Carbon buildup on exhaust valves.

Hi all, I'm finally getting around to rebuilding the engine on the ZXR250 and I'm currently up to cleaning up the top end. I've pulled out all the valves and there seems to be an extraordinary amount of carbon built up on the exhaust side. Never having gone this far into an engine rebuild, what seems to be a large amount of carbon, might actually be normal. On 3 of the cylinders, the carbon also looks quite oily. What I want to know is, does anyone know why some of the buildup looks oily, what can I do to fix it, and is this a normal amount of buildup. Keep in mind that its a '92 C model with an unknown amount of k's on the clock (I think it shows around 50K). I have always run 98 oct through it, with regular servicing. I know that the high octane fuels will cause carbon buildup if the engine is constantly used at low revs, however this thing has been flogged to death since I bought it. The valve seats appear to be fine with no pitting or burning, and thats about as far as my expertise goes. A couple of pics attached, not sure they show the buildup real well but they give the idea.

Cheers
Marty

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carbon build-up in the exhaust ports is entirely normal mate, and that looks like normal build-up to me. You could go to the trouble and expense of having the exhausts cleaned right out of all carbon build-up, and polished (to reduce any further build-up) - but honestly, its a waste of money on a small hp bike like a 250.

The important things are condition of valves and valve seats. If these are good, and the clearances are still able to be set in the middle of the range - then all you need to do is check if there is any excess play in the actual valve-stems, and replace all the valve-stem seals as a matter of course.

Thanks Gos. I've now got the majority of the carbon out and it looks a shit load better. As I'm doing all of this by myself, by hand, and on the cheap, there's no way I'm getting it ported and polished (unless I can get it done cheap ). The stem seals look to be in good nick, but I gather it's better to replace them anyway? Are they a "one size fits all" thing for bikes or do i have to have them matched to my valves. I've had a bit of a browse on the interweb, expensive little suckers. Can you get away with not replacing them?

i'd give 'em a quick hand lapping too

...... I gather it's better to replace them anyway? Are they a "one size fits all" thing for bikes or do i have to have them matched to my valves. I've had a bit of a browse on the interweb, expensive little suckers. Can you get away with not replacing them?.......

With the head off, and with unknown k's on the motor - I would spend the extra coin and get them replaced now. They can be exxy, and you will probably find that this size seal is common to a range of motors in this capacity. But you need to get the right ones. Yes, you could get away with not replacing them now - but down the track, if you get any blueish-white smoke on startup, then that will be the valve-stem seals failing - and you will also be up for replacement head gasket etc when you do them at that time.....

far better to do it now while the engine is apart. JT is on the money about hand-lapping the valves back into the seats, this ensures a good seal for the valves, and removes any minor pitting etc. Cheap to do as well

No worries. Thanks for the info. I'll take one of the seals for a ride to make sure I get the right ones. Cleaned all the carbon out last night, it almost looks new.......almost I already had the stuff for lapping, but thanks for the heads up. Hopefully I can get the seals around here.

Replacing the stem seals may cost a bit of money, but doing it now while the rest of the engine apart is going to be a lot less work than removing the head and redoing them later on. Effort wise, doing the head is one small step short of just taking the barrels off and checking the rings for wear. Cost wise, its a noticeable chunk of change for a 4 cyl 250.

At a cost of between $80 - $120, I'm thinking the old ones will do for now. My guess about 50K being on the clock was way off, it's showing 36665. I think when I bought it, it had around 25k on it however being a grey import, I'm not sure how honest that is. Regardless, I'm not sinking too much cash into it, it is only going to be a tracky after all. Mike, the whole engine was stripped down so I've already checked the rings, bores etc. The whole engine appears to be in quite good shape, so at this stage I'm not too concerned about the seals, not at that price anyway. I guess I'll find out down the track if it's the right move.

i'd spend the penny to save the pound mate