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Just got a free CB100, it's sat in an airplane hanger for the last 20 years and has around 5k miles on it. Its gotten a brand new carb put on and still wont idle, starts and runs at wide open fine. Any ideas?
Jetting miles off, float height miles off, air leak?
Air leak.
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First thing is to replace the intake boot from the carb to the engine. You can test this by running the bike and pointing a garden hose of running water at the boot and listening to the engine. If the sound changes it's leaking. A 40 year old piece of rubber is probably spent.
Is it a brand new carb? If it's a rebuilt be sure to check the jets. If new make sure it came with the same jets as the ones in your original carb. They usually are clearly marked.
Google around and see if you can find how to video or threads on the subject.
CB100/CB125 Hybrid aka CB112.5
Last edited by ducatirdr; 10-25-18 at 07:10 AM.
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I would use a bit of starting fluid / ether instead of water personally.
I’ve always used the gas from an unlighted torch to find vacuum leaks on hoses or cracks in intake manifolds for cars. You slowly move the torch head along side the hoses, fittings and manifold. When the engine rpm changes you’ve likely found a leak. There’s usually more than one. I could see it being a bit trickier with a bike as the air filter intake is quite close. I’d probably try using a small hand held propane torch as the nozzle is pretty small and easy to direct into tight spaces. If using a cutting torch just use the volatile gas, not the oxygen and just a bit is all that's needed.
I suspect water would work but it might be a bitch if it sucks too much in and stalls.
-Christian LRRS/CCS HasBeen ECK Racing
2011 Pit Bike Race CHAMPION!
Just run it pinned all the time
Does it stay running on starter fluid
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1994 Yamaha YZ250 CA Street Legal 2-smoke :smoke:
I have built/rebuilt several of these. They are not super complex but need to be set up properly.
It will need the right fuel mix - if that is a China carb there is suspect #1. Make sure the tank/ fuel tap are clean. Clean and adjust the points - bonus points if you replace them with OEM (see carb reference above). Make sure you have a fresh battery.
Check also that you have no restrictions in the airbox. In my experience air leaks caused hanging revs and wild idle speeds - check the basics first. My $ is on battery/points.
Last edited by klp; 11-05-18 at 04:08 PM.
Fleabay new carb for $20
Smear/coat carb boot with black silicone
Its a 100 should only run wide open![]()
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