This is for advanced users only... or people willing to says "whoops" at the proper time.
IF you don't want to pull the entire top end to replace one or more leaky valve stem seals, or IF you want to test and see if a particular valve is "sticking" because it may be slightly bent, here's a way I found to do it.
YMMV, be careful, don't blame me if you break something, etc.
The pic attached tells the story pretty well. This is on a CB550, so it's the same for a 500, 400 and 350-4. You have to remove the Cylinder head cover which lifts the rockers away from the valve stems of course. (see other posts about the proper way to do this so you don't create a new problem while trying to solve an existing one).
Remove the spark plugs. Rotate the engine (using the kick starter or stator bolt or very carefully/gently use the bolt at the end of spark advancer on the ignition side) until the cylinder you wanna work on has its piston at TDC. You can figure out how to lock it in this position I'm sure.
Cover all the ways a small thing could fall down into the engine with tape or cloth or paper towels...Do this around the cam chain galley especially. Be ridiculously careful about this. Imagine these little tiny parts are trained saboteurs who have legs, and they are TRYING to RUN to the smallest gap to find their way into your engine and transmission's nether regions where they can do the most damage. Taken seriously this will provide the proper motivation to fully idiot-proof your engine from falling detritus. Oh yeah, I also put the spark plugs back into the other three cylinders at this point because I don't want something falling in one of those holes either.
You will need a way to keep the soon-to-be-free valve from dropping down into the combustion chamber. I had some clean cotton rope (no loose fibers) available. You can also buy a roll of clean lamp cord or window shade cord. You've already pulled the spark plug and that's the hole you are gonna use to feed that clean very clean/no-loose crap (don't use cheap twine) rope down into the combustion chamber. Use a sharpened chopstick to squish as much in there as you can. You wanna fill that domed space up as completely as possible, and if you do it right, the valve will not move down AT ALL when you release the spring tension that is now holding it up/closed tightly. Got it? Ok, now you want to compress the valve springs (there are two, nested).
I found a piece of shaped steel that I had from some other project (I think this came from a garage door opener mounting kit or ?? whatever). The key is that it is strong and has slots for the bolt heads. I made a valve spring compression "tool" by notching out a 1" piece of abs conduit -- enough so that it retained its strength but also would allow me enough window to reach in with a magnet and remove the keepers (honda calls them "cotters" in the fiche).
I sourced a couple longer M6 bolts and then looked for best opportunity to get compression with the tool onto the spring retainer with pressure as straight down in the valve's direction as possible. This particular piece of steel had extra notching in it, so it helped that I could tilt the trailing edge of the Abs "tool" 45 degrees into the steel bridge. This way as I tightened the bolts straight down I was able to get compression on the valve spring without the tool sliding off the back. It took a couple tries to get right.
GO very slowly and evenly with the hold down bolts and you will soon see the spring(s) compress. Have a long thin magnet ready to pick the little keeper semi-circles off the top of the valve retaining top washer. When you have both safely pulled out and put aside, gently reverse the pressure on the valve springs by alternating between the bolts as you unscrew them a bit at a time. Soon you will feel all the pressure is off, and you will be able to pull the plastic valve tool you made out of the way, and snatch that top retaining washer with your magnet. Now you can remove your entire bridge rig to access the valve itself. Lift off the two springs. There are "seats" for the springs, one sized for each spring, sitting down in there. Sometimes they stay there, sometimes they stick to the spring and come along as you pull the spring. Just be sure you have one for each spring and set all 4 parts aside. If you have the service manual you can measure the springs to be sure they are still in spec.
You can now see your valve stem, and probably can also see your worn out, possibly side-ripped valve stem seal.
Remove the seal, and now you can work the valve up and down a bit... you'll feel right away if it is sticking/hanging up (usually just before it gets to the fully up/closed position) or if it is loose/smooth/easy to move up and down and you get a nice little "pop" sound as it tops out into the valve seat. You might have to pull a little bit of the rope back out to get enough "play" to do this fast up-down check, use a strong magnet to hold the side of the stem as you feed out the rope, and don't take all the rope out by any means. I am pretty sure would still be enough valve stem showing that you could grab it with your fingers and pull it back up, but why tempt fate? This is when you can also check to see if your valve guide is shot or loose. Pinch the top of the valve stem, lower it half an inch or so and try to wiggle it from side to side. It should NOT move laterally. The "perfect" valve/valve guide fit is quite snug, without friction, and not loose enough to allow the valve to wiggle as it travels up and down a million times. There is of course a measurement for this (in the factory manual) and you would need a dial indicator properly oriented and calibrated to measure it. But - if you wiggle it and can hear click/click as the stem smacks the side of the valve guide, you gotta pull and service the cylinder head. If it doesn't move side to side or BARELY moves at all, you're probably ok. You can ALWAYS pull the top end in the future, the point of this is to avoid having to do it now.
Here's where you wanna clean it all up, pour a little sea-foam down the valve shaft so it penetrates and coats between stem and guide. Install a fresh and nicely oiled valve stem seal. Pull the valve up as high and tight as you can with your fingers (the new seal will help hold it there) and feed any rope you removed back into the combustion chamber. PULL HARD straight up on the valve. PUSH HARD with your chopstick to get as much rope in there as you can. If the valve drops at all when you release it with your fingers, do it again/more. You want that valve as high as possible so it is properly located/seated and centered when you go to put it back together now.
Replace the valve spring seats (those two little thin flat hardened washers) and then place the inner and outer springs on top of them. Place the valve spring top retainer on top of the springs. Now put your bridge rig back in place where it was before, and reverse what you did. Slowly compress the springs evenly, checking to be sure the valve stem is sticking out the center of the top retainer and your compression isn't shoving the springs sideways into the valve stem. Compress ONLY enough to reveal the top of the stem - the lip there and a bit of the shaft so you can use your long thin magnet and the chopstick to deftly place the keepers where they go. They have a specific orientation, fat side/lipped edge is UP. Get em both in there at the same height centered, slowly release the spring pressure a little on each bolt to lock em in place...You might have to wiggle your rig a bit as you do this to make it all go back together correctly. When there is not more spring pressure, look carefully and compare the one you just worked on to the others. It should appear to be identical. Same exact amount of valve stem sticking out, same exact spacing between cylinder head wall bosses and the springs, etc etc. Remove your rig. Use your thumbs to press the valve down and let it come back up. I do this a few times - -it goes pop pop as it releases and then re-seats itself. It gives me final confidence that when the cam lobe swings around and lifts the rocker which then slams violently into the end of the valve stem, it won't resist the impact any more than the spring pressure is designed to create.
It sounds like a lot, but it's easier /faster and way cheaper (no new base gasket, cyl head gasket or o-rings needed) than doing a full top end job.
