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Air lock in fuel line?

Discussion in 'The Garage' started by gpounce32768, May 20, 2023.

  1. gpounce32768

    gpounce32768 XS400 Enthusiast

    I've been shaking down the '82 XS today- took it out on the highway and pushed it hard, it rides pretty well but I got stranded twice on the high-speed runs (70mph) both times for 5 mins or so- apparently so the bowls could refill at which point the bike started right up.

    I had put a transparent filter in which I <think> was keeping an air bubble in the fuel line causing the bowls to empty under the WOT. The symptom was sudden loss of power and stall, attempts to start on the side of the road which eventually succeeded- apparently after enough fuel trickled into the bowls.

    When I made it home I pulled the carb end of the hose and let the hose drain back into the gas-can, I did see evidence of air-lock up around the filter- so am fairly confident that was the problem.

    I replaced the filter assy with a simple hose, hoping that the air bubbles will have a chance to escape. I have some transparent fuel hose coming- I do want to be able to see the gas.

    The petcock does flow well so I don't think the constriction is in there. So I <think> its handled but am wondering if there is any other experience out there with somewhat weird fuel flow issues.
  2. CaptChrome

    CaptChrome XS400 Addict Top Contributor

    The only fuel delivery related issues can be from (in order from the tank):
    Petcock net filter in the tank is fouled with debris.
    The petcock itself is blocked with internal buildup or not opening fully under vacuum.
    Too small of a fuel line causing a restriction.
    Too restrictive of an in-line filter.
    The net filter below the carb bowl fill valve seat is clogged.
    The carb bowl fill valve is not opening fully due to incorrect float level setting or leaking float.
  3. gpounce32768

    gpounce32768 XS400 Enthusiast

    It stranded me again the same way with a plain hose, so I don't think its air-related. But I have some transparent fuel line coming so I can see whats going on in there. I opened up the petcock, a bit of crud in there and the o-ring was tired so I have a mikesxs rebuild kit coming. Yeah I'm thinking at the moment that its old crud washed down into the fuel rail and is interfering with flow thru the float valves.
  4. gpounce32768

    gpounce32768 XS400 Enthusiast

    Got the carbs out today, nothing conclusive in the fuel rail- nice and open, no crud anywhere- compressed air didn't blow anything out and it flows fine. Any chance of the little conical screens over the float valve bodies constraining flow or somehow being naughty? I'm inclined to think not- brakekleen went thru with no problem.

    The floats are empty and moving free, so would be floating fine and they'd both have to be jammed up to cause the bowls to empty. Level is set as per the external tube test in the service manual- its right on the money.

    The symptom common to all 3 times I got stranded is after 5-10 mins or so the engine started right up and ran normally w/ its usual power but before then was acting like no fuel. I even had it happen sort-of a 4th time in front of the garage complete with an epic backfire. The first 2 times I was flogging the bike down the highway, 3rd time was idling waiting for a light, idling in front of the garage too..

    Will put it all back together once I get the petcock rebuild from mike's. Kind of makes me wonder about something else perhaps heat related in the engine... next time it happens I'm going to test spark and check the plugs for wet/dry. Compression sounds fine as per the starter, it cranks like a full charge on the battery and the headlight is at its usual bright-ish luminosity.
    Last edited: May 22, 2023
  5. CaptChrome

    CaptChrome XS400 Addict Top Contributor

    So, what were the symptoms of the stranding? Like were you running at high speed and then the engine started loosing power over a period of time, or did it just quickly stop while at speed?

    Looking at the plug color would be important here to help in the diagnosis. Getting a bike really heated up to temperature can sometimes induce rich or lean conditions that you might not see. I just had a lean condition where the bike started up normally and then went lean after slightly loose carb holder bolts heated up and expanded some allowing an air leak (and the bike did not want to restart without choke when hot). Look at the entire intake system, even outside of the carbs
    gpounce32768 and tstidham like this.
  6. gpounce32768

    gpounce32768 XS400 Enthusiast

    The 1st two times were a rapid stall at near WOT like it ran out of gas. At the light I was just getting going after a green, but the same rapid stall. It wasn't a reduction in power as in running out of gas and needing to switch to reserve- the engine just rapidly and totally cut out. During restart attempts on the roadside, enrichener and petcock prime made no difference though I don't think I can exclude flooding as a cause- seems a stretch but it would make more sense than carb bowls running empty. Perhaps that might suggest the float valves NOT closing for some reason; that stuff is all new from a carb rebuild kit.

    Yeah I like the weird rich/lean thing. The boots and all are snug but I know what you mean about vacuum leaks- I had that dance on my old Bandit for a while. Given the unclogged fuel line I'm definitely going to look into the plugs/spark etc next time it happens.
  7. tstidham

    tstidham XS400 Addict

    I don't feel like that is fuel. When I've forgotten to turn my petcock on, I can go a few blocks before she starts acting up and starting to bog and get jerky. Straight cutting out is usually a loss of spark as capt chrome is alluding too. Also, it would be unlikely both carb bowels would empty at exactly the same time.
  8. gpounce32768

    gpounce32768 XS400 Enthusiast

    Agreed. Going out with tools next time to check spark and sparkplug next time it quits.. oh well this is an excuse to get the emulsion tubes out of the carbs anyhow, I cheaped out and didn't pull them 1st time lol
  9. gpounce32768

    gpounce32768 XS400 Enthusiast

    Put on a transparent fuel line and brought a spark tester with me on a ride, got it die on me again. Definitely not fuel, the petcock flows fine. It takes about 10-15mins ride with moderate throttle eg 20 to 45mpg roads. The 1st warning was an odd pop, then about 15secs later it died on me in the usual way; no gradual sucking just a stall and dead. I put the spark tester on, got no spark- it doesn't fit great but I'm 80% sure it would have shown a spark- will have to test it later tonight when I have a chance.

    Voltmeter showed battery at 15v peak, 13.5 or higher at normal rpm's- no signs of overvoltage. The regulator was not blazing hot, nor was the ignition module.

    But after sitting by the side of the road for 10mins or so it started up nice and easy and I rode home. So I'm thinking ignition module... going to pull it, perhaps see if I can get a salvage replacement and try that?
    tstidham likes this.
  10. gpounce32768

    gpounce32768 XS400 Enthusiast

    lol now I see the TID12-07 scarcity for the Seca... yeah thats super-fun, so will dig into the unit and see if its sick and if so, repairable. I have it off the bike and the lid is off, going to test the pickup coil before desoldering the board... just like working on an old stereo lolz
    tstidham likes this.
  11. gpounce32768

    gpounce32768 XS400 Enthusiast

    I opened up the TID12 and took out the board. I sure love the old-school Hitachi construction- good stuff in there. No sign of solder fatigue or cold joints under the binocular scope or leaky caps/burned parts on the inside. The pickup coils measured fine and looked good visually, so I don't have any strong leads. Kind of thinking that the way forward is part swaps for the TID and/or pickup coils but I'm kind of tired of spending real money on the bike... unless anyone has other ideas?
  12. tstidham

    tstidham XS400 Addict

    Since it seems to be heat related, the pickup coil should be suspect. Maybe pull the cover off and ride, see if air cooling helps it go longer?
    gpounce32768 likes this.

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