Yamaha 20hp 4-stroke outboard problems...

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steve_l

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Hi all! I don't often ask for advice here but now I'm stuck. I'm trying to fix a friend's Yam o/b.

The problem is at full throttle opening. The motor will run for about 10 secs then dies. Idles ok and part throttle is ok. Starts again ok.

To me it sounds like fuel starvation at full power.

I've changed the line from the tank to the o/b. Cleaned the coarse filter in the tank. Changed the in-line filter. Checked the diaphragm in the motor's mechanical fuel pump. Even spliced in an electric fuel pump (rated at 170l/min) and by-passed the mechanical pump but the problem is still there.

Now I'm starting to pull my hair out! A new pump costs 190€ (diaphragm costs 170€ on it's own so no point just replacing that) so I don't want to spend that and still have the problem.

So, what have I missed? Or is there some other clever system in the motor which needs replacing? Any ideas?

-steve-
 
botheras said:
If it was a motorcycle, I'd say blocked mainjet, as carbs tend to run on idle circuit up to 1/3rd throttle, throttle lift mid 1/3, and mainjet size WFO....
Click to expand...

Ok, forgot to say, had the carb apart and blew everything through with an airline.

However, if it's something really stuck in the jet then maybe the airline didn't dislodge it.

Carb coming apart againg tomorrow.

Thanks! :)
 
steve_l said:
Hi all! I don't often ask for advice here but now I'm stuck. I'm trying to fix a friend's Yam o/b.

The problem is at full throttle opening. The motor will run for about 10 secs then dies. Idles ok and part throttle is ok. Starts again ok.

To me it sounds like fuel starvation at full power.

I've changed the line from the tank to the o/b. Cleaned the coarse filter in the tank. Changed the in-line filter. Checked the diaphragm in the motor's mechanical fuel pump. Even spliced in an electric fuel pump (rated at 170l/min) and by-passed the mechanical pump but the problem is still there.

Now I'm starting to pull my hair out! A new pump costs 190€ (diaphragm costs 170€ on it's own so no point just replacing that) so I don't want to spend that and still have the problem.

So, what have I missed? Or is there some other clever system in the motor which needs replacing? Any ideas?

-steve-
Click to expand...

The tank vent is clear ? Tried talking the cap off?

You seem to have eliminated the pump from the equation

You have changed the fuel line but there is, I think, a quick release connector on the engine. Have you renewed that along with the fuel line? If not maybe its O rings are knackered and it is sucking in air at that point. Likewise at the tank end where i believe there is a similar connector

Any possibility that there is an obstruction at the carb inlet.
 
VicS said:
The tank vent is clear ? Tried talking the cap off?

You seem to have eliminated the pump from the equation

You have changed the fuel line but there is, I think, a quick release connector on the engine. Have you renewed that along with the fuel line? If not maybe its O rings are knackered and it is sucking in air at that point. Likewise at the tank end where i believe there is a similar connector

Any possibility that there is an obstruction at the carb inlet.
Click to expand...

Yep, took the tank cap off, just in case. Replaced the whole fuel line including the connectors at each end.

Seems it must be something in the carb. Strip down tomorrow...
 
The easiest way to check the fuel pump (if you have a plastic fuel tank) screw the breather screw down on the filler cap, then run the engine, If the fuel pump is good it will suck the sides of the tank in! when you release the breather you will hear the air being drawn in. If this does not work, try running the engine up to full throttle whilst pumping the fuel bulb.
If the egine is running hot it has a fail safe shut down built in that knocks the revs back (it does on the 30hp) also worth a check. It does sound like others have suggested a carb problem.
 
Davy_S said:
The easiest way to check the fuel pump (if you have a plastic fuel tank) screw the breather screw down on the filler cap, then run the engine, If the fuel pump is good it will suck the sides of the tank in! when you release the breather you will hear the air being drawn in. If this does not work, try running the engine up to full throttle whilst pumping the fuel bulb.
If the egine is running hot it has a fail safe shut down built in that knocks the revs back (it does on the 30hp) also worth a check. It does sound like others have suggested a carb problem.
Click to expand...

Tried pumping the fuel primer bulb while running. The motor then surges a few times but dies eventually. That's why I initially thought it was the fuel pump.

Don't think it's running too hot. The problem happens right off the dock and the cooling water seems to be flowing well from the tell-tale and the outlet amyway.

Yep, consensus seems to be the carb.
 
Carb stripped today. Someone has been in there before! The float needle valve brass housing was a bit butchered and too much silicone sealer under the rubber seals. Oh, and looks like the float has been warmed a bit and the tab that holds the needle has been pushed up, 'cos there's a thumb print in the plastic! Not impressed! :)

Anyway, seems the needle valve wasn't opening enough and someone's tried to fix it before.
The brass housing was quite a way proud of the casting which didn't look right so I made a tool to push it back in by about 1mm or so.

Now the needle valve opens when the float is at the bottom of it's travel :)

Re-assembled and run on the water muffs - seems ok, although I might have to change the float tab again to get the chamber fuel level right.

On-water test tomorrow to try full power on load... :)
 
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