Carby Carb running dry at WOT

Discussion in '2-Stroke Engines' started by lazylightning@mail.r, Oct 27, 2011.

  1. lazylightning@mail.r

    lazylightning@mail.r Active Member


    I've done alot of researching and I know the only possible reasons for my problem, just thought I'd try and get some feedback.
    So when I open her up, full throttle at once, it responds pretty fast. Pretty good torque for all stock F-80(66cc)-(except for NGK B6HS plug). 35 tooth rear sprocket, so top end is not bad. I have no problem on steep hills if I peddle hard in the proper gear. But the strange thing is that about ten seconds after winding her up, it starts to cut little spurts and then it bogs down and dies if I keep holding the throttle wide open. Exactly the same way it does if I turn off the fuel and wind her out. Okay, this is a no brainer right? I even quickly removed the plug to make sure it's not soaked, nope, as close to dry as just a little oily can be (still using 16:1 at about 350 km).
    I have checked the fuel flow rate from the end of the hose after the inline paper filter cyclinder and it flows to the carb inlet at a good rate, or so it seems. I'm sure it's more than enough for that little pin hole of a jet, lol! I have adjusted my float, and I really cant get it any higher because of the push pin that is supposed to push down the float and drive fuel up and into the barrel. I've read 21mm float height here, and my float is so thick, plus the 4 mm length of that push pin pointing down. There is just no way I can get that outer(lower) float edge to be 21mm from the mating surface of the carb to float bowl. It's probably about 25mm. Geeze the float is so fat, it's about 20mm thick. I'll do an exact measurement when I pull it out again soon. I'm thinking of how to bend that tang so that it just barely closes the flow needle-valve before the float bottoms out on that push pin end. But it's already 1 mm from it now. WTF is with that? Can that float be sticking on the outside of the jet tube so regularly? How can that inflow needle-cone valve be sticking closed? If I keep running it, it takes a long time to refill, if I turn it off for a second (10 seconds), then the bowl is full of gas again, enough to wind her out for another ten seconds or so. Why could that inflow valve be so restricting the inflow? Maybe it was mis-machined and they assemble carbs with such cone valves with absolute disregard to the size allowances that should be obeyed in assembling a carb of this type? Or maybe the seat/brass screw in inlet tube has too much meat on/in it?
    Just food for thought.

    Thanks in advance for your ideas,
    Last edited: Oct 27, 2011

  2. Al.Fisherman

    Al.Fisherman Active Member

    Post too long to read, for me anyway. What carburetor are you working on?

    Carburetor running out of gas...
    1. Float needs to be adjusted
    2. Blockage in the fuel valve/screen
    3. Tank needs to be vented.
    Last edited: Oct 27, 2011
  3. motorpsycho

    motorpsycho Active Member

    possibly your float level is set too low or you have a blockage in the screen filter inside the tank. (like Al already said)
    the float bowl is running out of gas faster than it can be re-filled. if there's a blockage in that screen filter, it will impede the fuel flow. It may visually look like it's flowing enough, but it is possible that it isn't.
    when a float is set right, the float bowl should never run dry.
    personally i ignore that 21 mm float adjustment rule with these carbs(the n.t. carbs that is). I adjust the float until the carb floods over, and then drop it a little at a time until it no longer floods over. I never measured the height, but this is what works for me. no 2 carbs will be indentical, so i adjust them this way and it works.

    or your gas cap vent isn't venting like it should.
    try running it with the gas cap off and see what happens.

    you are positive that the float bowl is empty after the engine dies?
    when the engine died did you check the float bowl right away with the fuel petcock still open?
    if so, you could have a flow problem. After the engine dies, the fuel flow shoudl fill the bowl back up rather quickly.
    if it's running too lean, sometimes it will do a similar thing. Normally tho, you will get a bad bog at w.o.t. rather than the engine dying out and the float bowl will be be full.
    Last edited: Oct 27, 2011
  4. blue 48

    blue 48 Member

    i took the in tank filter out on my 2 bikes and put in quality NON paper filter in line. for some reason ive always had filter closer to the carb than the tank (well that works for me)

    yep check ur filler cap too, i get better flow from the 3l tank i got most likely due to being heavier when full???
    Last edited: Oct 28, 2011
  5. gothicguy64

    gothicguy64 Member

    hi if as u suspect to be ur carby have u not thought of these ?

    i myself run a dellorto on my rse motor
    and sbp gear
    on a cell bike
    8 gears ,expasion , fully aussie rebuilt an aussie billet head , inlet . clutch arm,cover on a chinese frame ,jap gears on a aussie bikeshop sticked bike.
    ohhh its a rse hp1 with extras to hp2 stds 48cc of fun
    an im loving passing cars on it

  6. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Thanks for the advice guys! I'll try to clean that tank screen and get a new inline filter, maybe non-paper too. My float is adjusted to its upper most position and it seems the float bowl just cannot overflow in any float position. Strange and keeps me thinking. Carb is stock on mine.

