Happytime NT carb Slide Jamming

Discussion in '2-Stroke Engines' started by AussieSteve, Nov 9, 2009.

  1. AussieSteve

    AussieSteve Active Member

    I haven't seen any other posts regarding this.
    Today, I tightened the top cap of the carby on my 66cc engine's NT carb,( a bit tighter than usual using a 22mm open-end spanner), after adjusting the needle height and went for a quick spin only to find that the slide was jamming full open. (To make it worse, it didn't happen immediately, but waited until I was a couple of miles from home. I got home with minimal pedalling by starting it and hitting the kill swich when it got revving too high. Then, when it slowed enough, dumping the clutch again.)
    After getting it home, I pulled the carby off and checked it out but there's nothing visibly wrong - neither the cap nor the carby is distorted. The slide moves freely until the top cap is tightened fully, then jams. Even loosening the cap by 1/4 turn solves this. It appears that the inside of the cap goes off-round near the top.
    Tomorrow, I'll make a thick gasket for inside the top cap, to stop it tightening as far and see what happens.
    Anyone else struck this?

    Boy, these things really are a learning experience, aren't they?

    ... Steve

  2. machiasmort

    machiasmort Active Member

    Your clip is on wrong! There's a trick to putting the assembly together! The spring is supposed to push the slide back down and seat the needle!

    You've got it backwards, so did I when I put mine together!
  3. Chris Crew

    Chris Crew Member

    either your clip is on wrong or . . .

    you may have an air leak somewhere in the carb----too lean makes the engine rev

    good luck

  4. machiasmort

    machiasmort Active Member

  5. AussieSteve

    AussieSteve Active Member

    No, I don't have it assembled incorrectly. I'm well aware of how these things go together. Also, it's not an air leak. As originally posted, the slide sticks when the top cap is fully tightened. With the carby off and the slide in place without needle, C clip or spring, I can screw the cap almost tight, then turn the carby upside-down and back and the slide moves freely. Then, if I tighten the cap the last 1/4 turn and repeat the procedure, the slide jams near the top. If I push it the last bit of the way up, it jams firmly. Then, if I loosen the top cap a fraction, the slide instantly drops.
    A thick fibre washer will cure it. I'm getting one shortly.
    I was just wondering if it was a common problem.
    ... Steve
  6. AussieSteve

    AussieSteve Active Member

    Just fitted a fibre washer - problem solved. I can do the top cap up tightly now and the slide still moves freely. As a bonus side-effect, the washer seals the cap thread as well.
    Pic attached.


    Attached Files:

  7. Al.Fisherman

    Al.Fisherman Active Member

    I had the same problem...the tighter the cap was put on the more distorted the carb got (not the piston). Solution...take a small amount of silicon (as in gasket sealer) and coat the threads. Screw cap on finger tight, this does not hinder removal at all.
  8. AussieSteve

    AussieSteve Active Member

    After I added the fibre washer, I found that the mixture had enriched considerably, too.
    I had to lower the slide needle to it's lowest setting. I still need to get a smaller main jet to suit.
    These carbs must suck air past the cap. Not any more in this case. It follows, though, that some air probably still leaks past the cable fitting/adjuster on the top. More silicon, I guess.
    ... Steve
  9. motorpsycho

    motorpsycho Active Member

    the reason why the mixture got richer is because the washer is allowing the jet needle to come out of the jet further. The jet needle is being raised out of the needle jet by the thickness of the washer that you used.
    I don't think silicone is the answer, most silicones will not cure when they are exposed to gasoline or gasoline fumes.
    Personally, i would use teflon tape around the threads and just tighten the cap by hand.
    Last edited: Nov 14, 2009
  10. Al.Fisherman

    Al.Fisherman Active Member

    Maybe bad choice of words...RTV silicon and only a thin coat on the threads. Use of this product is fine. Not to be used in areas where it will be immersed in gasoline.
    Last edited: Nov 14, 2009
  11. AussieSteve

    AussieSteve Active Member

    Don't be silly. (Excuse me) The washer is in the TOP of the cap. It has nothing to do with the needle height!
  12. AussieSteve

    AussieSteve Active Member

    Sorry MotorPsycho, I didn't mean to sound rude. I don't think that you understand where the washer is. Here's a quick sketch:-


    Attached Files: