Determining Main Jet Size

Discussion in 'Performance Mods' started by (Ian), Jul 1, 2010.

  1. (Ian)

    (Ian) Member

    I hear the default is .7mm, is this true, or do certain engines come with different sizes? I've got a Grubee Skyhawk 48cc, although I'm pretty sure the main jet is the same size as a 70cc.

    How do I tell what size it is, and how should I go about choosing a smaller jet?

  2. GearNut

    GearNut Active Member

    The stock Chinese jets are not marked. You need to take it to a machinist or friendly auto repair shop and they can size it for you with a wire gauge.

    Choosing a jet is trial and error. Observation of how the engine runs, and the color of the spark plug ceramic is the norm for tuning these Chinese engines.
  3. AussieSteve

    AussieSteve Active Member

    Ian, mine's a 66cc and had an 0.79mm main jet originally. Seems to depend on source as to whether you get an 0.7 or an 0.79. (Might be other sizes too.)

    If you can't get it measured, you could buy an 0.74 or 0.75 and see if it makes it richer or leaner than it is now, then work from there with the 'known' size. (Pretty much what I did. I bought an 0.76 and the mixture was leaner, so I knew that the stock one was larger than that. ie 0.79)

    Is it 4-stroking, or is the plug just a little dark and/or oily?
  4. (Ian)

    (Ian) Member

    When I go WOT it doesn't quite four-stroke but it feels pretty close. It definitely bogs down and doesn't accelerate right. Plug is oily/ dark. When I just go 0-3/4 it is a bit less oily and is more brown/tan, though still dark. I've had to set my needle to the richest setting to get the most out of it 0-3/4. Seemed to lose a lot of juice the next slot leaner, but WOT was smoother.
  5. AussieSteve

    AussieSteve Active Member

    I can't argue with your diagnosis - sounds like the main jet is too big to me.
    Forgot to mention earlier, Sick Bike Parts have a range of main jets to suit, here. (Dellorto 5mm)
  6. Pablo

    Pablo Motored Bikes Sponsor

    Every stock jet I have measured is right at 0.70mm or very close. Measured with a SmartScope (more accurate than a pin gauge, but pins/wire are fine). Anyway, sounds like you are on the right track. If you go to the downloads section of our page and click on tuned pipe installation instructions there are some carb tuning tips. Pipe Installation Instructions.pdf
  7. AussieSteve

    AussieSteve Active Member

    Paul, the ones from ZBox here in Oz are 0.79mm.
    Tony Cicchio at Rock Solid Engines first told me that, then when I got an 0.76mm the engine definitely ran slightly leaner, indicating that the original was >0.76.
    (Mine actually runs best with an 0.72mm jet.)
  8. Pablo

    Pablo Motored Bikes Sponsor

    Good to know!! Now I know - you actually went leaner. THANKS!
  9. AussieSteve

    AussieSteve Active Member

    Yeah, even an 0.74mm causes four-stroking in my engine.

    Incidentally, Tony's surname is Riccio, not Cicchio as I wrote earlier.
    I just found Tony's email re the jets. (In the end, by trial and error, I ended up at 0.72mm, his original recommendation.)
    An excerpt:-
  10. Fabian

    Fabian Well-Known Member

    Well i like to be different, or at least my engines (all 5 of them) like to be different.

    After spending a lot of time messing around with the jetting of my last 66cc engine (as all the previous engines failed due to lean jetting) supplied with the standard carburettor (approx No 70 main jet), which happens to be the same carburettor used on the 48cc engines.
    The solution was to rejet and my best result was with a Dellorto No 82 (0.82mm) jet, and a No 84 (0.84mm) jet having the smoothest power delivery, not to mention a noticably cooler running engine.

  11. Virginian

    Virginian Member

    I may be ignorant, but what is "4-stroking?"
  12. AussieSteve

    AussieSteve Active Member

    Hello Virginian, I doubt that you're ignorant - 4-stroking is a very specific term.
    A 2-stroke engine '4-strokes' when the fuel/air mixture is so rich that the engine can't fire on each revolution and instead fires on every second rev, like a 4-stroke. (It's an indication that the slide needle is set too high or that the main jet is too large, depending on the throttle opening that the '4-stroking' happens - if at 0-3/4 throttle, the needle is too high, whereas if it happens at WOT, (wide-open throttle), the main jet is too large.

    There's a really good video illustrating 4-stroking better than I could possibly describe it here:- 4-Stroking {YouTube 6:15} (Well worth watching.)

    You might also find this thread useful:- NT Carb Tuning Basics

    *** I'm assuming that you're talking about a Chinese HT engine, although '4-stroking' applies to any 2-stroke, (or for that matter, a properly-running 4-stroke engine).
    Last edited: Jul 4, 2010