Can't get it to run more than a few seconds - help adjusting carb?

Discussion in '2-Stroke Engines' started by TheMightyGoat, Apr 8, 2011.

  1. TheMightyGoat

    TheMightyGoat New Member

    Different problem. For those of you who responded to my chain issue, thank you, I think I have that fixed (I did move the rubber spacers inside and also changed the tension on the chain).

    On to the issue. I'm trying to start my bike for the first time. 48cc Grubee Skyhawk 2 stroke. The first time I put gas in the tank after installing everything, the result was gas pouring out the carburetor from the overflow tube as well as the air filter. I know nothing about carburetors aside from what I've searched for and read on these forums the past couple days, but I was fairly confident gas should not be flowing from it, so I took the cover off, found what serves as a valve for the gas to enter the bowl, and found I could adjust the flow by bending a piece of metal inside.

    I've had that cover on and off a hundred times in the last two days fiddling with that piece of metal, and the results of my efforts are these:

    A) The bike doesn't start at all. I presume this means it is getting no gas. To test, I put a few drops of gas under the spark plug and try again - it fires up, and then dies. So, I open the carb again and bend the thing out some.

    B) The bike starts, runs a few seconds (the longest it ran was about 20 seconds, most attempts have been more like 4-5 seconds), then dies, and immediately after it dies gas begins pouring out the overflow tube and the air filter again, and will not start again (flooded, I imagine). So, I open the carb again and bend the thing back in some, and we go back to scenario A.

    I've been assuming that there must be a happy medium between the two and that I need only find the right tension on that piece of metal, but I've been between scenario A and B a hundred times now, so maybe I'm missing something. Here are some pictures to illustrate what's going on.

    Overview of the setup:


    Cover off:




    Any ideas?

  2. motorpsycho

    motorpsycho Active Member

    are you kidding me?????

    If you have the carb mounted all the time like you have it in the pictures, it's on sideways. the intake tube also appears to be on sideways. How is the intake tube mounted to the engine? that looks like nothing i have ever seen before on one of these engines. it should look more like this (please note that this pic. was taken on a work in progress, so the fuel line is not hooked up yet)
    The float bowl needs to be facing down. (my carb may look different than yours but they all function the same way)
    With it on it's side like that, the float is not able to do it's job, which is to open and close the needle & seat when the gas level in the bowl gets low enough.
    It's flooding over because the float is made to move up & down, not in and out.
    all of that bending you have been doing to the float tab has done nothing, but now when you get the carb mounted right, your float level will be way off. it may still flood over (if the float level is set too low) or it may not get enough gas into the bowl (if the float level is set too high).
    I'm not positive but i think the float level should be 21 mm from the bottom of the carb to the bottom of the float when it's in the full down position.

    what's up with that spark plug wire? if that's just a peice of copper wire wrapped around the spark plug, you really need to get a correct spark plug wire with a rubber boot on it. You are losing voltage with it that way.
    Last edited: Apr 8, 2011
  3. TheMightyGoat

    TheMightyGoat New Member

    ...I did admit to knowing nothing about carburetors.

    The extent of the supplied instructions regarding carburetor orientation was that it should be "as level as possible". I didn't understand what "level" meant, but now I do, in regard to the float. I mounted it sideways because it was the only way it would fit in my frame. I will probably have to make up some copper fittings to mount the carb outside the frame now, as I have read some people have done.

    (The spark plug deal was temporary by the way.)

    Moderators feel free to delete this thread! :euro:
  4. motorpsycho

    motorpsycho Active Member

    well, not knowing anything about carbs. is a good excuse i guess, but all you had to do was look in the picture thread here and you would have seen that all carbs are mounted with the float bowl facing down.
    when in doubt, look at pics of stuff that other people have built.
    as for using the copper tubing...that could be a big problem if you don't get it 100% sealed at the joints. it has to be air tight or it'll suck in extra air making your engine run really lean (too much air, not enough gas). this can make the engine rev out of control, make the bike unrideable, and it can cause you to burn a hole in your piston over time.
    your best bet is to look around here because someone posted a thread somewhere about an off-set intake tube for this exact problem (not enough frame clearance). i think you can buy the off-set intake tube from one of the vedors in the list to the left, but i'm not sure which vendor it was.
    you have to have an air tight seal on the carb to intake tube, and intake tube to cylinder and a one peice off-set intake with a correct gasket is the way to do it.
  5. HeadSmess

