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A good friend who is also an excellent builder - recently thinks he also found another issue - but I can't share that out of respect for him and his hard work in trying to solve this.
Long and short - we've seen hardly any failures with an upgraded valve spring set up - the OEM springs themselves are durable - but the rate - and preload amount yamaha went to for 16-18 is problematic - at least from our experience.
I do believe certain users cause the bike to experience some form of resonance within the chain and valve/valve spring.
And of course - some people are just unlucky.
There are plenty of them out there - with zero problems.
The ole 5 valve turd - was so reliable in hindsight...it was the perfect engine at the perfect time to get the sport converted over.
The Shop
The old ones were still rock solid oil tank removed...and down at 1 qt - while flow was shit and ultimately power potential
Someone here said it happened twice to them in less than 20 hours?
I wonder if they just rebuilt the head and went with the original chain and tensioner unit after the first rebuild?
Sounds strange that it could be just some riders causing a weird resonance ( is that really valve bounce ) problem?
If the problem were "easy" to understand - or solve - yamaha would have never had it. They are "smarter" and certainly more well funded in researching the specific root cause of an issue than just about anyone. I may be "solving" the issue with a spring package change - that isn't actually solving it at all - just a bandaid that holds it together.
Some of which DIDNT abuse - and barely even USED the bike - we had one at like 7 hours with a vet guy.
Then there are some that went 150 hours - lost a rod bearing (totally normal) rather than a snapped rod - and had the best of luck.
Personally - anyone with a 14-15 I was suggesting at the time a carillo or falicon rod - and when the 16 came out - a 16 crank assembly.
I DO believe you are on borrowed time - and when they let go -it's disastrous. more often than not it claims the cases, cylinder, sometimes the head and valves, and of course the crank.
It's far cheaper to buy a new oem crank - bearings, top end - and just go through it all. The counter balancer got an update (throwback to another thread) to match up with the newer crank's different balance factor to alter the perceived vibration - so you can get that too if you wish - although it still works ok with the 14-15 balancer.
My 14 went 2.9 hours locked up. Turns out, first batch of 14's had an issue with the water pump shaft breaking. Yamaha was pretty cool and provided all the new parts necessary to rebuild it. Ran it the next season and a half traded for a 16.(I have two friends that had 14's, exact same thing.)
The 16 was fresh when I got it, only ran 44 hours traded for the 2017 I now have. So far its been great. 97 hours of MX and woods riding. I'm just about to freshen it up. Believe i will do a big bore kit on it.
However I know of somebody else that went to Yamaha's for 18, and literally had issues from hour 5 on. I'm not sure just how many times the bike failed. But it diffidently was the timing chain. They finally started to change the timing chain after 1 hour of racing. They have now went to a different brand. So i believe the 18's were an issue. I just bought one, and when I do rebuild I am going to have piston mfg. add .002-.004 to valve relief to just add a little measure of safety. And I will replace timing chain a little more often to. I still like the YZF's. I seem to be more comfy on them than others. Though a few years back I did a CRF270F BB and it was the most fun bike I have ever ridden. Fast Core mods head, Hot Cams stage 2 cams, injector mods, remap, FMF, and sat on Enzo suspension.
Wrekt...lol
16 yz250f Vortex Ecu, Stock comp vertex piston, kibblewhite Ti flat top valves, reground oe cams.
Fast novice rider, moto.
Low hours on rebuild.
Pit Row
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