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Air/Idle Control Problems


brianreed62
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Hi everyone! Having some problems with my 06 Scorpa SY250.

During a riding session, I wheelied the bike out on a steep hill section causing it to land hard on the right handle bar twisting the throttle cable completely around the handlebar. Prior to realizing that the cable was out of place, the bike was started, initiating an out of control idle climb that ended in a very loud backfire when the fuel line was closed because the kill-switch would not cut it off. Once the cable was returned to its original position, the bike started and ran, however it had extremely poor throttle response.

So my thought was to change out the throttle cable, which in turn did not change a thing. Messing with the adjustment nuts on the cable itself I was able to achieve one of two results; either it would idle fine on choke with normal throttle response then when the choke was switched off the revs would immediately start to climb rapidly towards a maximum; or it would not idle on choke without constant throttle attention and then idle fine when the choke was turned off and then when revved with low rpm it would either not return or it would bog and not transfer the throttle increase to the engine, but if let idle it would end up stalling out after 20 seconds or so. And both scenarios had no trouble starting, eliminating the theory of reeds. I checked the slide and jet needles, both of which looked to be intact with no grooves or imperfections. Then I tried starting it and sprayed all connection points with carb cleaner to check for air leaks, but really didn't notice a difference and obtained the same rev climb as before

My thought now is crank seals but I've never done those and wanted to see if anybody had any more thoughts before I start diving into that one.

This one's been a real brain teaser and I feel like it shouldn't be this difficult so any input would be greatly appreciated!

:wall:

 

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Unlikely to be crank seals.

Sounds like gunk in the pilot jet.

If it is air ingress, check the cylinder base gasket before you go pulling anything apart.

These bikes have a weakness in the seam of the main chamber of the exhaust which sometimes splits when there is a decent "backfire". If you had said that the exhaust sounded louder I would have suggested checking this out.

If it revved high for a long time before you got it stopped it may have done some damage to the cylinder, piston or rings. You can check for damage by looking up the exhaust port with the pipe off.

Sounds a bit odd that you say it stopped with a backfire when the fuel supply was shut off. It usually takes quite a while to stop an engine just by turning the fuel off.

Next time I suggest you block the exhaust outlet or turn the bike completely upside down to stop a runaway motor.

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I agree with FeetUpFun, make certain the carb is clean.  These bikes do not like any debris in the carb or jets.  Next, ensure the throttle and cable are rigged correctly with a small bit of freeplay in the grip with throttle closed.

I will add, my own SY recently had some weird stuff happening.  Turned out to be the rubber disc in the choke plunger had fallen out of the brass plunger.  Not lost, but would randomly reposition itself, the bike ran pretty good, but would die on any descent.

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