HEADS UP!!!

Site upgrade in progress... Core site functions are working, but some non-critical features/functions will be temporarily unavailable while we work to restore them over the next couple of weeks.

Please post any bugs you encounter, but before you do, check to see if it's already listed.

Thanks for your patience while we work to improve the community.

Sign in to follow this  
Followers 0
aidan1990

125 Pit Bike Carb Problems

11 posts in this topic

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!

Thanks

Aidan

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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:

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites
If the screw is located in the front of the carb, usually underneath by the fuel bowl, it is a fuel screw and turning it out richens the mixture. If the screw is located to the rear (airbox) end it is an air screw and turning it out leans the mixture.

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!


Register a new account

Sign in

Already have an account? Sign in here.


Sign In Now
Sign in to follow this  
Followers 0