hi guys wonder if any one can shed some light on this problem, when i start my bike it ticks over nice but if i put a little throttle on it the revs increase and hangs there  for a while if i turn the idle screw down it ticks over nice again until i rev it then its high again so i turn it down some more but it still continues to be up and down.


all jets are stock and have been cleaned with compressed air and mixture screw is 1 3/4 turns out i have sprayed the carb boots with carb cleaner to check for leaks and its all clear.


any advice would be appreciated 

What kind of bike? What shape is throttle cable in? Make sure it slams shut if you hold it WOT and release real quick. (Bike off)

hi rex thanks for response its a 03 yz450f the throttle cable looks fine no kinks or rust and it routed properly it springs back nice and crisp 

Does that bike have a manual decomp valve? Or cam operated decompression? If manual, make sure no air leaks.

its a auto decom 

With engine running, I use an unlit propane torch and probe around to find air leaks...when it revs up / settles out your close.

Edited by RexRider

ill give it a go cheers rex

"Hanging idle" is generally caused by a lean condition.  If your engine ran correctly with the current jetting at one time, the first two things to look at are the fuel mix trim and the valve clearances.  If you cannot trim the pilot screw back to normal and get rid of the problem, and the air leak checks above don't show anything, check the valves.  Tight intake valves can cause the problem by allowing compression to leak back into the intake and dilute the incoming charge.


If all of that is OK, you may have a varnished pilot jet.  A film of dried fuel residue no more than .002" thick can effectively turn the pilot jet from a 45 to a 35, and even keep the engine from running at idle without the choke on, in spite of the fact you can still see through the jet.



brilliant i suspect its the valve shims for some stupid arse reason i didn't check these when i did a top end rebuild ill get on to it and let you guys know the outcome :)

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