Throttle Stuck Wide Open, Now Backfiring & Won't Start

Joined
Mar 23, 2010
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#1
Hello All. Thanks for taking the time to read this. I was riding a couple weeks ago at Bear Creek and the throttle on my 1990 KX500 stuck wide open. Engine kill did not work but I was able to get the bike layed over and it stopped after 8-10 seconds of rev-out (the bike is jetted quite rich). After getting home and inspecting it, I realized the intake boot had split at the clamp and allowed atmospheric air into the cylinder. Poor maintenance and inspection on my part. I installed a new boot and throttle cable and began the pain-staking process of a cold start (I really do need to look at jetting and perhaps a decompression valve). I noticed right away backfiring upon kicks. Then the same when I was roll-starting it (that's usually how I get it going at the start when it's cold). Backfire upon backfire. New plug installed before last ride and plug was quite wet when I pulled it. Fuel making it past combustion cycle and out exhaust port- I can see it on the pipe. Something has obviously changed substantially since the throttle stuck. I suppose a tear down on the top and and checking rings/piston/cylinder would be a good idea but perhaps after checking a few other things first? Any ideas and suggestions would be greatly appreciated. Thanks!
 

RM_guy

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Damn Yankees
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Nov 21, 2000
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#2
That can be terrifying when the throttle sticks...especially on a 500!! _OMG_You are definitely getting too much fuel. Did you check the float height? or the needle valve and seat? It could be leaking badly and just dumping gas into the carb.

It could also be the timing is off. If it was retarded it wouldn't ignite the fuel until it was almost out of the cylinder.

Neither of those would come about just because the throttle stuck but they are worth looking into. The float valve would be the first place I would loo.
 
Likes: Okiewan
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Mar 23, 2010
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#3
Thanks for that. I pulled the pipe off and found the rings seized so I guess that rev-out took its toll. Piston seems to be in good shape but the cylinder wall does have some scoring on the front side. I'll take the cylinder and piston into the dealer and see what they think. Last time I seized the rings in this bike (had it jetted too lean) I free'd them, circa 2007, and the bike has run ever since! Hard to believe..
 
Likes: Okiewan