15 YZ450F Hard start problems

relentlessracing769
Edited Date/Time 5/31/2020 11:07am
Hey guys I'm super stumped right now. I have a 15 yz450f had a decent amount of hours on it. Haven't really kept track. Started getting terribly hard to start. Checked my valves they were out of spec, shimmed them we're good. New plug new gas. I have a hard time getting it to fire, when its cold it fires on kick 1 or 2 but when I've been riding it makes it terribly hard to start again, every now and again it backfires back with the Kickstart and whips back up at my foot. I have thrashed my Gaerne 10's from kicking it so much. Took it to my local shop, they took three weeks to put a new plug in and tell me there is nothing wrong. They said valves are in spec and did a drip down test. Everything to them checks out, clearly not because nothing has changed. Any leads? Any ideas??
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Factor E
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Factor E 5/31/2020 5:26pm
5/31/2020 5:26pm
You had no change after adjusting valves.
Then had backfire after or before adjustment?
With unknown hrs, (decent amount)- needs new piston

Shop should have given you % of leak down to help determine if new piston or valve job.

Yamaha-ck spark plug cap
relentlessracing769 5/31/2020 5:34pm
5/31/2020 5:34pm
No change in backfire and hard start after I adjusted the valves. If I was to guess I would say 50+ hrs on bike since I've purchased it. They did not give me a % but said that it cleared the leak down test. And didn't throw any codes. Timing is on, plug has been changed.
Factor E
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Factor E 5/31/2020 9:27pm
5/31/2020 9:27pm
Yamahas know for coil to go bad
BReid271
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BReid271 6/3/2020 3:50pm
6/3/2020 3:50pm
Definitely do a leak down test, I stretched an exhaust valve on a 16 YZ450F after 30 hours and the bike wouldn't start after that. But I also raced the bike and would rev to the moon...

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