:
Oh .... You have done a compression test on it for the OP while he was not looking have you ?


Its possible though.... I expect the poor thing has been run on 100:1 fuel mix all its life
Click to expand...

No, but I have been around 2 strokes long enough to know the symptoms. I'd change rings on my race bikes every weekend ... and pistons every two weekends.

A 1988 engine, that's been well used? Rings would be my first point of call, as a matter of course as part of a general "freshen up" ... it's well past due.

I also have the same engine, of a similar vintage ... so I know the ways of the beast. The other two areas to check:

1) build up of crud in the exhaust port and leg. You have to split the engine off the leg to get at this, but it is not a big job. It will give you a chance to get at a few other bits too.

2) how is the waterflow, it could be overheating? They do tend to crud up, while the engine is off the leg, clean out what crud you can from the cooling passages around the engine, I've rinsed mine through with brick acid before now to dissolve out calcified crap in the cooling and it's restored cooling flow.

There is a lot of total rubbish on the web about cleaning the passages down at the pump/gearbox end of things as they fill up with crap and salt ... yes they do, but they are not flowing water, ignore the nonsense you will see on youtube with people cleaning the area around the pump, its misguided foolishness not linked to the flow of cooling water.

While you have it apart, pull the gear shift actuator out, clean it, and run a reamer through the hole in the casing .. they are only plastic shift levers and prone to seizing up and a complete git to get out once they do seize. Pulling it out is easy when the engine is off the leg, I'd recommend you do it, if you decide to pull it apart.
 
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rszemeti said:
No, but I have been around 2 strokes long enough to know the symptoms. I'd change rings on my race bikes every weekend ... and pistons every two weekends.

A 1988 engine, that's been well used? Rings would be my first point of call, as a matter of course as part of a general "freshen up" ... it's well past due.

I also have the same engine, of a similar vintage ... so I know the ways of the beast. The other two areas to check:

1) build up of crud in the exhaust port and leg. You have to split the engine off the leg to get at this, but it is not a big job. It will give you a chance to get at a few other bits too.

2) how is the waterflow, it could be overheating? They do tend to crud up, while the engine is off the leg, clean out what crud you can from the cooling passages around the engine, I've rinsed mine through with brick acid before now to dissolve out calcified crap in the cooling and it's restored cooling flow.

There is a lot of total rubbish on the web about cleaning the passages down at the pump/gearbox end of things as they fill up with crap and salt ... yes they do, but they are not flowing water, ignore the nonsense you will see on youtube with people cleaning the area around the pump, its misguided foolishness not linked to the flow of cooling water.

While you have it apart, pull the gear shift actuator out, clean it, and run a reamer through the hole in the casing .. they are only plastic shift levers and prone to seizing up and a complete git to get out once they do seize. Pulling it out is easy when the engine is off the leg, I'd recommend you do it, if you decide to pull it apart.
Click to expand...

No.
 
I suspect VicS is correct.

I used one of these outboards for many years.

The outboard regularly developed exactly the symptoms you have indicated and it was always solved by cleaning the carburettor. Occasionally the first clean did not produce any improvement and an immediate repeat of the process was needed. Presumably there was still a small dirt particle that I failed to clear on the first attempt.

The motor was in daily use. Initially the carburettor needed dismantling about every three months. By adding a larger fuel filter under the cowling and filtering all the fuel with a coffee filter while adding it to the tank I was able to extend the cleaning time to about once a year.
 
afterpegassus said:
50:1 in my 14 years of ownership.
Click to expand...

Good. Many just use 100:1 as specified in the manual . That's OK for a lightly used engine provided it is given extra oil or fogged when laying up. The OMC "Oil metering system", and VRO 2 before it, reduced the mixture to 100:1 at low revs but progressively increase it to 50:1 at high revs. IIRC the original, but troublesome , VRO reduced it to 150:1 at low revs

( VRO = variable rate oiling)
 
RichardS said:
I'm sorry .... but where are you getting this stuff from? :confused:

Richard
Click to expand...

Every time you service your car engine you change the piston rings, the sparkplugs and the air filter dont you ?
Similarly you'd renew rings, plugs and air filter in a small outboard wouldn't you ?

Not many people know about small outboard engine air filters....... The fact that roszemeti recommends changing them shows that he is on the top of the game.
 
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