kymco 500 uxv

[k.r.hall]

my kymco 500 uxv, will not start it turns over but wont fire up, it made a loud pop going down a hill and cut off, also it back fires threw air breater and some fuel runs out there as well. anybody know what it could be. thanks k.r

[CROSSBOLT]

What year, how many hours and miles, did it ever start again after the "pop" and cut-off, how long has the air filter been in there? Stuff like that....

[k.r.hall]

will not start new air filter 400 hrs been doing great , from what i have read so far could be a timing thing, valve train problem just need some input on how to try correect it

[k.r.hall]

it is a 2015 model

[CROSSBOLT]

Well, first, some q & a has to be done: how much do you want to do to diagnose this?

[k.r.hall]

well dont know what you mean i said what happen , i would like to try and fix it myself if it not to serious. just an idea of what would cause it to not fire up. valve train maybe decompresion  unit or timing off somehow. thanks

[CROSSBOLT]

That's what I wanted to know, that you wanted to fix it yourself. My UXV500i has no compression release that I know of. I would tend to agree that it may be a timing issue. You will have to remove the plastic covering between the seats to access the valve cover, remove the valve cover and the spark plug. Turn the engine by hand until the valves are closed (there should be a series of three holes on the cam sprocket, two should be parallel to the flat part where the cover seals). There should be a screw cover on one side of the engine that will show a mark "T" that shows the crank has the piston at top dead center (TDC). You may have to rock the crank forward and backward to find it but once the "T" is centered in that hole check the cam sprocket and valves as before to see if the valves are closed and the holes are lined up with the machined surface. The valve timing is correct if all this checks. Inspect while you are doing all this for an exhaust lobe on the cam looking rough or worn down. Check valve clearances to verify they are at least close to .004" intake and exhaust. Sparkplug appearance may be checked. Some of these other guys actually have more experience with this exact machine than I have and they may chime in with some other ideas. But this will get you going. One other thing: when you have been trying to start this does it sound the same as it always has?

[k.r.hall]

thx karl good info, and yes it does sound the same as always no funny noise.

[Stig / Major Tom]

Karl, are you thinking the timing chain may have jumped a tooth on down hill engine over run?
Stig

[CROSSBOLT]

Yup, something along that order, Tom.

[k.r.hall]

ok karl , got it apart it jumped time 2 valves need replacing there is a worn place on exhaust cam , the timing chain was extremly tight i dont see how it did it but apparently it did, piston looks ok. anything else i need to check? thanks ken

[CROSSBOLT]

That was FAST! The worn place on the cam, is it pretty severe? You should replace the cam if the shape of the cam has been changed or is galled badly. Cam lobe hardness was a problem in the 500 cc scoots that was never addressed by kymco-they just replaced them several times without FIXING the hardness issue! Bent valves, right? I hate to admit I have not opened mine up so I have to ask: two valve or four valve? Roller followers or slipper followers? 

[k.r.hall]

4 valves roller one cam has a worn place like a chip or plating came off an yea it has flat spot both do.what a cluster deal .

[CROSSBOLT]

That's what I thought, KR. Roller followers should never have cam problems like that and have not in the 300 cc 4-valve heads. I know the warranty is out but this is a fault of manufacturing and Kymco has not fixed it. It may be a waste of time but maybe a call to your regional Kymco guy. They have been a reasonable lot to date. Find someone in your area with a hardness tester, a Rockwell as they are called, and test each surface of the two cam lobes. I will bet the one screwed up is real soft. Ball and roller bearing race paths are hard, hard steel and last a long time.

[k.r.hall]

ok will do , yea the exhaust cam had flat spot very pronounced and and a chip of plating about size of little finger nail other cam flat spot not as bad still should not have been that way