    HeadSmess Well-Known Member!

    oh well. now you gotta get the float height right again...

    for starters, bend it to where the valve closes with the floats(white plastic bits) about 3-5mm from touching the body of the carb...

    and from there it just gets tricky with those types of carbs... but you was on the right track (if only 90` out :p )...leaks, not enough bend fuel at all when you undo the drain screw on the bowl...too much bend!
  6. yeah 90 degrees out. bowl down, its called gravity fed carbs. remember that and work out that gravity does all the work for you.

    sealing carb is easy i have wrapped ptfe tape around the intake tube and the clamped the carb over it. makes a good seal and it doesnt corrode away by the fuel. oil resistant.
  7. motorpsycho

    motorpsycho Active Member

    now that i look at your intake tube again, it looks right. when i first looked at it, it looked totally different than most, but i didn't see the nut that holds it to the cylinder.
    now, i see the nut and i see that you have a "normal" intake tube.

    another thought is that you can buy a billet intake tube which is stright and i think shorter than the stock one. it looks like you only need a couple of mm. clearance, and this might do the trick.
    or better yet...if your strapped for $$, take off that air filter cover, and cut the bottom of it off (it's just plastic). that might get you enough clearance between it and the frame.
  8. TheMightyGoat

    TheMightyGoat New Member

    Okay, so last night I soldered two 45 degree 1/2 inch copper street elbows to my intake to get the carburetor mounted on the side. To fit one end of the copper fitting in the intake, I put a Dremel grinding bit in my drill press and worked the inside of the intake tube in even circles, to make a smooth surface of the correct inner diameter. The coupling end of the fitting was just barely smaller than the intake tube the carburetor is designed to clamp on to, so I also had to heat that end with a torch and pound a tapered object (upside down 9mm socket) into it to flare it just a bit. That all fits now, and is air tight, so I believe the carburetor mount to be solid.

    However, on the bike's first test drive, I find it has no power. I have a spedometer on the bike (tested to be accurate with my portable GPS). When I pedal up to speed and engage the clutch, the bike immediately accelerates up to about 14 MPH. Shortly after that, it peters off and speed decreases to about 9 MPH, and it will not accelerate. The throttle cable (pre-assembled, mind you) has no effect. After some time of puttering about at this speed, the engine begins to sound like it does not want to stay running, and indeed if I pull in the clutch, the engine stalls - evidently having been only re-started by my forward momentum still turning the engine over whilst the clutch was engaged.

    If I start the engine with the bicycle jacked off the ground, the engine roars away with the rear wheel spinning and does not appear to ever slow down (as it does shortly with me riding it).

    Thinking this was due to my fiddling with the float, and that the engine would not be getting enough gas, I pulled the carburetor apart and adjusted it out some more. Another test drive. No difference. Again, I adjusted the float. The float is now further out (letting more gas into the bowl) than when it was delivered to me. No change in performance.

    Assuming this is not a problem with the carburetor, what would cause the bike to bog down this way and not want to carry a rider? Perhaps a clutch issue? Also, when I said the throttle makes no difference, this is including when the engine is running with the wheel off the ground - at least, judging by ear; the engine sounds to be at high RPM from a start and sounds no higher as I twist the throttle.

