Help Lapping Valves - ZXR250C

I agree those would need to be ground unless you want to spend a bunch of time lapping.

 

In every TV ad that I've seen for washing powder, it makes people unnervingly happy (not quite right in the head in my estimation), I guess you're the exception.

 

Who can forget mister sparkle

 

You may have needed a citrus blast...minus the crooked lawyer

 

Ideally, you would cut the valves until there is no pitting visible, same with the seats. The exhaust valves and seats are usually worse for this.

Cutting the seats allows you to move the position of the sealing surface on the valve to the correct position, roughly in the middle

If the surface is too far towards the face of the valve with little margin, indicating valve recession, then you would crown (30/31 degree cut) and then face (45 degree cut) to move it closer to the centre.

For valves and seats in good condition, you just cut the valves so there's no pitting visible, and do a light face, crown and throat on the seats and check the width of the sealing surface with some bearing blue or similar.

Without any cutters, the best you can do is clean up the valves and lap them in until they seal.

 

The trouble is that the valves and seats are so small on these i4 250's that most commercially available cutters are too large to fit the valves and the seats.

Here are some images that will help with some valve terminology









As always, specifications for your bike will be in the service manual, cylinder head section in this case.


When you cut valves to restore the surface and remove pitting, you lose margin. Depending on how bad they are, you can end up with not enough margin. This is bad because the valve springs could overcome the pitiful amount of material left, especially at high RPM, you will either burn a valve and lose compression if you are lucky (seat width affects this too) or it will drop the valve and grenade the motor.

 

That valved appears dished (gone concave) but I think it's just got carbon buildup on the back angle and radius area. It's too hard to tell.

The first step is to clean it up for assessment. I usually use a wire wheel on a bench grinder. If you don't have that available, some scotchbrite and wd-40 or similar will work. Clean the stem as well.

It is critical to have the stems as clean as possible, as when lapping any grinding paste, grit or dirt on the stem will cause the valve guides to wear, which is bad.

Here are some pics from my ZXR250C thread, before and after cleaning. The dull coloured seats indicate it was not sealing.






This is a bad valve from a 2nd hand 'serviceable' head I bought. You can clearly see how concaved it is, very thin margin, and the fact it's not a straight 45 degree angle along the face. Junk.

 

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I'd clean them before anything else so that you can see the metal minus the carbon, ditch the ultrasonic, put them into drill and spin them at low speed with a squirt of Inox and a green put scourer, or if you have one a dremel with a soft wire brush against the direction of the drill - only issue with that is that the soft dremel wire brushes don't last long.

 

Perfect solution - now you can see what's what and you know what you're dealing with.

They don't look too bad at all, with the carbon caked on they looked like they were seriously worn