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dies on acceleration,

Discussion in 'Airheads' started by trabbart, Mar 22, 2013.

  1. trabbart

    trabbart hellrider

    Joined:
    Mar 8, 2013
    Oddometer:
    47
    i beginning to think i bought a basket case,, now the bike dies when accelerating.


    i did a short testrun when i bought the bike and then it accelerated nice . did some work to make it a cafe racer, after some changes i did ,,like clip ons etc it would not start easy and mostly runs on one head when it did, i just replaced the old coils, sparkpugs and wires , starts a lot faster, nice idle. run on both cilinders again,,,i was happy with the result!!!
    points look like brand new, and are installed by a dealer at po (i have the bill) ,,i left them ,,,,
    but the testrun was not wat i expected. when accelerating it just dies at around 4500/ 5000 rpm, the red alternator light lights up when the engine dies and then suddenly it fires back up while rolling. alt light go's out ,,bike runs fine but dies again when giving full throttle,,,, its just a second or two like running out of fuel,,but once in a while it just accelerates fine but mostly sputters and dies for a second or two to pick back up after rpm dropped. fuel filters never seems empty. tank is vented moren than should be.

    i just dont know if it is fuel related or an elecktrical isue. checked kill switch, wiggled the key , did not touched the carbs, so nothing changed on the fuel thing,,,,,,

    new sparkplugs where black as hell after a short testrun,,,i think i an not finished with this bike :huh

    clutch is slipping also, i have made some more play at the lever , but i think it will cost me a new clutch verry soon,,,, :( not a drip of oil to see ,,
    i think this bike has more miles on it then i did in 25 years on the road.
    #1
  2. regomodo

    regomodo Been here awhile

    Joined:
    Jul 14, 2012
    Oddometer:
    188
    Location:
    Bristol, UK
    Check the float heights, my bike would cut out for a while when going flat out for ~30seconds. Turned out both float heights were about 4mm too low.
    #2
  3. trabbart

    trabbart hellrider

    Joined:
    Mar 8, 2013
    Oddometer:
    47
    was my first plan,,,but,,
    when i open the floatbowl,,will the gasket be ruined?
    any helpfull link would be high appreciated on how to set the floathight
    #3
  4. ME 109

    ME 109 Long timer

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    Maybe, if the bowls haven't been off for a long time. You can at least try, undo the clip and give the bowl a few taps with a piece of wood. (solid, but soft)

    Does it cut out immmediately, or sputter and fire, sputter and fire?
    #4
  5. regomodo

    regomodo Been here awhile

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    The way I did it was to turn the petrol off, unclip the bowl , empty the petrol, place the bowl back holding it in place with my hand, turn the tap on and wait, turn the tap off, carefully remove the bowl, measure the height of the fuel from the center of the bowl with some kind of dipstick. Bend the float tab to suit.

    The float height itself depends and Snowbum's site probably has it.
    #5
  6. trabbart

    trabbart hellrider

    Joined:
    Mar 8, 2013
    Oddometer:
    47
    ok i checked float hight,,all ok, bike has new floats,,wite as snow. it was at 25 or 26 mm ,,maby a litle higher than recommend.

    i dont know the bike verry well. i noticed now that i was running on reserve fuel. but only one petcock was pionted to the reserve,,the other to full. maby that was causing the problem. maby one petcock cant deliver the fuel fast enough at high speed?
    #6
  7. ME 109

    ME 109 Long timer

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    Maybe,:wink:

    The tubes that form part of the fuel tap can become blocked. Some bikes have brass screens over the outlets which can also block.
    #7
  8. Plaka

    Plaka Brevis illi vita est

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    Do a fuel flow check. NFO and directions.specs on snowbums site. Takes 5 minutes and all you have to do is pop the float bowls, run gas into a measuring cup and time it with a watch. if not enough gas, fix it, starting from the petcock screens at the top, then the fuel filters then the carb passages.
    #8
  9. Disston

    Disston ShadeTreeExpert

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    What year/model BMW motorcycle do you have?
    #9
  10. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Sounds like an electrical problem.

