Stalling during decelleration

Discussion in 'Battle Scooters' started by techguy, May 7, 2011.

  1. techguy

    techguy Scooter Trash

    Joined:
    Oct 13, 2008
    Oddometer:
    845
    Location:
    Sacramento, CA
    I have stalled twice in the past week... once on the interstate. I am riding my Kymco P250 with just about 35,000 km on it. Carbureted engine. I thought it might be low gas and a faulty tank sensor but it was not. I filled the tank with the amount expected based on the needle position. I tried to start and the battery started getting low. It would run the starter quickly would and try to kick over but would not start (felt like it was out of gas but I had just filled up). I got tow to the dealer and they looked it over and found no glaring problems. It did start under its own power without any external additional support. It was just stubborn to start. I ride it home, even made one stop without stalls or issues. I kept it below 50 MPH the whole ride home and was very mellow.

    This past week ( within 3 days of the previous event) I was commuting to work and i was coming home. I had just finished a stretch of about a mile of 55 MPH and I had rolled off the accelerator and was coasting towards the traffic light in moderate traffic.. I felt the bike lug and felt it wanted to stall. I made a right turn to get off the 3 lane (each-way) expressway and was going to parallel i the smaller roads to avoid the traffic dense areas in case I had another full stall. In the end, the stall eneded and I was able to continue home.


    1. Is it possible these two events were related?
    2. Would coasting (riding with the accelerator in the full neutral position, without acceleration and without any braking)
    3. it is possible that I am seeing the first stages of a failing fuel pump failure? The coasting could prevent a vacuum from pulling the fuel up from the tank. Riding under power will draw enough vacuum but coasting under may not draw the fuel required to support combustion.
    I wondered if at the gas station, I was not able to create enough of a vacuum on the gas lines to support combustion.

    Any ideas out there?
    #1
  2. Tacoma

    Tacoma Been here awhile

    Joined:
    Feb 27, 2011
    Oddometer:
    233
    I wondered if at the gas station, I was not able to create enough of a vacuum on the gas lines to support combustion.

    Any ideas out there?[/QUOTE]

    Thats the right idea, try running it with the gas tank cap off, also you have some type of filter screen-filter between the gas tank and carb, that could be clogged, also check for an intake -vacuum leak, mostly where the carb mounts to the cyl, you can check for a vacuum leak with a can of WD40 with the engine running, (I use starting fluid instead of WD40 to check for vacuum, intake leaks but it could be very dangerous)
    also check the head bolt torque , you'll be amazed the difference it would make when the head bolts are only 5 lbs off, in time the head bolts looosen up cause the hot and cold expansion and contraction of the cyl and heads and where it bolts to the engine (every part takes heat different)
    I am not up on the Kymco, but if you have an in tank electrc fuel pump you have some type of pick up screen and that could be clogged
    you should also run a engine compression or leak down test.
    #2
  3. MODNROD

    MODNROD Decisions, decisions

    Joined:
    Feb 20, 2009
    Oddometer:
    2,210
    Location:
    Midwest, West Oz
    Low float level.
    Tiny dusty stuff in idle air bleed.
    Dirty fuel outlet filter in tank.
    Bit of "flaky stuff" in tank, occasionally covering outlet on decel.

    Check float level, most mechanics at dealerships don't bother, time is money.

    Put decent fuel treatment (I've heard the US guys mention "seafoam"?) in tank.
    #3
  4. YamaGeek

    YamaGeek Ancient trailbike padwan

    Joined:
    Jun 24, 2007
    Oddometer:
    6,798
    Location:
    western oregon
    As a 'techguy' at a scoot dealership I'd think this would be something you could figure out.

    But, start at the fuel tank and work your way towards the carburetor, don't rule out a failing CDI module.
    #4
  5. techguy

    techguy Scooter Trash

    Joined:
    Oct 13, 2008
    Oddometer:
    845
    Location:
    Sacramento, CA
    I am a techguy with the computer... not the motorscooters. I wish I had more cycle skills. I do okay but no 'pro' skills.
    #5
  6. gogogordy

    gogogordy Long timer

    Joined:
    Feb 25, 2009
    Oddometer:
    2,043
    Location:
    Winchester, Southern California
    I'd be leaning towards the idle "circuit" in the carburetor....specifically the idle Jet sounds clogged. Perhaps the carb is icing-up, there may be a faulty warming coil on the carb too, or the electric choke could be stuck open.

    I wouldn't expect a fuel pump on a carbureted scooter, as typically carb. fed scooters use gravity to feed fuel, and fuel pumps only for EFI systems.

    Your Kymco may be fuel-pump equipped, but I doubt it.*

    Good luck, look for the simple first.

    *EDIT: Just checked the parts/svc manual, you DO have a fuel pump since the tank is mounted below the engine on your bike....so that may be a consideration as you suspected. In my experience, a bad fuel pump will show its weakness for a higher demand for fuel, not a lesser one but who knows? G'luck.
    #6
  7. bbishoppcm

    bbishoppcm It ain't a moped.

    Joined:
    May 19, 2010
    Oddometer:
    666
    Location:
    Bennington, NH
    I'm with Gogogoggordy on this; your idle circuit may be to blame (clogged, improper mixture, etc). A clogged idle circuit will cause it to run lean at idle; a worn idle mixture needle valve will cause it to run rich. Fuel pump failure? not likely; you'd see that more at higher speeds than anything else (electric fuel pump, which recirculates fuel through the bowl). The vacuum controls the EVAP emissions valve position. If you had poor vacuum, you'd be running lean all the time, and performance would be mediocre at best. Your bike isn't old enough to have cracked vacuum lines, but a cracked intake boot is possible; check for cracks around your intake boot (do Kymcos have rubber boots?). How's the spark plug look? It should be a light brownish color, not black and sooty (and not white and chalky).
    #7