    Yes, I've thought of the Delorto carb, and the Jaguar ignition and expansion pipe. I'm on a student budget here and this stuff will have to be shipped to me when I do get money for it.
  7. lazydog

    lazydog Member

    Try this next time it happens, loosen the cap or take it off.

    This drove 2 friends and myself insane for close to a month, Everything runs great low to mid all day, open it up and it only takes a few minutes. Opened the tank on the side of the road just to make sure he wasn't outta gas and "psshht". Seems some gas caps are a little to tight and don't vent right.
  8. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Nope. I had that problem figured out to start with. For a while I would just open the cap for a second every few minutes and then later I drilled the tiny hole to vent it. That was back last spring ;-) Thanks though.
  9. HeadSmess

    HeadSmess Well-Known Member

    everyone i talk to asks "you hear about how theyre trying to ban these?"

    slow the **** down, use ya brain, or well all lose the priviledge! go fast on empty roads in the sticks, not in traffic in populated areas!

    probably one of these people that rides on the footpath expecting everyone to get out the way too...

    this is nsw, the nanny state. remember that.

    *end rant*
  10. lazylightning@mail.r

    lazylightning@mail.r Active Member

    I had to drill out the brass inlet feed tube that the needle closes the flow in. It was just too narrow. Now it works like a charm. I carefully removed the burs so the needle could sit well. I swear, it's even faster now too, though I dont know how that could be...
  11. gothicguy64

    gothicguy64 Member

    hmmmmm where to start .....

    aussie law is 200 watts of power
    aussie law is the sign posted speed limit unless on path then its 20kph
    pity im a sandgroper living in dirty sydney full of bloody arab imports
    an i usally do 40 kph but i am able to go much faster an do if its safe to do so like in places i do 60 on the hume goin into the city an it freaks cars an m/bikes on a pushy doin 60 ........
    the rd from penrith is much better for doin 70 its downhill
    up i do maybe 55 ...
    an yes i am worried i ride a outlawed pushy (its well over 1800 watts )

  12. olow

    olow Member

    :helmet: so hows it running now:poop:
  13. lazylightning@mail.r

    lazylightning@mail.r Active Member

    I drilled it out to a 1.5 mm. Yes, it was actually smaller than that. I don't know if all of the stock carbs have such a narrow inlet tube/float bowl valve seat, maybe I just got lucky. It may be kind of a top speed governor mechanism.
  14. lazylightning@mail.r

    lazylightning@mail.r Active Member

    Hi Guys!

    I think I made a significant discovery here. The brass inlet tube/float bowl valve seat had a very narrow feed hole down in there. It was about 1 mm in diameter and drilling it out to 1.5mm did the trick. Has anybody else had troubles with getting enough fuel into the stock carb float bowl at high speed? I had to use a larger bit to widen the orifice so the needle valve could be pushed up in it easily and have a better seating surface. To which I carefully tapped an object with the same cone form and size of hard metal just as the valve seat. Don't hammer on that valve, it has a very narrow support at the end. Now the valve never misses the hole and it seals perfectly and I get enough gas to feed the motors needs. I think that this is signifigant because that 1 mm orifice was just not feeding enough gas in there fast enough and my jet is bored out to only .71mm.
  15. Al.Fisherman

    Al.Fisherman Active Member

    No, I've never hat that problem out of 5 builds, all using a NT carburetor.
  16. motorpsycho

    motorpsycho Active Member

    Me either.
    4 builds ann using n.t. carbs.
    they will run at w.o.t. as long as you want them to.
  17. lazylightning@mail.r

    lazylightning@mail.r Active Member

    That's strange, but then again, that which the Chinese ship to Russia may differ from what is shipped to the US for some unfounded reasoning on the shipper's part. Maybe they think the Russian government is strict or something and they don't want people driving fast, or maybe it's just the kind of thing that is required of them inside of China and the same got shipped to Russia. A 1 mm inlet hole. Check your's the next time you have it off.