NX250 valve clearance too small

[kuroguro]

post separated and opened up a separate thread - admin

Near 87000 the bike started to be really hard to start. There were some hints of this in the past, I thought it could be either the carb or the timing chain. In the end the top was taken off. While the chain is quite worn, it does not look too bad. However measuring the valves revealed that the intake had 0 clearance (exhaust perfectly in spec). Even with the shim removed the gap is still too small, hopefully swapping the valves will fix it. I don't see the OEM parts anywhere so I guess I'll try China.

Interestingly unlike most other bikes, the intake valves seem to suffer more often on the NX... odd.

https://www.advrider.com/f/threads/nx25 ... t-53257743
https://www.advrider.com/f/threads/nx25 ... t-18413993
https://www.advrider.com/f/threads/nx25 ... t-35233531
https://www.advrider.com/f/threads/nx25 ... t-33808645

[scrambler]

However measuring the valves revealed that the intake had 0 clearance (exhaust perfectly in spec). Even with the shim removed the gap is still too small.

That's actually very strange. I have no idea how that could happen. Somehow, the valves must have suddenly worked its way deep into the seats. Or were the valve heads eaten away? What do you see? Pictures would be nice .
But there are solutions without Chinese valves You could try swapping the bucket tappets – according to the workshop manual, they can have four different strengths. However, I have never measured it.
Or you could start with the camshaft. A few years ago, the engine overhauler messed up and milled the valve seats too deep, so I had the same problem – too little valve clearance viewtopic.php?t=69 My salvation was a MD25 camshaft – same lobe lift but a 1 mm smaller base circle. This allowed correct valve clearances.

I could provide used valves or MD25 camshafts for your repair. Or, if there's a metal shop in your area that can do it, you could have the camshaft reground megacyle style.

[kuroguro]

I see, I'll have a bit of a think about the options. Thank you!

Somehow, the valves must have suddenly worked its way deep into the seats. Or were the valve heads eaten away? What do you see? Pictures would be nice .

I have a feeling it was out of spec for a really long time since I never checked. It probably just reached the point where there's a bit of blow-by? Maybe.

The chain rattle from the other thread didn't match your videos as well, blipping the throttle didn't produce any rattle, just specific RPMs - I suspect the cam was already "dragging" on the valves as it was fairly hard to turn by hand.

I don't know how to diagnose what exactly happened - I can take more pictures just need some instructions for what to check. A basic leak test didn't pass any liquid, however there are some deposits on the back of the intake valve.

The lip of the valve is slightly rounded but doesn't look terrible, I think?

The intake seems to be a bit longer than the exhaust 84.6 vs 84.2 mm - but I'm not sure, maybe it should be this way?

Btw I'll be offline the next week for a work trip, more pictures will have to wait until the weekend

[scrambler]

The workshop manual doesn't say anything about the length of the valves; I can measure them next week.

NX250 cylinderhead valves specifications.jpg

What seems to be important is the width of the valve seat. In your pict, it looks like it's too wide, so it was probably knocked wide by the valve and needs to be reground. I would ask an experienced engine rebuilder if the valves are still usable and if so, go with them. Then enlarge the valve clearance via camshaft. Perhaps that was also the reason why MD25 camshafts have smaller base circles.

Have fun on your trip, don't work too hard (or hardly working)

NX250 valve seat inspection .jpg

[kuroguro]

What seems to be important is the width of the valve seat. In your pict, it looks like it's too wide, so it was probably knocked wide by the valve and needs to be reground.

You're right, good catch! I somehow missed this in the manual. It looks like the contact area is about 2mm. Also the valve lip is not rounded as I first thought, in the right light I can see it has a sharp angle in the middle of the contact area. This makes me wonder if it's the valve that's losing the material, not the seat/head? But the valve should be stronger... hmm. Either way I'll try to find someone to look at it.

seat.png

Have fun on your trip, don't work too hard (or hardly working)

Thanks!

