Carby High idle, no air leak

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#1
Hey forum!

I had an issue with my old (Now broken) 50cc china engines. The idle was so fast, that I was literally scared to pull the clutch, but instead I used the kill switch (Yes, I know it's bad.)

What are the causes? Can a lean condition cause high idle? Also the idle jet would not do a thing. Fully in it wouldn't even turn more, fully out and I couldn't use the throttle.
 



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#3
Crassius is correct.

The idle screw not having an effect is a tell tale sign of a vacuum leak. Warped intake flange, gas damaged intake gasket, carb intake collar connection, crank seals, and even the top of the carb throttle adjuster where the cable goes in can all be sources of a vacuum leak that may not be initially apparent.

If it is not an easily located vacuum leak try pulling the magneto cover and check there for signs of a leak. Self welding rubber tape is cheap and works great to seal the throttle cable carb adjuster (along with weatherproofing CDI connections both at the case entry and the wire connectors).
 

gary55

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#4
All of what FNT said and you might wana check that the cable casing isn't hanging up on the elbow where it goes into the throttle assy. or the adjuster nut where it goes into the carb.
 

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#5
All of what FNT said and you might wana check that the cable casing isn't hanging up on the elbow where it goes into the throttle assy. or the adjuster nut where it goes into the carb.
Doh, great point - often it's the simple things overlooked.

Too tight zip-ties on the throttle cable and forcing it to make sharp turns can also cause an idle hang, and if it is holding the slide up then the idle adjustment also would not function properly.
 

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#6
I had Hi-temp red silicone applied everywhere. Didn't fix the issue. If heads were loose, (I never torqued them, didn't know they can come loose!) could it be a head leak?
 

gary55

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#7
I had Hi-temp red silicone applied everywhere. Didn't fix the issue. If heads were loose, (I never torqued them, didn't know they can come loose!) could it be a head leak?
The after market heads hold their torque well, but the stock heads constantly lose their torque. I go 162 to 168 in. lbs.. Could a head leak suck enough fresh air to cause your super high idle? Possibly but I think if it was that bad a leak the compression would be so low it woulden't run at all. If you are certain their is no intake leak, the cable is working properly, and the slide in the carb. is aligned properly with the index pin I would suspect the seal under the magneto magnet.
 





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#12
I have a cheap banana pipe, the upper engine exhaust hole is done correctly. It's the same, but instead of the rectangle exhaust port, it's a circle like the actual exhaust.

Also, I'll buy the generic "High Performance" carb they sell on Ebay: https://www.ebay.com/itm/High-Perfo...e+carubretor&_from=R40&rt=nc&LH_TitleDesc=0|0

Other than that a CNC head (Wich one has better cooling,the cubic or the round one?) wich I'll sand down a bit for higher compression.
 

gary55

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#13
I'll check the seal under the magneto. But- can a very lean mixture affect idle?

I had the muffler off and didn't re-jet my carb.
yes very lean makes uncontrollable high idle. Leaving the muffler off is just a really bad idea. Both of these most likely caused the chrome being destroyed. Good luck with the new engine.
 






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#19
I alredy got a new exhaust wich is a banana pipe. That's why I want a higher air flow exhaust. But this time I'll re-jet it. BUT! Could I just put a bigger air filter on it? I currently have the one that is a black circle and has 4 pipes comming out. If I put on one that is bigger, like on the link I sent, that would be enough right?
 

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#20
I tried the Keihin clone and its a pain to tune vs the simple and cheap NT carb. If you want a bit more flow and better looks you can throw a small cone filter on it with the long offset intake. Will give a little bit more power and mid range too.

You should ALWAYS be tuning your bikes since not only is every person's location different but every motor will like slightly different things as well.
 


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