Severe running on KX500 carb?

[I_Duddits]

Severe "running on" KX500 carb?

I'm sick to ...kin death of trying to sort this out
Why oh why, when I rev her up and let go the throttle, does she not slam shut the RPM (although the throttles all the way closed)?
Oh no, the dog rolls on for a second or two. Doesn't sound like a lot but it fekkin is when you're getting to the end of a road and stop means stop

So if I start her up and idle, then blip the throttle she goes back to Idle (usually) but if I ride and take her through the rev range in a couple of gears then stop (Should go back to idle) the thing screams :ohmy: full RPM. I hold the kill switch in till she almost dies, then let go and she screams again (with the throttle fully shut off) I repeat the process 3 or 4 times till eventually she settles back down to idle.
The boots either side the carb are in good condition.
The jetting is factory.
The fuel is fresh.
Spark plug new.
Air screw out 1.5 turns
Cable not sticking
Any ideas guys?

 

[Beuford]

Just a thought, have you checked the throttle slide for a lot of wear? maybe being a old big bore the slide is becoming "cocked" and the spring cant push it back.Similar to why 4 strokes need two throttle cables, one to return it.
Also check the crank for slop, my YZ490 has worn bearings and at WFO it takes a little bit to get back down with a lot of "surging" as it comes down.
I know im pushing fate by not fixing my bearings and seals but i just cant seam to get around to it.
One last thing, though im sure you have already checked completely but is the throttle cable perfect with no drag and not getting pinched?

 

[whenfoxforks-ruled]

When it does this check the plug for a possible lean condition. If not, make sure the cable is properly located.

 

[CarnageZ28]

Boots might be fine, but how bout the gaskets on the reed cage? Start the bike, let it idle, and spray all around the boots, reeds, cylinder base, carb, etc. with starting fluid. If idle fluctuates, you have a leaning condition.

When it starts to do this, cut the engine ( Kill switch ) while it is running at full RPM. Pull the plug immediately and see if it is leaning (White-ish coloration of the ceramic portion of the plug)

Be careful cause 1, it is NOT safe. And 2, if it is leaning, your gonna have it let loose on you sooner or later and be doing top, and bottom possibly, rebuild.

 

[I_Duddits]

I'm wondering, the carb (I think) is bigger than std ie: 38mm not 36mm, is it possible then that she's running out of fuel? Would that cause the increase in uncontrolable RPM?
It fits then that the bike behaves ok stationary but screams after a run due to fuel demand. How could I tell (while riding) if the bowl remained constantle full?

I'm clutching at straws here, does any of this follow logically?

 

[whenfoxforks-ruled]

I would try and verify if it has the correct carb or not. Any carb running out of fuel, will burn lean. So check the plug to verify the lean condition. Even a larger carb jetted and adjusted properly, should run properly. The condition of the engine plays a big role also.