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  1. #1

    AFR not consistent at different times of day

    OK this is a weird one.

    I've been noticing that on all day rides my AFR is where its supposed to be in the morning but at the end of the day on the way back to the ramp it's higher by a significant amount to the point I have to baby it home.

    Today was another day like that, in the morning it was all good but late in the day on the way back it was up by close to 1 AFR point at the same RPM point. I thought maybe the MAP sensor is acting up and sending wrong signal but logs show its consistent with the morning value and so is the throttle position so it couldn't have been getting more air. Timing was also the same.

    Only thing I can think of is fuel supply. When I got home I hooked up an accurate digital pressure gauge and checked it. It looked normal, idles at 58.2 and quick blip to 5k RPM it was at 57.9 I believe. This was close to 3 hours after the ride so everything was already mostly cooled down and the quick throttle blip might have not been long enough for fuel to drain from the rail to show significant pressure drop.

    I have a walbro pump which has about 20 hours on it and a Aeromotive regulator which is probably 5 years old. One of these must be having some issue. I have a theory which might be a little out there. On a all day ride a ton of heat builds up in the engine bay, the fuel itself must be super warm. Is there a chance the "rubber" diaphragm in the regulator, somewhat old now, is getting softened by the warm fuel/heatsoak and is not able to hold the set fuel pressure?

    I can't log fuel pressure but here is a log from today, first pic is from the morning and second is from afternoon.

    I know "install a pressure gauge for next ride" response is coming but I don't really want to go on another all day ride with this issue. Next time it might be worse.


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  2. #2
    tiderider's Avatar
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    How much of a weather temperature swing was there from morning to late in the day?


  3. #3
    Quote Originally Posted by tiderider View Post
    How much of a weather temperature swing was there from morning to late in the day?

    Air intake temp is in the log screen shots, there was no difference basically. All the logged parameters are very close except for AFR. There's no way weather can swing AFR that much or there would be motors blowing up left and right.

  4. #4
    JT jpt7779's Avatar
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    IMO, this is due to heat soak. Do you have a cold air intake or is it buried inside the hull.

    Year and model info would help.

  5. #5
    Quote Originally Posted by jpt7779 View Post
    IMO, this is due to heat soak. Do you have a cold air intake or is it buried inside the hull.

    Year and model info would help.

    This is on a RXP 260 with the usual kanaflex above the gas tank intake.

  6. #6
    ptscon's Avatar
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    If air temperature was rising the AFR would be going in the opposite direction, richer. It does sound like not enough fuel is getting to the cylinders and since the tune did not change than fuel supply must have.

    Could the walbro be starting to show signs of failure already as it heats up through the day?

  7. #7
    Bob 1tommygunner1927's Avatar
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    Had a similar situation years ago. In tank fuel line cracked spilling off fuel pressure and shortening the life of the Walbro, which was producing lots of noise. Not saying this is your situation but its worth inspecting.

  8. #8
    Quote Originally Posted by 1tommygunner1927 View Post
    Had a similar situation years ago. In tank fuel line cracked spilling off fuel pressure and shortening the life of the Walbro, which was producing lots of noise. Not saying this is your situation but its worth inspecting.

    You were seeing pressure drop after things heated up? I'm going to pull the pump today to have a look if there's anything obvious.

  9. #9
    Bob 1tommygunner1927's Avatar
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    Quote Originally Posted by aperxp View Post
    You were seeing pressure drop after things heated up? I'm going to pull the pump today to have a look if there's anything obvious.
    At the time, my ski did not use a fuel pressure sensor. I kept needing to increase pressure manually, and the pressure would bleed off immediately after shut down. But it makes sense the fuel pump would heat up with higher pressure and bleeding off fuel which could explain your change in AFR's.

  10. #10
    Took the fuel pump assembly out and everything looks normal. Not gonna waste time here, I'm replacing the pump, since they're not expensive, as well as the oem plastic pickup tube with submersible fuel hose.

    Seeing how simple the internals of the regulator are I don't see how fuel warming up could affect the spring enough to drop pressure so much to cause close to 1 AFR point change.

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