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Causes of engine surging/hunting at speed?

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skyflyer
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2020/03/24 19:07:25 (permalink)
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Causes of engine surging/hunting at speed?

Kawasaki FB460V engine, which has a speed governor so normally throttle set to max and the governor controls the throttle to mainatin speed.
 
Idling is fine bit at higher speeds the engine surges quite significantly - maybe plus or minus 500 rpm? 
 
Its an old engine (about 1990 I think) so i bought a new carb and its now worse, almost sounds like a kid on a motorbike revving it!
 
I discovered by accident that if I remove the air filter it almsot cures it.
 
Spark plug s normal brown colour so i think the mixture is broadly correct.
 
The idle jet adjustment screw is very difficult to get at without removing the carb. I set it to the service manual recommendation of 1-1/8 turns out from fully closed. However i notice on the old carb it was nearly two or three turns out. But surely this would have no bearing on the fast running anyway?
 
Whats really frustrating is that when i replaced the old carb it hasnt improved.
 
Ive got a suspicion that I need to back off the idle screw a lot more, but with the difficulty in trial and error I am looking for some guidance.
 
Could it also be anything else? Valves, timing etc?  I'm not the original owner but i doubt its had a proper service for years!
 
Thanks

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    Roy
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    Re: Causes of engine surging/hunting at speed? 2020/03/24 20:47:16 (permalink)
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    Hold your finger on the throttle shaft, if it smooths out the governor is too sensitive. If it stumbles and dies, the fuel system is at fault. Does it smooth out under load? If it does the idle circuit is partially plugged, even at wide open throttle the high speed does not come into play untill load is applied and the throttle plate opens to it's maximum.
    skyflyer
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    Re: Causes of engine surging/hunting at speed? 2020/03/26 08:52:21 (permalink)
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    Thanks for that; I eventually realised that if old carb was OK but not great, and then new carb was worse and replacing old carb was worse still then the problem wasn't the carb but the governor!
     
    I've reset everything as per the manual, throttle cable adjustment, governor arm setting and so on and its all running much better. Still hunts a bit when nor under load but once under load is pretty steady.
     
    Top speed seems a bit on the low side, just need to get new battery for my tacho and I can set that up too.
     
    Thanks for the pointers
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