what's that screw for anyway??? carburetor - idle screw

Discussion in '2-Stroke Engines' started by pianoman8t8, Jan 29, 2007.

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  1. pianoman8t8

    pianoman8t8 Guest

    ok, so i got the engine kit from dax. Is there a way to adjust the carb., because from what i see, there is only the "idle adjusting screw" (or that's what it seems to be), and something that seems to be like a primer pump thingy??? and there is also a hole next to the air intake underneath the air filter. I took apart the carb, and there is a float with an arm extended to it, like a close off valve that goes to the fuel inlet, and there is a hole that connects to the other hole that comes out underneath the air filter....does that serve any purpose? but I see other people saying they adjust some screw to get it running right?? what screw are they talking about? or maybe it's the same one as the idler screw??? please tell me if you know anything about these carbs...

  2. cyclemotors

    cyclemotors Guest

    Hello pianoman,

    the only adjustment screw that im aware of is the one your talking about.. Factory and manual state it should be turned 4 half turns anticlockwise from fully closed..

    But in my experience with every setup you will need to adjust this slightly to get the correct mixture.

    I usually listen to the engine while turning the screw untill it sounds just about right when idling.
  3. Tom

    Tom Active Member

    Aug 4, 2006
    Likes Received:
    I think thats the idle screw. To adjust the mixture, you have to pull out the pin in the carb and adjust the floater thingy to the correct pin setting (there are 4 of them). Hard to explain, I know someone posted pictures before, might want to try a search.
  4. srdavo

    srdavo Active Member

    Nov 4, 2006
    Likes Received:


    this is copied directly from my grubee instructions:

    "Note: The air/fuel mixture screw should be preset at about 3.5 turns counter clockwise from the totally closed position.
    *NOTE: Do not back-off screw more than 4.5 turns or vibration may loosen the screw and cause it to fall out."

    I agree that 3 & 1/2 turns is not set in stone.

    Another idle adjustment I've found is the cable adjuster on top of the carb. any ideas guys?

  5. Steve

    Steve Guest


    I don't even pretend to know why, but if I set my idle screw at around 4 turns out, the engine has no power at wide open throttle. However, if I turn it out to 5 turns, the engine makes good power at wide open throttle. :???:
  6. tirebiter

    tirebiter Guest


    That's wierd Steve. All the idle screw does is stops the slide as it goes down into the throat of the carb. It hits a slanted slot so that screwing it in or out lets the slide stop up or down a little. As soon as you give it any throttle above idle it doesn't do anything.

    Maybe you've got one of those secret carbs that the oil companies bought the patents to. :shock:
  7. Blaze

    Blaze Guest

    You have the same carb I do, and I know how to adjust it.

    Different manufacturers have different carburetor setups. From Pianoman's description, it sounds like he has the same carb I do.

    Tom and Tirebiter are correct for this particular carb.

    The only adjustment screw that my carb has is the idle screw. That's it.

    The push-button looking thing is called a "tickler". I have an old Moto Guzzi motorcycle that has the same push-button ticklers on the carbs. Their only function is that when you hold them down for a couple seconds they slightly flood the carburetor on purpose to help it start when it's cold, but I never use mine, and actually plugged it with a screw to eliminate a possible source of leaks. (In case you are curious, they work like this: The button is connected to a pin that goes into the carb. Pushing the button pushes the pin, which pushes the carb float down. Pushing the carb float down makes the carb think the bowl is not full yet, so it pours more gas into the float, thus overfilling the carb. Holding this button down too long will cause gas to start leaking out through the button.)

    Adjusting the mixture is pretty easy. Unscrew the cap on top of the carb where the cable goes in. Pay attention to what it looks like when you are taking it apart. The washer with the slot in it goes OVER the E-clip when you reassemble it. Compress the spring, unhook the cable from the sliding cylinder, and take the needle out of the assembly. On the needle you will see 4 grooves. Putting the E-clip on the top groove lets the needle sit lower into the carburetor, which blocks the fuel flow, which leans out the mixture. Putting the E-clip on the bottom groove gets the needle up out of the way a little, which clears the way for more fuel to go through, which enriches the mixture. Top groove = lean. Bottom groove = rich.

    I live about 3 miles from the ocean, which pretty much puts me at sea level. I find that my bike starts easiest and idles best with the E-clip on the second groove from the top. The top groove makes my bike harder to start and the bottom two grooves make it so rich that it spits a lot of unburnt fuel out of the tailpipe.

    If you are at a high altitude, I have heard that you can get a new jet (the nozzle that sprays the gasoline) to adjust for altitude. Changing the jet would be really easy, but figuring out which one you need might be a pain. Hopefully you won't have to do this. Mine runs fine with the stock jet, so I have never had to mess with it.

    Anything past carb adjustments might be some other engine stuff, so let us know what kind of problems you are having and I'm sure somebody here can help fix it.

    So to sumarize:

    The only thing the screw does is set your idle. Screw it in to make your bike idle faster, screw it out to slow the idle down. This screw will not affect anything else on your motor except idle speed.

