You can be pretty rough with the valves. You need a propper valve spring compressor to remove them though. The spring compressor is like a big G-clamp. Once the springs are compressed, pick out the retaining collets and lift off the valve cap. Once the valves are out you can simply scrape the carbon off and clean them up with fine emery paper. Both the valves and their seats are bound to need recutting though; this is a job for the machine shop. You can "lap" the valves in for final seating.

Wind some old spark plugs into the holes to protect the threads. Carefully clean the combustion chambers with something like oven cleaner. Don't get heavy with a scraper or wire wool on the alloy head!!

The pistons will undoubtedly be shagged and need replacing!

To get the pistons out you need to remove the sump. You can often unbolt the big-ends and lift the pistons/rods out the top of the block without removing the crank, but you will want the crank out anyway to look at the main bearings. Not sure on the FZR250, but the engine block may be in 2 parts, with the cylinders lifting off as a unit.

If it's purely an exercise in pulling it apart to see how it works then great, but if you are going to recon' it, don't bugger about, get a Yamaha Workshop Manual. I would imagine it's very very similar to an FZR400, so get a manual for that too, as it will be in English and the information will be very relevant.

Finally, don't ask how I know anything about 4-strokes... it's my Forum, and I can ban anyone for asking the wrong questions!!!!!
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Andy.
NSR-WORLD.COM

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