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When to Check Valves Again?

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So my 2017 KX250f is my first 4-stroke and I just hit my first 15-hour mark (yay!). That said, I am paranoid about its maintenance. I have an hour meter and have been keeping a detailed log about when I clean air filter, change oil, etc. Any maintenance at all.

Anyway, I have the manual of course. I measured the valve clearances and came up with these numbers:

Intake Left: 0.13mm
Intake Right: 0.13mm
Exhaust Left: 0.17mm
Exhaust Right: 0.18mm

The intake valves appear to be comfortably within tolerance (intake tolerance 0.10mm~0.15mm), but the exhaust valves are on the tight side of the spec (0.17mm~0.22mm). This brings me to my question: when should I pull off the head cover again and re-measure the clearances? Since my exhaust side seems to be on the tight-end (even though still in spec) should I still go another 15 hours (like the manual says)? Or check again in 2, 5? Shim them now?

Thanks for the help!

-RageD

P.S. I'm usually one for just "trusting the manual" (and I do) but I'm wondering if your valves are nearing out-of-spec if you shouldn't just blindly follow the maintenance interval. I'm not sure whether or not the engineers have put 0.17mm as a lower limit for any riding or the lower limit for riding another 15 hours? :)

Edited by RageD

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Every 20 hours of run time for the typical owner is plenty. If it is a track only, race bike, then every oil change/day. You may also want to include leak down tests at the same time. Rings wear even in the best maintained machines.

Valves tend to stay in tolerance unless sheer wear (100's of hours) or poor servicing (air leaks, filthy filter). But once they start to close up, they tend to do so faster and faster until catastrophic  failure. My WR has 180 hours it now. The valves have not moved.

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The valves on my '16 tightened up in a similar fashion. I would definitely re shim the exhaust, personally i would do the intake as well but im sure you could go a little longer on those. Shim to hit the higher end of the spectrum

Sent from my LGLS991 using ThumperTalk mobile app

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Great thanks for the advice guys. I usually only ride track, but I'm a mediocre C rider at best :) (I don't currently race-- so hard to say exactly), so I never bounce of the rev limiter or anything like that.

I was planning to go out tomorrow for maybe an hour or two of practice; I may give it a whirl since it's still in spec but order a shim kit to get all the valves closer to spec ASAP.

Thanks for the info!

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But once they start to close up, they tend to do so faster and faster until catastrophic  failure.


Actually I've been thinking about this-- does this imply that the wear tends to be quick from the moment you start to reshim? I.e. Your valves are either good and they hardly ever need to be reshimmed or you reshim more and more frequently until the valve needs to be replaced? Also, in that case, when you replace a valve do the seats also need to be recut to match that specific valve?

Thanks!

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19 minutes ago, RageD said:

 


Actually I've been thinking about this-- does this imply that the wear tends to be quick from the moment you start to reshim? I.e. Your valves are either good and they hardly ever need to be reshimmed or you reshim more and more frequently until the valve needs to be replaced? Also, in that case, when you replace a valve do the seats also need to be recut to match that specific valve?

Thanks!

Pretty much. Once the clearances start to go, the speed at which the tightening up increases dramatically. Most valves have a hard coating on them. Once that coating wears away, there goes the layer of protection. MY rule is, reshim once and check again within ten hours, if they moved agin, stop riding and get the valves done. A competent shop will know if the valves can simply be replaced or if seat cutting is needed. If you pull the head and give it to them to do (make sure they use a SERDI machine if the seats have to be cut), be sure to include your shims, buckets and cams and let them reshim. Once teh head on the bike, recheck the clearances and then again at the next oil change. Sometimes, a new valve job has to 'settle into place;' and clearances may widen. When you reshim, do not just go by the markings on the shims, double check with a micrometer.

Now, I do not necessarily think that may be happening you in your case. Valves have a range that is acceptable. If when the engine was built, the clearance feel inside of that range, that is it. There is no effort to make the clearance right in the middle of the range.

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Awesome thanks. I've got a hotcams shim kit on its way now, so I'll probably get to reshimming in the middle of the week.

Of course when I bought the thing I didn't think of checking the valve clearances to get a baseline :( so I'm not sure whether the valves are wearing yet or if that is just where the clearances have been. Either way, I'll pull the head cover off again at around 25 hours and measure after the reshim.

While we're on the topic, when replacing valves what should I replace? I usually buy my own parts and bring them to the shop. Should I replace the springs, buckets, retainer, seals, etc. or basically just the valves themselves when needed?

I usually do all of my own work (been a 2 stroke guy in the past) except for cylinder replating which I would send out in the past. Point being, I don't really have a shop in the area I trust yet :) so I'm always weary about going to someone without the parts in hand. Anyway, if it's usually hard to say without seeing I guess I'll have to just trust someone at some point xD

I mean hey, I'm trusting some guys on the internet (but at least I can go look at your old posts and see a track record of sound advice). Boy it's tough to establish that relationship :) But TT hasn't steered me wrong yet!

Anyway, the 4t is fun I must admit. I'm excited to have the chance to do some more work on it too!

Thanks again guys!

Edited by RageD

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