    The only way to adjust your mixture is to move the E-clip up and down the needle. Since it works so well for me, I'm going to suggest starting with the second groove down and work from there.
  8. pianoman8t8

    pianoman8t8 Guest

    Hey, thanks for the info, blaze. When I assembled the engine and the carb and everything else, i don't recall seeing more rings, but i will definitely take a look when i go home this weekend or friday. I can't wait. Thanks again!

  9. Steve

    Steve Guest



    I know, it should not, and technically speaking, cannot happen. But again, on mine it does.
  10. gone_fishin

    gone_fishin Guest


    ...when i first got mine, i also thought it was for mixture, a closer look before install and i would've known better.

    funny thing, tho:

    when i thought it was a mixture screw, it behaved like one. then i learned it was for adjusting idle, now that's what it does :lol:

    maybe i've got the magic carb....
  11. Blaze

    Blaze Guest

    I just thought of this because of the fuel/oil ratio discussion in another topic:

    I should have noted that I run a 25:1 mixture. Since you just got your motor, I'm sure your mixture is pretty close to that anyway, so it doesn't matter for this conversation, but if somebody else reads this and they are running a different fuel/oil ratio, it could make a difference.
  12. pianoman8t8

    pianoman8t8 Guest

    Good call, blaze, i never woulda thought of that. Also, i messed with the carb alot over the weekend. It came setup on the 2nd groove from the top, so I decided to mess around with it a little. I put it on the 3rd groove, and it had more bottom end, more power, but top end suffered. I also tried the first groove on the top, and it ran like crap. So i still have it back on the 2nd. I also tried the 4th just for the he** of it, and it ran like crap also. I have a good mind to get that upgrade one from the Grubee place.
    Thanx guys for all the input, i apreciate it!
  13. mrsaxman99

    mrsaxman99 Guest


    psychological self-fullfilling prophecy

  14. I think there are 2 different carbs being talked about. The stock carb which has only one screw for idle adjustment & premium carb that has idle & mixture screw side by side.
  15. gone_fishin

    gone_fishin Guest

    yup, that's right, but check the date of this topic. back then, we were dealing with the stock carby and only hearing vague rumors of something better. there was a bit, heck there was a LOT, of "mixture/idle screw" confusion when MBc first got busy. you had to be there but it pretty funny, and it's what inspired this thread :)
  16. Scottm

    Scottm Guest

    I love reading these old threads. Good job likemyspoiler in all your research.
  17. Hollywood

    Hollywood Guest

    I have done alot of R&D on carbs and they can be a pain to get them to idle right.I have seen two types of carbs in engine kits.The old style what has one adjustment screw and a bigger(longer)carb with two adjustment (one air screw and one fuel screw) screws (to use this type carb you will need a lot of frame room to get it to fit).The first thing i will start with is air filter and air filter housing.What i have found by testing is that the filters that come in the kits are not much good.They do not let you get a good air flow.What i have found to work the best are filters you can make out of Scotch Brite.The Scotch Brite comes in four colors green,red,gray and white ( they can be had at any good hardware store or body shop supple store)I think the green and red give you the best air flow.You may want to use the gray or white ( the white is the finest ) if you live in a area where their is a lot of dust and dirt in the air.I put a light coat of oil on them to trap the dust.As for the air filter housings i seen and tested there are four types.Most kits come with a black plastic housing or a steel one with a white plastic cover.The black plastic housing gets its best air flow if you cut or grided off the four plastic flutes as short as you can get them.The steel housing with the white plastic cover works good by just changing out the filter with a Scotch Brite one.There is a all steel housing after market one you can get(i get mine from Dennis Foster).I have found this one to work the best for air flow of the ones i've tested when you change out the filter with the Scotch Brite filter.The new style carb works great with the K&T type filter that comes on it.This carb also works the best of the ones i have tested if you can get it to fit in your frame. Well my finger is getting sore(i am a one finger typer)Next time i will reply with what i have found out by my testing on the jet and screw adjustment.
  18. Sockmonkey

    Sockmonkey Guest

    I think it would be good to oil a Scotch Bright pad if U use it for a filter, as this will help it to trap more dust. I used to do this with my RC airplane engines.

    As for the idle screw adjusting the mixture... What if by turning it out 5 turns it loosens it up enough to allow air to be sucked by it at high speeds thus leaning out the mix???
  19. Hollywood

    Hollywood Guest

    Don't turn your screw out five turns or you are going to loose it.If you want to play with the fuel/air mixture move the e-clip on the jet.The sweet spot for fuel/air is between 3 to 4 turns out to get a good idle spot.
  20. Jim H

    Jim H Guest

    Lost my idle screw the first week I ran my 80... replaced it with a wood screw until I got a new one... ran like carp though. There was a young fella on this forum a while back who swore that his idle screw increased hp, torque, low end, top end, midrange and everything... I miss his wild stories...