Yamaha 4hp struggling for power

RichardS said:
I'm sorry .... but where are you getting this stuff from? :confused:

Richard
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Oh I dunno .... 30+ years of experience running 2 strokes? 50 to 100 hours seems about typical for when they start to go off, you can run them a lot longer, but the compression going south ...

On the race bikes, I used to change the rings at around 6 hours and rings and pistons at 12 hours, as peak performance was key, but in "utility" applications like road bikes and outboards, 50 to 100 hours is fine. Rings cost very little, and take about an hour to do ... what the big deal?
 
VicS said:
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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You must be the only bloke on the forum with a 2 stroke car then ... or maybe you just don't understand the differences between 2 strokes and 4 strokes?
 
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?
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Buy it a new plug.
NGK.

If you've checked the main jet again and again, then look at fuel supply.
Float level.
Float valve.
Cleanliness of passaages from fuel pipe to float bowl.

Fuel tap and any filter/gauze above it.
The good old tank air vent.

Does this engine have a fuel pump to allow for a remote tank option?
These are a diaphragm malarkey connected to crank case vacuum.
A clean and possibly a new diapragm.

If the feed to the carb is simple gravity, you could remove the float bowl and check the flow rate into a catch bowl. It should be several times max fuel consumption.

You could check the gearbox isn't seizing...

You could run it as hard as it will go, then look at the plug. If the plug is clean, that suggests a fuel shortage, if it's black or wet that suggests an ignition problem.

The other possible issue which springs to mind is the exhaust. A build up of carbon and oil products can choke the exhaust and stifle power.

I think this is a reed valve engine, but like the piston rings, crank seals and so forth any issues generally affect starting.
 
rszemeti said:
Oh I dunno .... 30+ years of experience running 2 strokes? 50 to 100 hours seems about typical for when they start to go off, you can run them a lot longer, but the compression going south ...

On the race bikes, I used to change the rings at around 6 hours and rings and pistons at 12 hours, as peak performance was key, but in "utility" applications like road bikes and outboards, 50 to 100 hours is fine. Rings cost very little, and take about an hour to do ... what the big deal?
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Pretty silly reply, we're talking about outboards, not racing bikes. Most 2 stroke outboards will by now have done many hundreds, even thousands of hours since new and still run fine. Change rings in an old outboard in an hour? Dream on.

I'm not familiar with the OP's engine but my suspicion would be fuel supply, possibly the tank filter.
 
rszemeti said:
Oh I dunno .... 30+ years of experience running 2 strokes? 50 to 100 hours seems about typical for when they start to go off, you can run them a lot longer, but the compression going south ...

On the race bikes, I used to change the rings at around 6 hours and rings and pistons at 12 hours, as peak performance was key, but in "utility" applications like road bikes and outboards, 50 to 100 hours is fine. Rings cost very little, and take about an hour to do ... what the big deal?
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You’re right about high performance high power 2 strokes. Jet skis etc will need a top end rebuild every 100 hours.
But it’s not the case with these wee things at all.
 
QBhoy said:
You’re right about high performance high power 2 strokes. Jet skis etc will need a top end rebuild every 100 hours.
But it’s not the case with these wee things at all.
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It may be longer, but according to the OP, he's had it since 1988 and does about 150 hours a year ... while they may last longer than 100 hours before the rings need doing it is not indefinite, it will be overdue a set by now.

The problem is the area around the exhaust port ... you often find the ring measurement at opposite the port is fine, but the rang where it passes over the port is knackered.
 
rszemeti said:
It may be longer, but according to the OP, he's had it since 1988 and does about 150 hours a year ... while they may last longer than 100 hours before the rings need doing it is not indefinite, it will be overdue a set by now.

The problem is the area around the exhaust port ... you often find the ring measurement at opposite the port is fine, but the rang where it passes over the port is knackered.
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It'll be due new set of rings if a compression test returns a low figure
 
VicS said:
It'll be due new set of rings if a compression test returns a low figure
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Assuming the rings are the problem!
I pulled apart a 2 stroke once, I think the rings were OK, it was mostly the barrel, piston, small end which were the problem.
Oh, and the head, it had dents in it from bits of small end roller.
The rings looked OK to use, but I never needed to.
Luckily I wanted the gearbox, not the engine.
 
Is someone taking the mick here ? I must have got lost along the way.
One minute we’re suggesting that we change and replace rings every time we service and engine....next we are suggesting or making similarities to the commercial use...someone needs to give in and stop talking none sense. It’s getting silly.
I’ve had and still have msny 2 strokes from the 80’s. (Currently Yamaha Malta, 2x 8hp and a Suzuki 2.2, all 2 stroke) ran them fairly frequently for years and years. If they are treated right and good oil used...to suggest these needs new rings every 50-100 hours is just plain silly. C’mon now. Even at that. Who replaces just rings on a 2 stroke. You do at least a whole top end.
Some totally clueless suggestions along the way here.
A Yamaha 170hp screaming 1300 GPR 2 stroke jet ski...totally. Every 100 hours would be good preventative practice...but that’s not what we are talking about at all. Far from it !
 
If it’s down on compression...that will be down to running lean and lack of oil. It won’t be just and only as a result of “ the rings being due a change” it’s not even a thing. Really surprised at such a suggestion. It’s not even on the service schedule, in isolation, as suggested. Ridiculous
 
QBhoy said:
If it’s down on compression...that will be down to running lean and lack of oil. It won’t be just and only as a result of “ the rings being due a change” it’s not even a thing. Really surprised at such a suggestion. It’s not even on the service schedule, in isolation, as suggested. Ridiculous
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it's 'a thing' on high performance racing motorcycle engines, which tend to be a bit generous in the piston clearance and only have a single ring. When they're doing over 10k rpm, there's little time for the gases to leak past a single ring, but a worn ring will leak enough to prevent starting and may break in the ports, which take up amazing amounts of the cylinder wall. But that's 30BHP 125's , not 3HP Yamaha Maltas.
 
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