[kuroguro]

Small update - I had ordered some Chinese valves before I made the post, just because they were cheap, and they finally arrived.

Good news - inserting and rubbing the valve on some red marker reveals that the seat is making good contact and it is about 1mm in size. So it looks like most of the damage is indeed on the old valve and not the head. I'll try to consult still to make sure, might have a lead where to find someone. (Excuse the bad camera focus )

marker.jpg

Bad news - there are at least 4 different Chinese valves for some reason... which means it will be hard to track any reviews to know which ones are any good if someone tries. After digging trough multiple sellers, the ones I got (marked "KW3") are the only ones that had a complaint . It is not clear however if it's not an installation error.

kw3_bad.jpg

The other Chinese ones are marked "DQ" (by Dadong for Shineray), "Y" and "3" like OEM. More on the KW3 I got - they are both exactly 84mm in height. The stem diameter on the exhaust seems slightly smaller than OEM, my caliper is not precise enough but I can feel less air resistance if holding the valve guide closed with a finger. I was not planning to use the exhaust but the size difference is worrying. It could be within spec tho, I have no way to measure atm.

kw3.jpg

So between the bad review and stem diameter mismatch I probably won't be installing these.

I did some digging around to see if any other intake valves could potentially fit. Of course there is no way to fully know unless obtained and checked carefully but here are the best candidates so far:

TKRJ YY0410A: https://www.tkrj.co.jp/goods/m-YY0410A-2GS-E2111-00.php

Inexpensive even with shipping from Japan, however they are too short by about 4mm. Even the thickest shim of 3.5mm would still be too short (+ stock shim difference). I was hoping the different thickness buckets you mentioned might help but according to valvecrusher it's likely a mistranslation: https://www.advrider.com/f/threads/nx25 ... t-33879245

Maybe if the seats are extremely recessed or if some aftermarket bucket is found.

Ferrea F0170: https://www.ferrea.com/yamaha-competiti ... alves/p563

Oversized competition valves (+1mm dia) for 98-01(?) YZ400F/WR400F/YZ426F(?).
The dimensions are basically perfect. But... quite expensive for an experiment, would probably be over 100e to get 2 of them in Europe.


From a practical point of view I should probably take up your offer for the used valves but on the other hand it would be good to find a decent alternative for the future as the OEM stock has ran out. I guess there is no rush for me to decide as the season is basically over.

Edit: btw do you have some secret recipe to get the carbon off? I've been scrubbing it forever with carb cleaner and wd40 and I'm barely making any difference

[scrambler]

Thanks for the basic research. I was worried that the Chinese valves wouldn't fit either, and it seems to be the case now.

I've never actually done anything on the valves myself. I always have the cylinder heads overhauled by a professional; if they just clean and reground the valves, it doesn't cost much. Unfortunately, that wasn't possible with the cylinder head of my AX1; the professional was booked up for the rest of the year.

So I removed the valves myself for the first time. I checked them beforehand with brake cleaner. They seemed to be tight, so no damage was expected. I clamped them in the drill and then cleaned them with soft steel wool. This worked well for the soft carbon deposits on the exhaust valves, but not for the hard crusts on the intake valves.

[kuroguro]

Scrambler, could you confirm a good OEM intake valve's length if you have a spare moment? I can't figure out if my intake valve has gotten longer somehow (~84.5mm after cleaning carbon from bottom) or if the seat has recessed in the head. And for some odd reason the Chinese KW3 (84.0mm) has perfect clearance

I've confirmed that KW3's stem gets fatter on the bottom and is out of spec (max 4.490). That likely caused floating valves for the reviewer after warming up the engine, so definitely not usable (maybe only if valve guides are measured and worn out as well ).

kw3_fat.jpg

[scrambler]

I knew I had a new OEM valve lying around somewhere; I just had to find it . It doesn't look like yours has gotten any longer.

[kuroguro]

Thanks! Agh... so the seat is bad then too. Definitely gonna need a machine shop to replace those.