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High Throttle Stalling, Yamaha 9.9 4 Stroke

fritz3000g

New member
Hi there,

My 1989 9.9 hp 4 stroke is having trouble when above about 1/4 throttle. Below that, it runs great and has for weeks. But over 1/4 throttle, especially when it's warmed up, it acts like it's running out of gas. Any thoughts? The carburator has been soaked and cleaned carefully.

Thanks!
 
Did your carb cleaning include removing the pilot and main jets? The orifices of these 2 jets need to be completely clear of debris. What is the status of primer bulb when engine starves for fuel?...collapsed, normal? You should be able to feel fuel flow through the bulb just by lightly holding the bulb in the palm of your hand as the engine is running. The higher rpms, the more flow you should feel. Try pumping the primer bulb as you get over 1/4 throttle, see how the engine reacts. Reply back with results.
 
Thanks ClassicAQ!

Yes I removed the pilot and main jets. I took the whole thing completely apart, sprayed every hole (including the teeny tiny ones) and soaked it. Since then I've only used ethanol-free premium.

The bulb is relatively hard at low throttle, then it becomes soft at higher throttle. It doesn't collapse, but it also doesn't have any pressure on it. When I pump it (at high throttle), the engine runs just the slightest bit better, but still acts like it's running out of gas.
 
Thanks ClassicAQ!

Yes I removed the pilot and main jets. I took the whole thing completely apart, sprayed every hole (including the teeny tiny ones) and soaked it. Since then I've only used ethanol-free premium.

The bulb is relatively hard at low throttle, then it becomes soft at higher throttle. It doesn't collapse, but it also doesn't have any pressure on it. When I pump it (at high throttle), the engine runs just the slightest bit better, but still acts like it's running out of gas.


Are you running the fuel line off a portable tank or below deck tank? Sounds like a fuel line restriction between tank and bulb. The bulb should not be soft at higher rpms. These new portable tanks with their EPA safeguards cause havoc on the fuel system, same goes with below deck fuel tank pick up too. If you have a portable tank and it's got the newer EPA cap, try getting a hold of the older style vented cap and see if it makes a difference.
 
I'm running a portable tank. Lifting it up to be level with the motor doesn't help.

It has a vent in the cap. I've looked at the fuel hose inside the tank and it appears fine but I can try someone else's gas tank and hose this weekend to see if that helps. Thanks for the suggestion - it would be fantastic if the issue was something so inexpensive to fix.
 
Ok, so I ran a test last night where I opened up the carburator drain, and pumped the fuel line pump. LOTS of fuel flowed through. I think that rules out the tank, line, and fuel pump (am I mistaken?). From past experience with lawnmowers, snowblowers, and this motor, i've never seen a carburator issue which didn't result in idling problems. My motor idles fine.

So does that mean the issue has to be downstream of the carburator?

The engine compression is good. The one thing I've noticed is that the oil on the dipstick has a faint gasoline smell.

Any ideas where, given the remaining options, my poor high-throttle operation might be coming from?
 
Well... based on this article: http://www.ebay.com/gds/5-Common-Symptoms-of-a-Malfunctioning-Fuel-Pump-/10000000177634796/g.html and working through the troubleshooting guide in the service manual, the next thing for me to check was the fuel pump.

After pulling off the hoses and running the starter I definitely feel that it's generating pressure. I don't have a gauge, but it seems like plenty to move the necessary fuel. I also can't blow through the wrong direction, so the one-way is solid.

So... that leaves me at a loss. To recap:

- Fuel coming out of the tank fine - hose ball pushes sufficient fuel through to carburetor bowl, and stays hard when motor not operating.
- Fuel pump seems to be working
- Motor has good compression
- Motor runs fine for first 10 minutes of operating and then sputters above 1/4 throttle.

Any other ideas?
 
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The only way to know if the fuel pump is functioning correctly is to hook up a vacuum meter. I know you think it's functioning correctly, but a meter test is required in order to completely rule it out. When you opened the carb drain screw and pumped primer bulb, this would not mimic the function of the fuel pump. Because you're pushing in fuel, while the fuel pump pulls fuel via crankcase pressure.
 
Thanks ClassicAQ.

I don't have a meter, and if heating up is the source of the problem I'd have to test it hot. In the meantime, I figured there are two tests I can do after the motor starts sputtering.

One is to bypass the fuel pump and hoist the fuel tank above the level of the motor.
The second is to hose down the fuel pump with water till it's cool (I guess the new fuel pumps are water cooled to avoid vapor lock).

Hope to try them next time I'm in the water. Thanks again!
 
Thanks ClassicAQ.

I don't have a meter, and if heating up is the source of the problem I'd have to test it hot. In the meantime, I figured there are two tests I can do after the motor starts sputtering.

One is to bypass the fuel pump and hoist the fuel tank above the level of the motor.
The second is to hose down the fuel pump with water till it's cool (I guess the new fuel pumps are water cooled to avoid vapor lock).

Hope to try them next time I'm in the water. Thanks again!

I'm confused, or you're confused, about how your fuel pump operates. As you can see in the parts blow-out, your fuel pump is not a mechanical pump, therefore it would not heat up from friction caused by moving parts. The fuel pump pushes/pulls fuel via the diaphragm due to changing pressure in the crankcase. The first test you mentioned of gravity assist is a useful diagnostic test for troubleshooting fuel pumps. The 2nd test is not.
fp.jpg
 
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Well, I did the tests described above, and neither of them made any difference on the motor stalling at higher throttle when it's warm.

As a matter of fact, I got a misfire while doing the experiment, which I think is the result of excess fuel. So that leads me to believe that it's got enough gas.

So... the other options are electrical and oxygen. I know there's a thermostat, but I was under the impression it only adjusted the idle speed. Does it do anything else?

Back to the drawing board, I guess.
 
Well, I did the tests described above, and neither of them made any difference on the motor stalling at higher throttle when it's warm.

As a matter of fact, I got a misfire while doing the experiment, which I think is the result of excess fuel. So that leads me to believe that it's got enough gas.

So... the other options are electrical and oxygen. I know there's a thermostat, but I was under the impression it only adjusted the idle speed. Does it do anything else?

Back to the drawing board, I guess.

Hi, I'm sorry to bother you and butt in on your thread but I seem to have exactly the same problem with my Yamaha 9.9 4 stroke. Did you manage to get yours sorted? If so, could you please let me know what was wrong with it.
 
I am also having the same issue. It is almost like the timing is not advancing?? I have the green oil pressure light so I know I have good oil pressure. I have heard that if there is an oil pressure issue that it will retard the timing?? is this controlled by the ECU? I am getting plenty of gas as when it bogs I can smell unburned gas from the exhaust and my primer bulb is hard? My motor is a 1989 9.9 4 stroke? Thx for anyone that can give advice on this.
 
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