125 Pit Bike Carb Problems

10 replies to this topic
  • aidan1990

Posted January 18, 2008 - 08:37 AM


Ive recently snapped my throttle cable on my 125cc pit bike and have had it replaced with no problems to start with, after a few days I noticed the kick start was a lot stiffer than usual and when I rev the bike hard it sounded boggy, I checked the spark plug and it was slightly black so I cleaned it and put it back in. it didn’t really make a difference so I mucked about with the screws on the carb to try get a better throttle response. I turned 1 on the screws to get a higher rpm And the other screw made the engine steam when I turned it so I left that screw alone.
Things got a little better, but then the bike became hard to start, some smoke came from the air filter when I tried starting the bike, I am able to get it started but it cuts out if I don’t open the throttle at least ¼ turn.
I am able to higher the rpm by turning the screw on the carb so the bike doesn’t cut out, but its way to high and isn’t exactly purring, I’m sure its not doing the engine any good.

As you can probably tell I don’t know much about bikes, I don’t really want to try making things better incase things get worse. :cool:
An easy solution will be taking it to a garage but getting charged a fortune.

If anyone has any advice PLEASE LET ME KNOW!



  • chirorider

Posted January 18, 2008 - 09:26 AM


Your plug may be fouled and replacing it would work better than cleaning it. The idle screw is only a patch not a fix as you have found. It sounds like it is running rich.

Is the carburetor a mikuni? If so, there is a screw where the carb attaches to the manifold. It is very hard to reach on the bottom of the carb. Try turning that screw in until it bottoms (lightly, don't force it) and see how many turns in it went. Should be around 2.5 but may vary. If you turn that in, it will lean the idle mixture a bit. If you have to go any leaner than a total of 1.5 turns out from bottomed, then your pilot jet needs to be richened.

If it runs best at more than 3 turns out then you should probably use a leaner pilot jet.

Make sure you have fresh fuel and try the new plug to start with. The cable should not make any difference as long as you are getting smooth full movement of the slide. (you can remove the air filter and look in as you actuate the throttle and see if it moves from the bottom to all the way open.

Good luck.

  • aidan1990

Posted January 18, 2008 - 05:06 PM


i cant see a screw where the carb attaches to the manifold, there is however a screw on the oposite side of the idle screw, on the exhaust side.
i havent touched this at all and dont know exactly what it does.:cool:

  • chirorider

Posted January 18, 2008 - 06:25 PM


Hard to say without seeing the carb. On my bike, the idle screw is on the pipe side.

Can you find out what carb you have or take a picture of it?

Anyway, commonly there is an idle adjust screw that is pointing towards the slide. In fact if you take the air filter off and screw that screw in you can see the slide moving up. The other screw is usually a fuel or air screw. Usually (but not always) on a four stroke it is a fuel screw and turning it in leans the mixture. Turning it out richens. If, however it is an air screw, the opposite applies.

I'm no carb expert, but if you post a picture, maybe someone can tell you more.

  • dirtbkr188

Posted January 18, 2008 - 07:52 PM


If the screw is located in the front of the carb, usually underneath by the fuel bowl, it is a [COLOR="DarkGreen"]fuel[/COLOR] screw and [COLOR="DarkGreen"]turning it out richens [/COLOR]the mixture. If the screw is located to the rear (airbox) end it is an [COLOR="Navy"]air[/COLOR] screw and [COLOR="Navy"]turning it out leans [/COLOR]the mixture.

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  • chirorider

Posted January 18, 2008 - 08:06 PM


If the screw is located in the front of the carb, usually underneath by the fuel bowl, it is a [COLOR="DarkGreen"]fuel[/COLOR] screw and [COLOR="DarkGreen"]turning it out richens [/COLOR]the mixture. If the screw is located to the rear (airbox) end it is an [COLOR="Navy"]air[/COLOR] screw and [COLOR="Navy"]turning it out leans [/COLOR]the mixture.

That's how I've always heard it also. I do think there are exceptions to this rule though.

  • Pitster

Posted January 18, 2008 - 10:09 PM


Give us a call I have tons of Mikuni 22 mm stock carbs $25!!! Perfect for a 125cc!!!

  • aidan1990

Posted January 19, 2008 - 04:28 AM


there is a screw on the fuel bowl and seems to be closed, i suppose the other screw next to the idle is the air screw which is almost closed.

i found out it is a mikuni carb.

can you give me a standard setting to work from with both air and fuel screw?

i will be getting a new spark plug soon and will see if its any better.

  • dirtbkr188

Posted January 19, 2008 - 05:20 AM


Personally, I've never heard of a carb having both an air screw AND a fuel screw, too. Post up a few clear pics of your carb, if you can, and indicate which is the air screw and which is the fuel screw.

  • aidan1990

Posted January 19, 2008 - 09:14 AM


i dont have any picture but have found a carb the same as mine on ebay.but the screw next to the idle isn't visible i dont think.

heres the web site...


  • chirorider

Posted January 19, 2008 - 10:41 AM


That carburetor is not a mikuni. The screw at the bottom is just to drain the float bowl.


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