Modifying stock NT carburetor air filter cover

Discussion in '2-Stroke Engines' started by groove9028, Sep 2, 2010.

  1. groove9028

    groove9028 New Member


    I'm a proud owner of an 80cc zb engine. GREAT LITTLE THING. I've been riding her for the past week. Runs great, leaned her out with the needle, threw in an NGK. Running 40:1. When riding, it accelerates smoothly but occasionally the exhaust goes from a smooth purr to a raspy BRR. I've been made aware i might be 4 stroking. Since leaning out the mixture with the pin is at the leanest, (top notch) can i drill holes into the black air filter cover to promote better air flow, perhaps leaning out the mixture? Any recommendations would be GREAT!! HAPPY RIDING!

  2. AussieSteve

    AussieSteve Active Member

    Drilling holes won't make a lot of difference. If I were you I'd be looking at a smaller main jet.
  3. motorpsycho

    motorpsycho Active Member

    yes, your best bet is to re-jet the carb. when 4 stroking occurs, there is too much fuel and not enough air getting into the cylinder, and if you go to a leaner main jet it might help.
    BUT, i re-jetted one of my stock nt carbs, and i had to go richer not leaner.
    everyone says that the most common jet that comes in a stock nt carb is a #70. well, mine was running too lean with the stock jet. there is not a number stamped on the stock jet so i am only assuming that it is a #70. I put in a delorto #72 main jet and it runs like a top now, with no 4 stroking at all.
    it did 4 stroke with the stock jet even tho it was a smaller jet that the delorto which is kind of confusing.
    usually 4 stroking happens when the air-fuel ratio is too rich, but mine 4 stroked with the smaller jet, and it no longer 4 strokes with the bigger jet.
    also, i could look at the stock jet and the #72 delorto jet side by side under a magnifying loupe, and i could easily see that the #72 delorto jet has a bigger hole than the stock nt carb jet.
    so i went richer with my main jet, and it made a world of difference.
    I live in the midwest, in the USA and the weather here is still hot (mainly in the 80's) and humid.
    weather conditions and air quality will affect how an engine runs. some people have to lean the carb out, some have to richen the carb up to get it to run right.
    but i am also running a modified long pipe and muffler (not an expansion chamber on this perticular engine) and a high flow carb stack and filter which could explain why i had to go richer rather than leaner.
  4. HeadSmess

    HeadSmess Well-Known Member

    i drilled the holes :)

    sorta slightly better, but then again... a long time ago the first mod i did was machine a plastic ring to sit inside the filter box, BEHIND the foam element. then the "cage" actually had something to press against, sealing the edge of the foam. a much more effective filter. honestly, with the stock setup? you may as well not have it at all!

    but, the now-effective seal did restrict it somewhat. so i drilled holes. it made some improvement over the four lil tubes. not much. maybe just imagination?

    ill take a pic of the ring i made....
  5. AussieSteve

    AussieSteve Active Member

    A decent after-market air filter is worth the effort. I fitted a Sick Bike Parts low-profile filter. Improved flow and better filtering.
    (They look pretty good, too.)
  6. HeadSmess

    HeadSmess Well-Known Member

    lousy camera, lousy pic, but an effective ring.... see the grit n gunk? thats what the filter DIDNT stop before the change...

    Attached Files:

  7. flashstar

    flashstar Member

    Ditto on the smaller/ larger jet. I had to jet down my carb 2 sizes and it still runs rich. I'm going to probably stick with a .64 mm jet (compared with the stock .70 mm).
  8. As the others said, rejetting would be good.

    I did drill holes in the cover though, gave me 3KPH on the top end.
  9. Lazieboy

    Lazieboy Member


    Nocomprenda. How do you re jet a carb i am un educated in this area. I need pointers.:dunce:

    Also, my carb is leaking somewhere, maby a screw. Where should i start should i take it apart?
  10. There are a few things you can do.

    1) Take the the pin at the top (located under the cap the throttle is attached too) and ajust where the clip is.

    2) Take the jet out and change the size of the hole. Either drill it out larger, or fill it with solder and drill it smaller then stock.

    I know I didn't explain it great, but thats the basics. If you need more detail ask me, or someone else that know how to do it.
  11. HeadSmess

    HeadSmess Well-Known Member

    youll find drills at an engineers supply, and take your wallet with you :lol:

    youll want 0.6 mm to 0.7mm usually in 0.05 increments... and a micro chuck.

    solder, an iron, or torch.

    screwdriver, new plug, plug spanner.

    start by soldering up jet. be careful not to over fill it (the size of the CAVITY in the jet is also critical...for other reasons.)

    now carfeully redrill the jet with the 0.6 drill.

    (find someone with a lathe, or buy a few spare drills...)

    reassemble carb, check that the clip on the needle is in the middle notch.

    go for a spin... you may need a bit of choke now...

    once its warmish, open throttle wide.

    if it dies. its too lean, redrill the jet to 0.65....

    if it goes hard....thats about it :)

    then try raising the clip on the needle one notch :eek: (leaner mids)

    better? try the next notch! if the clip runs best on the top notch, you could go smaller on the jet drill, then put clip back in the middle.

    if it gets worse, then try lowering the clip this time. if you need to go to the lowest notch, the jet is too small, go up a size.

    then with the new plug, go for a ride. kill the IGNITION with the throttle held in the area you want to check... usually just wide open :lol: BUT DO NOT LET IT IDLE! you have to kill the spark.

    you want tan/brown. white is too lean. black is use less oil and too rich...

    keep it just on the side of rich :)

    etc etc.

    its possible to do the low mid/idle mixture, and also "fuel enrichment" when opening the throttle, even on these basic carbs. but im not going to explain them here :p theyre irrelevant for this application.

    your aiming for a fairly smooth progressive acceleration, no flat spots.
    Last edited: Sep 23, 2010