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Last week I bought a used 05' 400ex. After getting it home, I noticed the quad was a little hard to start but with a little choke and a little blip of the throttle, it would fire and run great. On my third ride around the yard with my son, I came out of a corner in 2nd/3rd gear and the quad just died rolling to a stop. Tried restarting several times with no luck. The engine would turn over without any weird noises or anything but it just wouldn't fire. After playing with the choke and throttle, I got it to fire and run long enough to pull it into the garage. Since the quad came from a guy that had it stored for winter, I decided I would check out the condition of the carb. Took it apart and checked both jets but found no blockage, but I went through and cleaned the jets up anyways while I was still there. I also took this time to change the oil and oil filter. The next day, I pulled it back out to see if I could get it to start and run. At that point it was still hard to start, but I noticed an odd sound occasionally coming from the right side of the case. I would described the sound as a brief chatter that would only occur maybe one time for every 4 attempted starts. Eventually I was able to get the quad started so I took it for a ride down the road to see if there was any issues with power loss. About 1/8 mile down the road the quad just stalled and died again. Finally got it started back up long enough to ride it back home and into the garage where I replaced the spark plug just to make sure that the spark was strong although the old plug didn't seem to perform any less than the new plug. Once I was able to get it started after replacing the plug, I could get it started with a little effort and it would even idle, but as soon as I gave it any throttle, the motor would stall. Suspecting a valve adjustment was needed, I pulled valve cover inspection plugs. The intake seemed a little tight at .004 but the exhaust was pretty much spot on at .005. After getting the intake back into spec, I got the quad into TDC on the compression stroke (Timing mark on the T and the cam lobes facing down) so that I could inspect the valve train and cam timing chain. What I found is at TDC on the compression stroke, the notch on the right side of the cam gear (when looking right at the gear) was maybe one tooth above the surface of the head while the left side notch was one tooth below the surface of the head. To me is sounds like it skipped time. I have to pull and inspect the automatic timing chain tensioner just to verify that it is still fully functional, but to anybody who has more experience, what are the chances it jumped time causing a valve to hit the piston? Should I just replace automatic tensioner, set timing and try to run it again to see what happens or should I pull the head to inspect valves and pistons and possible replace the timing chain just incase the one currently in the machine is stretched. For what its worth, the quad was said to be entirely stock with the exception of a full Yoshimura exhaust, K&N air filter, 450r shocks, bigger capacity oil tank, and aluminum intake/battery box. I don't mind taking the head off, just not sure if that's necessary at this point however I understand that if the piston did hit the valves, I am looking at a new piston, valves, and possible a bore if there was any internal damage at a minimum. Thank you in advance for any info/suggestions.
 

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