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125cc Pit Bike

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Good morning....I have a 125cc Chinese pit bike that is difficult to start and loud...A little background

Last year the bike started and ran great. My son had a very minor fall (more like tipped over) which resulted in a cracked gas tank. I replaced the tank but the carb was now leaking through the overflow. It was toward the end of the season so I winterized the bike and stored...

A couple of days ago I pulled the bike out, tore apart the carb and cleaned it...assuming the overflowing gas was due to the float not working. Reassembled the bike and started it up.

Here is the problem:

- more difficult to start this year than last

- no more overflowing gas

- revs higher than it did last year on idle

- last year did not require choke to start the bike. ...even when cold

- louder than last year when idling

- turning the air fuel mix screw or idle screw seem to have no impact on the idle

- some minor smoke out of the exhaust NO SMOKE LAST YEAR

A couple of other points that I don't know if it can be a contributing factor;

- has a slip on silencer. ..looks old not sure if these type of slip on pieces need to have the packing changed?

- when I bolted on the carburetor, after cleaning, I noticed that the bolts in the port from carb to engine appear to be stripped and therefore cannot be "snugged" up..


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Sounds like a vacuum leak between carb and intake runner. This can cause high idle and hard to start problems. You need to fix the stripped bolt problem

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Engine and Carb quick check list - TBolt USA Tech Database!

Install Intake manifold to head and carb, oil gaskets and tighten bolts lightly at first then torque then down gradually in a cross pattern to provide a good seal and avoid air leaks

1)Throttle cable adjustment check:

a)Carb slide MUST hit bottom 

b)Test it , You MUST hear the sound of the slide hitting the bottom of the carb

c)You must have free play or slack in the throttle cable 

d)You must be able to turn the throttle a small amount WITHOUT moving the carb slide

2)Make sure the carb is 100% clean, free of debris and you can see light thru both main and pilot jets 

You must start with a known good and properly oiled air filter 

3)Double check you valve tappet adjustment .003intake and .004exh. 

4)Make sure your exhaust gasket is installed properly.

5)Make sure you have a good intake to head and intake to carb seal and NO intake leaks

6)Make sure all electrical connections are tight and you have a solid ground from the engine to the frame.

and make sure you have spark 

7)Check for good fuel flow and fresh fuel. 

8)See our jetting guide and do a plug read. 


Flooded Engine:

IF your flooded , pull the plug , Kick it over like 10 time WITH THE PLUG OUT 

Put in a NEW PLUG

One more thing

I find at times it helps to just crack the throttle a tiny bit to let air in , just a tiny bit 

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Thanks. This is great....I will follow up. I have reinstalled the intake manifold. Bike starts easily BUT it is much louder/rougher idle than last year....

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