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Old thread.
I have same exactly problem on my rally 125 with rotax 123 engine. It came with 140 main jet, vhsb 34 carb. It couldn't rev and spark plug was wet all the time. Putting bigger jet made it worst, even without air filter. Now with 120 jet it works almost fine. Plug is still slightly wet. Perfomance is not so good now. Engine is rebuilded. Rave valve works fine. Ignition was set going from 1.8mm before tdc to 2.0mm btdc and there is no gain in revving.
How is it possible that it cant work with jets 140 or 150+? Every bike with dellorto 34 works with standard jet or even bigger.
 

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The float may be bent or leaking.
With the result that the petrol level in the valve chamber is higher and that has a strong effect on the mixture.
There are 2 type,s of floats. And here is how to ajust them:
 

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Hallo,
I it could be that the float is bent or leaking. the fuel supply is then later shut off by the needle. with the result that the petrol level in the valve chamber is higher and that has a strong effect on the mixture
 

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What comes to mind,
There are 4 stroke and 2 stroke carburettors.
In a two stroke there is a partition at the throttle needle, which prevents the airflow from going past the needle opening twice and thus gives a leaner mixture.
Left is two stroke.
Yellow Household hardware Metal Auto part Font

And second is:
A leaking membrane gives a larger back and forth flow in the carburettor making the mixture too rich.

You can also look for it in your air filter. too thick foam rubber or too much (thick) filteroil. Or no engine foam but furniture foam rubber. I can't think of anything more at the moment.
 

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I have put dellorto 28 and problem was still there. Then i took spark plug out and set gap to very small, maybe 0.25 mm. And now it is running perfect, it revs to the end.
Maybe coil is weak?
 
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