    --Bill
    #10
  11. Padmei

    Padmei enamoured

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    +1

    take the tank off & carefully check the wires running to the coil
    #11
  12. Disston

    Disston ShadeTreeExpert

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    Also check the wires going to the diode board. Look for wires being pinched by the front cover.
    #12
  13. Plaka

    Plaka Brevis illi vita est

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    Alt light always comes on when the engine dies---no alt output.

    If the issue is dependant on speed and it is a "dies" issue rather than runs like crap, and it's consistant, then it likely isn't electrical. if it runs like crap past a certain RPM, you might suspect the ignition. The voltage regulator might be cutting out around 4500 and shorting something that kills power---sort of a long shot. Using more gas at higher RPM is a sure thing and if the flow is at a minumum at lower RPM you will always lose it higher up.

    A fuel problem is also the fastest thing to rule out. You could be working on an electrical issue for some time. So I would start by working on fuel.
    #13
  14. Bill Harris

    Bill Harris Confirmed Curmudgeon

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    Or, alternatively, the alternator output (to the blue wire to the ALT lamp) is good and the 12v reference (other side of the lamp) is off.

    --Bill
    #14
  15. Plaka

    Plaka Brevis illi vita est

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    The alternator output is never good if the engine isn't running. but I see your point, the engine is still spinning even if it isn't making power. So the question is when exactly and at what RPM does the light come on.
    #15
  16. trabbart

    trabbart hellrider

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    i dont think its fuel related. it dies to sudden, and on both cilinders at the same time.
    all the work i did was electrical, moved the headlight with after market clamps, , changed blinklers on the board, replaced the old coils , moved the egnition (key)switch.

    about that switch,,it had a bad contact inside before , when i wiggled the key, the power went off. i used some contact cleaner and it holds power when wiggeling.
    i suspect the wireing to the coils,because i have been working mostly on that wire bundle, i hope to find it this weekend.

    my next testrun cant be done this weekend ,,it just started snowing. next testrun i must be sure that all power is lost when it happens, i dont know if the headlight dies also. and i believe that the orange light came on also,
    #16
  17. trabbart

    trabbart hellrider

    Joined:
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    Oddometer:
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    found it!!!!!!!!!!!!!!!!!!! yehaaaa

    i started it up this morning in the garage, damn it fires right up from stone cold,,,its freezing and snowing,,,,that new coils and wires did a huge improvement.

    i started to wiggle with the big wireing harnass comming from out of the headligt going to the rear,,,it died, and not a single light,horn blinker wanted to work anymore ,,,,,,started messing with it again and poef,,power back...i now nknow its a broken wire or bad connector or connection in that bundle somewhere under the tank,,,just have to find the exact place with a testlight or ohm meter.... simple as that...

    at least i also learnt how to check float ,,,
    and the red controlelight does not mean you still have electrical power . if standing still and use the klill switch you dont have a red light,,,but i bet if you kill it while riding you will get the red warning light....
    #17
  18. chasbmw

    chasbmw Long timer

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    Dying at 4500 rpm is classic float bowl too low territory. How do you measure?
    #18
  19. ME 109

    ME 109 Long timer

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    8,648
    Location:
    Albury Australia
    Bingo!

    But we'll keep the half prize money.
    That was a 50/50 chance.
    (government lottery)
    #19
  20. trabbart

    trabbart hellrider

    Joined:
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    low bowl would give more sputtering and one cilinder at a time that has problems i guess. i had a complete and sudden cut off. and then comming back to life with some sputtering . i think vibrations and wind on the cables make it happen at that rpm.

    i have done a search how to mesure on this forum,,
    close the petcock, take the bowl off, remove gasoline, put back on, open petcock, mesure from deepest point. i had more than the 24mm on both carbs.

    and gas was comming verry fast out of the tank,,all new pipes also.
    #20