Yamaha Non starter.

[Mercury]

Hi, I am having trouble with my Yamaha 4 hp 2 stroke. I have cleaned fuel line and carb, reed valve look good new gasket on reed valve & carb, and I have spark. Tried a compression test but it seems low to me around 80 psi. but not sure what it needs to be, does any one know ?

[beamishken]

80 psi does sound a bit low but its notoriously difficult to get an accurate compression test result on a pull start engine even if it is as low as 80psi I'd still expect it to start. If the plug is okay & you definitely have spark I'd try some neat petrol down the plug hole & see if it fires on that, just dont go too mad & that it cant turn over. If it fires you know its fuel related if not then you need to look elsewhere

[jeffstevens763@g]

120/150 psi normal

[Fenlander]

Like Ken I'd be straight to proving if it's fuel delivery or not. Either his method into the plughole or mine where I put 2-stroke mix into a trigger spray and spray this into the intake while pulling over with the throttle part open, choke off or part open.

If this doesn't make it fire and you revisit the spark aspect I wonder if you've fitted a new plug? Assuming you tested the plug by observing it spark out of the cylinder I have known a plug that will show OK like that but not fire correctly once tightened in.

Re compressions yes you'd hope for higher but again as Ken hints there are variations in testing, including the accuracy of a particular gauge, that make comparisons with a target reading difficult.

Has this motor been fine up until a first start up this year?

[Mercury]

Many thanks for all replies.

[Mercury]

Has this motor been fine up until a first start up this year?[/QUOTE]

It's been laid up for a couple of years, but was ok when I put it away.

[Fenlander]

>>It's been laid up for a couple of years, but was ok when I put it away

That would have me thinking carb passageways/jets gummed up so it will be interesting to know if introducing fuel externally will kick it into life.

I wonder how far you went with carb cleaning? With these small motor carbs often they don't always clean totally first time, perhaps even needing an ultrasonic bath to do a full job.

[willk]

Originally Posted by Fenlander

I wonder how far you went with carb cleaning?

Fuel Fuel Fuel

I'm addicted to watching James Condon on Youtube. That's Condon, not Corden - easy to confuse them because they've both cleaned a lot of carbs...

[Fenlander]

>>>James Condon on Youtube

A shared appreciation of generators eh?

[willk]

Originally Posted by Fenlander

A shared appreciation of generators eh?

I can watch him all day. I enjoy his somewhat sideways approach and eventual job well done. TBH I was a little shocked by how simple some of the jobs were and it gave me the courage to do a bit more myself. I certainly have a better understanding of the ancillary components of a small engine now.

[User name]

Originally Posted by Mercury

Hi, I am having trouble with my Yamaha 4 hp 2 stroke. I have cleaned fuel line and carb, reed valve look good new gasket on reed valve & carb, and I have spark. Tried a compression test but it seems low to me around 80 psi. but not sure what it needs to be, does any one know ?

120 is normal for this motor, but it should still start with 80PSI. I'd double-check you have no leaks on the tester hose/gauge. A faulty reed will cause a loss of compression too.

I'd also try some starting fluid straight into the throat of the carb and see if you get ignition. If you do then it'll prove a fuel delivery problem.

Is your motor the one with or without the onboard tank? The petcock can be an issue on them. I'd start by verifying the fuel delivery is good up to the carb before moving on to a carb strip and clean.

[Mercury]

I sprayed some "Easy Start" in the carb but still no start. so I put some oil in the bore and compression went up to about 160. put the plug back in and it fired up after the second pull, it smoked a lot then died it seems it was just burning the oil. So I think the bore is the culprit. Just wondering if it's worth getting hold of a S/H power head. PS. it's the one with the onboard tank, I gave it a good fettle and new fuel before I started.

[jeffstevens763@g]

I had a chainsaw with the same symptoms turns out it was semi seized which I think was me not putting oil in the petrol by picking the wrong can senior moment

[User name]

Originally Posted by Mercury

I sprayed some "Easy Start" in the carb but still no start. so I put some oil in the bore and compression went up to about 160. put the plug back in and it fired up after the second pull, it smoked a lot then died it seems it was just burning the oil. So I think the bore is the culprit. Just wondering if it's worth getting hold of a S/H power head. PS. it's the one with the onboard tank, I gave it a good fettle and new fuel before I started.

160 sounds about expected with oil in the bore. It won't have fired on the oil alone though. It'll be the easy start that ignited. The smoke is to be expected since you put oil in there and it's burning off. I'd test the compression again now that you've lubed the rings and burned off the excess. It may have settled now at a higher value than you got initially, but the fact that it fired up after the easy start is pointing you towards a fuel delivery problem rather than the bore being the cause.

I'd check and clean both jets, the jet body, the diaphragm etc. The float height for these is 22±0.5mm.

[A1an]

Originally Posted by willk

Fuel Fuel Fuel

I'm addicted to watching James Condon on Youtube. That's Condon, not Corden - easy to confuse them because they've both cleaned a lot of carbs...

Likewise, that impact is like ASMR for anyone that works on small engines.

Its amazing how easy it is to get engine parts in the States.

[Mercury]

I think I read the compression tester wrong, it was not 80 psi it was 60 psi. So it's worse than what I thought. I would not fire with some fuel down the bore or easy start, the only time it fired was when I got some compression by squirting oil through the plug hole. So I think it is ready for the great big ocean in the Sky.

[User name]

Originally Posted by Mercury

I think I read the compression tester wrong, it was not 80 psi it was 60 psi. So it's worse than what I thought. I would not fire with some fuel down the bore or easy start, the only time it fired was when I got some compression by squirting oil through the plug hole. So I think it is ready for the great big ocean in the Sky.

60 is quite low indeed. If you remove the side plate you'll be able to see the piston/rings and it'll give you a better idea of the health of the engine. If you're lucky you might get away with a new set of rings. These are also very easy to strip down too.

I'd confirm for certain that the reed is not letting by before doing any significant work though.

[spartacus]

Good shout out for James Condon.

I spent last summer trying to fix an old Matika strimmer. New sparks, cleaned carb (multiple times), new piston ring. Getting difficult to source parts too as model now obsolete. Suspect worn piston bore, probably due to overheating. These things get very hot, very quickly if tackling large areas of weeds.

Anyway back to your Yamaha. These are easy to work on, and I would be tempted to put a new piston ring on it but only as last resort. With the spark plug out, if you pull the starter rope and it runs with almost no resistance then it does sound like it’s lost compression.

Before getting into the guts of the engine, rule out that the gasket behind the carburettor hasn’t failed.

https://youtu.be/Jz7-o5CEiss?si=GXt0yuVovG22Pr1s

While you’re at it, check the bullet wiring connections. They are prone to oxidisation. Also check you’re definitely getting a decent spark.

[Mercury]

I managed to get a S/H power head from E bay, it fire's up and runs ok now. So it must have been the original power head at fault. It's a wee bit noisier than the original but it work's just fine.

[Fenlander]

Thanks for rounding this off. Unusual for it to go from OK to such low compression over a couple of years storage but glad it's up and running again.