1. 2 Stroke Husqvarna Motorcycles Made In Italy - About 1989 to 2014
     WR = 2st Enduro & CR = 2st Cross

All 2st Idle adjustment Screw vs Fuel mixture screw

Discussion in '2 Stroke' started by Kevin_TE250, Jan 31, 2010.

  1. Kevin_TE250 Husqvarna
    Pro Class

    Location:
    Boise Idaho
    Since I'm a total newb to 2 strokes can you guys give me a 101 version of these ? is the Idle adjustment the same as air mixture ?

    Couldn't get the bike 04 CR125 to idle yesterday it was cold.

    So simply kicke up the idle adjustment (when it was warm) and all seemed ok.. now I read about the Fuel mixture adjustment screw is this like the screw on the bottom of my 4 stroke carb that works in conjuction with the pilot ?

    the manual said to adjust this also but it seems to be fine...

    Thanks guys :thumbsup:
    Kevin_TE250, Jan 31, 2010
    #1

  2. MattR 2T Forum Clerk

    Location:
    Temperance, MI
    Husqvarna Motorcycle:
    WB165, SM610
    Other Motorcycles:
    Yamaha FZ1
    Idle Screw vs. Air Screw

    The stock idle screw on the TMX carb is a Phillips screw with locknut. The air screw is located close to the airboot (air inlet side of the carb).

    Here is a picture of my 2008 CR125:
    Note: I have replaced the stock screws with aftermarket parts but I will use this picture for sake of discussion. The idle screw is the large silver knob (near middle of carb). The air screw is the smaller blue screw near the airboot.

    Idle Screw:
    The idle screw is adjusting the position of the slide. It's only job is to lift/lower the slide to make the bike idle faster or slower. It's doing the same thing as you turning the throttle and holding it steady.

    Air Screw:
    The air screw adjusts the mixture ratio of air relative to fuel. As you turn the air screw outward, you are adding more air and will lean the mixture. As you turn the air screw inward, you are reducing the air and will richen the mixture. Typically, you will have the air screw about 1.5 turns outward. Note: Air screws are on 2 stroke bikes, while fuel screws are on 4 stroke bikes. Fuel screws are adjusting the mixture too, but it is just the opposite since you are adjusting the fuel quantity... not air quantity (like with air screws).

    [IMG]


    Regarding your jetting question when cold...
    When it is cold, the bike will be more lean since the air is more dense. If you made no jetting changes, then you will need to turn the air screw inwards 0.5-1 turn (from your standard setting). This will reduce the quantity of air and richen the mixture (air:fuel ratio). Your "final" air screw setting will also indicate if the pilot jet is incorrect. If your air screw is turned out only 0.5 turn, then the pilot jet is too large. If your air screw is turned out 2.5-3 turns, then the pilot jet is too small. If you get the pilot jet correct, then the air screw will have a typicaly tuning range from 1-2 turns out. I hope this makes sense.

    EDIT:
    Here is a good resource for carb tuning:
    http://twostrokemotocross.com/2009/12/carburetor-theory-and-tuning/
    MattR, Jan 31, 2010
    #2