Carby filter and jetting

Discussion in '2-Stroke Engines' started by SdCruizer, Feb 1, 2012.

  1. SdCruizer

    SdCruizer Member

    Just got one of those china 69cc engines
    maybe its 66cc the epa sticker I have says 69cc im not sure

    Im not sure what jet is stock, from the little searching ive done I think I read 70mm like shown here

    is that correct

    im at sea level and wanted to use this filter

    anyone know how much upjet I need I was guessing at 72 or 74
    ive only jetted a 70cc large scooter once for a pipe so I know how to read the plug
    and would like to have a couple jets to try out
    just wondering what the norm is just for the filter at sea level

    thanks, trying to search through every page here but it could take weeks

  2. HeadSmess

    HeadSmess Well-Known Member

    usually go down on jet sizes... thats why we read the plug.

    im running something like .47mm
  3. SdCruizer

    SdCruizer Member

    thats strange
    I thought if you introduced more air it made it leaner

    so anyways was I correct about the stock size jet being 70mm?

    what altitude are you at
  4. motorpsycho

    motorpsycho Active Member

    well, the stock jets are not marked but the gueestimate by everyone here is that they are around 70mm.
    I added a high flow filter and an expansion chamber to my 49 c.c. 2 stroke engine and had to jump from the stock jet (presumably a 70 mm) to a 74 mm.
    I added a high flow filter and a custom made pipe /baffle combo that I made to my 66 c.c. 2 stroke engine, and had to go up to a 72mm.
    no 2 engines or carbs will work the same so what works for one engine, may not work for another. of corse, altitude, temperature and humidity all have a lot to do with how you jet a carb.
    just remember when you try to go leaner, only go down 1 jet size at a time because 1 jet size can make a HUGE difference.
    when trying to go richer, you can play around and try jumping up 1-2 jet sizes at a time because being too rich won't hurt anything internally
    Normally if you add a pipe and a filter, you should have to richen the air-fuel mix by going to a bigger size jet, not smaller.
  5. HeadSmess

    HeadSmess Well-Known Member

    yer, as i recall the stocker was a 70...

    i like my pipe more and more! its quiet, fuel efficient and powerful!

    ive never had to go up in size on a ht though, with any kind of exhaust... ive always seemed to be between .45 and .55

    admittedly i have bored a taper into my carbs throat so it does create more "suck", theoretically at least... :D

    im only about 100 metres above...
  6. SdCruizer

    SdCruizer Member

    well I put in my filter only
    and tried the 74 and 72 jet
    72 seemed better then 74 but it was very close
    never tried the stock though
    on mine its a little hassle to pull the carb out because the filter barely fit or I would of tried the stock jet too