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Thread: Fuel demand valve troubleshooting.

  1. 06-30-2022, 05:43 PM #1
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    Default Fuel demand valve troubleshooting.


    2018 Yamaha F70 wouldn’t run today. Primer bulb would not get firm, but would move some gas when I pressed the ball on the end of the hose. I went straight to the fuel demand valve, thinking it was a a on/off valve. It would release some vapor when I pressed on the top of it with a screwdriver, but bulb still would not get firm. I disconnected the hose downstream of the demand valve, nothing happened. When I disconnected the upstream side, it was pressurized. I removed the valve all together and the bulb would firm up, but I noticed a small drip on the fuel/water filter. I was able to replace the o-ring and stopped the drip.

    Is there any troubleshooting that can be done with these valves? Should I reinstall it, buy new, or just leave it out?


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  2. 06-30-2022, 06:21 PM #2
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    Forgot to mention some important details. Last time I ran it, it got on plane and then dropped off to about 2/3 WOT. After a few minutes, it would get back to normal WOT, but idles rough. I replace the fuel filter under the cowl and am waiting on the water/fuel separator because I ordered the wrong one at first.


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  3. 07-05-2022, 11:41 AM #3
    jigman09
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    You might want to ask this question on the Yamaha owners group on Facebook. The page is widely used and the information is spot on.


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  4. 07-06-2022, 08:22 AM #4
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    Might get lucky replacing the bulb. Low cost stab at it. Fixed my woes a while back, I went with an Attwood Bulb that claimed to be 33% bigger, definitely pumped up better and quicker. Cold starts much better.
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  5. 07-13-2022, 10:49 AM #5
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    Replace the demand valve. It is a glorified one way check valve. I know they aren’t cheap but kinda a safety feature plus is supposed to help keep primed.


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  6. 07-13-2022, 01:37 PM #6
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    Quote Originally Posted by Cray View Post
    Replace the demand valve. It is a glorified one way check valve. I know they aren’t cheap but kinda a safety feature plus is supposed to help keep primed.


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    Thanks Cray. I never did find a detailed diagram. Correct me if I’m wrong, but it seems like a regulator also, which allows fuel to flow when the downstream decreases below a certain value. When I first removed it was restricted both ways, I gave it a few love taps and it started allowing flow in the 1 direction. I’m tempted to reinstall since the valve doesn’t look to have many components to fail and may have had some trash in it.


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  7. 07-13-2022, 08:28 PM #7
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    Quote Originally Posted by TigerFanatic1 View Post
    Thanks Cray. I never did find a detailed diagram. Correct me if I’m wrong, but it seems like a regulator also, which allows fuel to flow when the downstream decreases below a certain value. When I first removed it was restricted both ways, I gave it a few love taps and it started allowing flow in the 1 direction. I’m tempted to reinstall since the valve doesn’t look to have many components to fail and may have had some trash in it.


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    That’s exactly how they are supposed to work. If going to try again I think I might try spraying it with carb cleaner in both sides. I figure it has some buildup in it and probably sticking. Had to replace one on a friends boat about 6 mo ago. Tried everything but could not get it to function right. Did some reading on it that’s when I figured out how they are supposed to work.
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  8. 07-14-2022, 09:38 AM #8
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    Great idea to clean it with carburetor cleaner.


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  9. 07-16-2022, 12:27 PM #9
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    Ended up being the plugs, runs great after replacement. I sure thought it was fuel related


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