    I am also confident that my fuel line and intake from the carburetor to the engine are not inhibiting gas flow, but I can post pictures upon request. Any suggestions are appreciated.
  9. motorpsycho

    motorpsycho Active Member

    here's my thought and this is a very common thing.
    take the top of the carb off and pull the slide out of it. look at bottom of the slide and you should see 2 notches cut into it (not sure exactly what carb you have tho).
    if it has 2 notches, a short one and a long one, the long slot should go to the right side of the carb. Look down inside the carb where the slide goes and see if there's a pin in the side of the carb. if there is a pin, line the long slot on the slide up with that pin in the carb. the slide shoudl slide down into the carb all the way pretty easily and it will be a nice smooth fit.
    the short slot should line up where your idle screw is at.
    the common problem is that the slides are put in wrong so they won't move. when you try to give it throttle it does nothing because the slide can't open up. again i'm not sure what carb you have and this is a common problem with the older n.t. carbs, but i think if you look at your slide you will find a similar problem.
    post some pics. of the slide once you take it out of the top of the carb so i can see what you're working with.

    another thing...are you sure that your choke is not partially closed?
    Last edited: Apr 10, 2011
  10. TheMightyGoat

    TheMightyGoat New Member

    Not entirely, no.

    The choke cable was also pre-assembled, but the shift lever to adjust it hasn't felt right to me. It doesn't kill the engine to bring it all the way down (which does relax the cable), even if I've just started it for the first time of the day. (Which makes for some precarious stops, given that my kill switch does not work correctly either.)

    I will have to take the other side of the carburetor off tomorrow and investigate the pin you've described, and perhaps how the choke cable is attached. I'll post pictures if I don't figure it out. Thanks for your help so far.
  11. motorpsycho

    motorpsycho Active Member

    so from what it sounds like you may have a "speed" or "cns" style carb with the chole plunger thing on top.
    I think that it's more of an air bleed than a choke, but it kind of works the same as a choke,
    when you do take the top of the carb off, there will be a couple of springs in there...don't worry the choke and throttle cable run through them so they won't go flying.
    if you do have this cns style carb, i'm pretty sure that the slide will have those slots in it and the pin inside the carb to line the slide up with. make sure that the slide moves up & down freely. when you pull the cable out to pull the slide out it should slide up & out very smoothly and easily and not be sloppy or binding up.
    this problem is just a gues on my part, but by what you're describing, it sure sounds like this is the problem.
    what name is on the side of your carb anyway?
  12. FylingScot

    FylingScot New Member

    Hi Mighty Goat:

    Classic SNAFU with the carburettor.

    I have a similar setup but I am on a lightweight steel road frame with the rear tire close to the seat tube - much tighter than a mountain bike. This forced me to put in spacers on the U-clamps to move the engine and jackshaft mount forward so that the jackshaft would be tight behind the seat tube and clear that tight rear tire.

    The side benefit was creating a little more space for the air filter forward of the seat tube. I made the spacer by sawing up some aluminum pieces that were part of the clamping system for the engine - they don't get used because you have the jack shaft.

    I think trying to space things might be a lot more successful than rolling your own intake so bear it in mind as an option. I would also look to mess around on the air intake side of the carb before going anywhere near the engine intake.

    I'll post a photo when I get home.

    Good luck.
  13. FylingScot

    FylingScot New Member

    Sorry Mighty Goat,
    For some reason I thought you had the jackshaft kit. However, my advice still stands, better to keep the standard intake and try to move the engine a little to make room for the carburettor and air filter. I see you have a big aluminum plate under your engine. Can't quite see why you have that and I wonder if that isn't part of the space problem.
  14. TheMightyGoat

    TheMightyGoat New Member

    My frame is too big for the supplied clamps. The upright portion of the frame was only slightly too big, so I was able to make the rear engine mount fit it by removing some material from the clamp. The lower mount however, was nowhere near close to fitting the frame - the supplied studs weren't even long enough to reach the other side. I could have replaced the studs with longer bolts, but I couldn't find any 6mm bolts that long. I could have used threaded rod, but I didn't want to attempt tapping the engine to a different thread, not being sure of the integrity of the metal. So, I used the stock holes in the frame where a water bottle holder had been attached, and mounted some aluminum I had laying around, and in the aluminum I drilled two holes to accept the studs from the engine (with some rubber in between). This way it is fully adjustable without modifying the engine itself.

    The aluminum does lose me 1/4" of space... I suppose that might be enough for the carburetor to clear the frame in the correct position, but I think the tube I've modified is acceptable.