First look up the clearance and go over to your local auto parts store and ask for a go feeler gage that covers that size. (It’s .10 mm) By the way make sure is narrow and bendable as some are not. I prefer go-no go types for beginners but you would have to get one from a tool truck or professional store. You will need valve cover gaskets and sealer and a scraper just in case. Start with the clear side with no sidecar to get a feel for it. After doing that one tilt the sidecar for more clearance by pulling the top bolts off each mounting strut.
Take out the plugs. Take off the distributor cover. Take off the valve cover.
Turn the engine over until the points just open. One cylinder should be at the top of its travel and its two valves should allow you to rock the rocker arms. Loosen the lock nut with a wrench and slide the go feeler gage between the top of the valve and the rocker arm. You may have to turn the adjusting screw back and forth a bit until you can slide the proper section of the gage in and slide it with a small drag. Now loosen the screw just a tiny bit as when you tighten the lock nut it always changes, usually tighter. You may have to repeat this several times with each valve until you get the adjustment lock nut tight with a slight drag on the feeler gage. If you must err do so on the loose side as a valve will tick and you lose a tiny bit of power if it is loose. If you go too tight a valve can burn. Check the lock nuts one last time and inspect the valve cover gasket. If the gasket is damaged carefully clean the cover and apply sealer on the side that meets the cover. Apply the gasket to the cover making sure it is on straight and let it dry. I clean the head surface and apply a light layer of oil to the gasket mating surface. This is so you can reuse the gasket if you have to remove it on the road. Lightly tighten the valve cover hold down bolts (don't break them) and you are done one side.
Roll the engine over until the points start to open on the next cam and you are ready to adjust the other cylinder. Use a bit on Locktite on the strut bolts. If you can get an old VW manual it might help as it is much the same.
Jeff’s answer was much more thorough and shows that there is more than one way to skin a cat. I was going to take the best of both and co-mingle but I think some folks might benefit from the slightly different points of view. I learned to use the points to adjust valves by on multi cylinder engines as timing marks do not work on all cylinders. Either way will work fine on a Chang and if you check the hole for the timing mark you’ll see when the points open will be within a few degrees. All you have to be is on the compression stroke and on the back of the cam, there is quite a bit of latitude. I did this job on my Chang Hong water pumper so I have included pix of my other bike’s points distributor too. On the Chang Hong I have a Hall Effect electronic distributor and you can see the pencil mark on one side of the top. This shows me cylinder number 1. When the cutout is next to the magnet on 1 I adjust that side. A full rotation and the other cutout next to the magnet and I can adjust number 2. This works just like the points just opening on number 1 and 2.
I have also added a couple of pictures of the small valve cover bolts that my red bike has but the Chang Hong does not. On the Chang Hong improvements were made to a better gasket and the small bolts were replaced with pins. They also added clearance to adjust the valves without tilting the sidecar. As an aside you may need to use a shortie flat screwdriver like the one in the picture on the sidecar side. Because of the additional room, the improved reusable gaskets and having no need to tilt the sidecar the valve adjustment took me fifteen minutes on the Chang Hong. Figure another hour if you have to tilt the sidecar and scrape off old gaskets.
Jeff’s answer:
Okay. It's kind of daunting if you've never done it before. If you mess this up, don't blame me. I'm trying to help but can give you no guarantees as I'm a self-taught mechanic...
Things you'll need:
Patience.
A dead cold engine. (Never do this with a warm engine. The gaps are so that heat can expand it without seizing everything up.)
0.10mm feeler gauge
Flat head screwdriver
10mm crescent wrench
17mm (I think) on a torque ratchet wrench
Replacement gaskets
Heat resistant gasket sealer
Place bike on center stand.
Remove sidecar.
Place a pan under the left cylinder head.
Remove the the 10mm nuts. As you face the cylinder they are at 10 o'clock and 4 o'clock hidden from your view. Make sense? It will when you find them.
Remove the cylinder head cover bolt with the 17mm.
If you got both 10mm nuts you should now be able to remove the cylinder covers. You may spill a little oil into your pan.
Remove all the old gasket from the cylinder cover and head. Clean both.
Now, adjusting the valves. As you look at the head, the valve closest to the carbs is your intake valve and the valve closest to the exhaust is your exhaust valve.
Remove the little black plug that is covering the little round hole which allows you to see your flywheel. Using your kickstart, gently and patiently rotate the flywheel until the intake valve starts to open and the "0" is dead center in the window. (This is TDC. You'll see marks for 36, 30 and 6º before you get to "0". While doing this, couldn't hurt to put a little dab of white paint on the "6". It'll help when you adjust the timing with a timing light.)
Valve clearances. Let's set the intake first. (The valve closest to the carb.) You'll see a lock nut on an adjusting bolt. Loosen that lock nut. Place the .10 feeler gauge in between the valve and the rocker arm.
Using your flathead screwdriver tighten the adjusting bolt until you get a .10 gap. Tighten the lock nut and doublecheck the gap. Readjust if required. Too tight and you'll run too lean. Too loose and you'll run too rich.
Repeat this adjustment on the exhaust valve.
Now, if you've got a sidecar, you'll need to have pulled it or at least leaned it by taking off the upper mounts. Better to take the time to remove it.
You're going to repeat what you did on the left side, BUT, you'll need to patiently operate your kick starter 360 degrees so that the "0" appears in the flywheel hole with the right intake valve now in the open position.
Once you're done there, place a reasonable amount of gasket sealer on each side of your replacement gasket. Place the gasket on the head. Replace the cylinder cover. Re-attach the 10mm washers and nuts.
Using your torque wrench, at 29 Nm, no more, no less, bolt the cylinder head back on. Don't overtorque! If you strip this bolt you're a little screwed.
Might as well change the oil while you're doing this.