KX250f Overheat and loss of power through gears

5/28/2021 5:58am Edited Date/Time 5/28/2021 5:33pm
Trying to diagnose what the issue is with my bike as I just got it put back together and its not running to full potential - the bike is a 2017 kx250f.  I took it for the first test ride yesterday to see how she ran after tooling on it for the last few months and it ran but not great. Here's a general overview of the ride and then I'll touch on the work done to it:

-It starts in 1-2 kicks and sounds/ idles fine.  If you twist the throttle and let off the rev calms back down to an idle.

-1st gear felt fine, but as soon as I shift into 2nd-4th the revs sound much higher than they should be and I almost instantly had to shift to the next gear.  5th gear rev sounded fine and at that point I was going pretty fast but it should be able to push more out for sure. I have the same bike but a 2013 model that I ride all the time so I have a good baseline and this just doesn't feel right. 

-With clutch pulled in and downshifting to prepare for a turn the bike would stall once hitting lower speeds unless I blipped throttle a few times to keep revs up.   Clutch seems to engage fine and bike doesn't pull forward if clutch is pulled in while sitting still in gear. 

-Right as I was pulling up to put the bike on the stand I felt a decent force of air blowback out of the airbox hit my leg and the bike shut off. This was the only time that happened and the engine was not locked up after (see next bullet as to why I say that). Could be related to the bullet point above??

-At end of the 20 minute ride the bike was overheating.  I checked coolant and it was full. There is oil in it too. I only checked for overheating because it shut off as stated.

Not sure if I have a few different issues happening or they are all related to one thing; such as timing being off, an air leak, or hopefully not one of those + I ran it hot for too long and damaged the engine. 

Back in December the bike was running fine.  I had to reshim the valves on my other KX and since I had never done before I opened this bike (2017) as a reference. The 2013 ended up being done right and runs like a top so I decided to reshim this bike since I already had it opened and proved I could reshim properly. Got it shimmed and closed up, but as I was putting the gas tank back on the fuel tube on bottom of tank got stuck on the nozzle it feeds into (that ultimately leads to the injectors) and the nozzle snapped. To replace this nozzle I then had to pull the air box and throttle body off the back end of the bike just to reach the screw to take the nozzle off.  Very unideal to open this much of the bike up before ensuring the timing is right but I had no choice since I had to fix the nozzle issue as the bike couldn't get fuel without it. So I'm thinking the issue may lie in one of these possibilities... timing is off a hair bit via my valve shims not being in spec like I think they are and have correctly done before, or my cam is off by one tooth...OR while reinstalling the airbox and/or throttle body I created an air leak causing a power loss...OR possibly something else happened during the 6 months the bike wasn't ran like gaskets drying up or common maintenance issues.  

I will say I think the shims are fine, but the cam sprockets and alignment on this bike wasn't as clear cut as it was on my 2013 bike. It seems to be an issue for others on the 2017 and 2018 KX 250s (see link to forum below). I count 28 pins across, but there are 13 pins between the top dot on the two cams where it should be 12.  The guy in the other forum had that exact condition and eventually followed up to his post and said he ran his bike with that setup and it did just fine for 60 hours after he closed it back up.  

https://www.vitalmx.com/forums/Tech-Help-Race-Shop,42/Is-my-timing-off-…



Any thoughts on the causes of the symptoms? Thanks!
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Paw Paw 271
Posts
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Joined
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Location
Benton, LA US
5/28/2021 5:33pm
Replace cam chain.
Reseal throttle body to intake and head.
Make sure your valve clearances are correct.
Clean fuel injector and fuel filters.

Paw Paw

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