I have a good amount of experience working on engines, but I wanted to get some other points of view as I sure don't want to have to waste any more money.
Bought a very clean 2018 KX250F from original owner, not running. Upon disassembly the head wasn't getting oil and smoked the camshafts and head. Bottom was in spec, everything else good to go.
Purchased new OEM head, installed old valves with new seals, new gaskets, new hotrod cams, new piston, new cam chain...
15.5 hours later had engine failure. Broken intake valve guide and looks like both intake valves are bent. You can see that the one valve made contact with the piston.
Trying to figure out what went wrong here. My thoughts are:
1) skipped time/faulty tensioner. These are basic OEM ratchet style tensioner and seems to be functioning when I removed it.
2) unknowingly reused a slightly bent valve that wiped out the guide?
Interested in hearing some thoughts from any guys out there with experience.
The only thing that will break a valve guided is the valve contacting the piston which induces a radial load on the guide. You need to figure out why the intake valves contacted the piston.
You put 15 hours on the engine before the failure so that rules out any error on assembly. My guess would be that it somehow jumped timing or you lost spring pressure and it ate a valve at high rpm.
What piston did you put in it? What hot rods cam ?
Stock sized vertex with stage 2 hot rod cam.
What would cause the loss of spring pressure or for it to jump time? Let's also say I put it back together and say it was one tooth off, would it even run...smoothly?
Just concerned whatever happened here doesn't happen again.
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Honestly, you have to inspect piece by piece and make sure everything is in spec. The fact that it broke an exhaust valve tells me the cam timing was retarded (cam chain moved forward relative to the cam sprockets). Did you take apart the bottoms end completely and check all of the oil galleys to make sure they were all clear and the oil pump/scavenge pickup were all ok?
When timing the cams, are you timing your exhaust cam first?
Also own a 18 KX250f just did a “crank-up” rebuild on it i’m at 45 hours steady.
did you replace cam chain guides? i always wonder if they wear enough to not put as much tension on the chain itself as it wears and thins out..
17 through 19 KXF250s break intake guides and chip intake seats when they feel like it. Hybrid copper guides and seats are the fix.
You need to check The exhaust cam for gear slipping (both cams). I have repaired 3 engines that had that problem and all 3 were stage 2 cams? one may have been stage 3 ? The gear is only pressed on. To repair cam you need to press gear off realign and press back on. Then 2 or 3 spot welds to eliminate happening again Take some pics of your cams and we can tell. edit= Lay the cams in head with the timing marks in time and the lobes direcection will tell.
Number one, DONT USE any thing other than OEM KAWASAKI piston or CP piston.
number two: Dont USE chinese Camshafts.
Number three: Dont re-use Valves. EVER!
That would make a lot of sense if that was the cause, I will check that out and get some pics - thanks!
Ok, so moving forward would you think it's best to replace all the valves with either OEM or should I be fine with the ProX titanium valves?
Oem: piston, rings, valves guides, cam chain, valve springs?
I have spent way more than I have available so need to try to only buy what's necessary to get the bike back up and running.
del west, Or OEM kawasaki, only if your gonna install new seats and keep bike, instal new seats, new valves, new retainers, new collets, new tappets, new shims,
thats why going pro x doesnt make sense, cause pro x titanium valves might say they fit inside the guide right, but the oem retainers collets and springs may not mate to the valve perfectly like oe kawasaki valve thats why i am huge on just using one brand for the entire assembly
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