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  1. #1

    Default High Throttle Stalling, Yamaha 9.9 4 Stroke

    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!

  2. #2

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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.

  3. #3

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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.

  4. #4

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    Quote Originally Posted by fritz3000g View Post
    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.

  5. #5

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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.

  6. #6

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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?

  7. #7

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    Well... based on this article: http://www.ebay.com/gds/5-Common-Sym...7634796/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?
    Last edited by fritz3000g; 09-25-2017 at 12:10 AM.

  8. #8

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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.

  9. #9

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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!

  10. #10

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    Quote Originally Posted by fritz3000g View Post
    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.
    Click image for larger version. 

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    Last edited by ClassicAQ; 09-28-2017 at 11:45 PM.

  11. #11

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    plz reference below picture.

    (butterfly valve opening grade of relate carburetor fuel parts)

    Click image for larger version. 

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  12. #12

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    I not sure why the fuel pump performs poorly when heated. It's attached metal-to-metal with the engine, so heat would certainly move through. In any case, it appears to be a problem others have experienced. Here's one thread:

    http://www.yamahaoutboardparts.com/f...ng-th3901.html

  13. #13

    Default Re: High Throttle Stalling, Yamaha 9.9 4 Stroke

    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.

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