When I take off the retaining screw #12 it is easy to adjust the valves by turning the threaded part#11, but I am not sure how to do this properly. Any advice would be appreciated. This engine has an unusual configuration for the valve train:

http://www.honda-engines-eu.com/en/images/39064.pdf If this don't load go to honda-engine-eu.com this be 1.01 mb to download pdf for all small honda engines the valve adjustment is give to be 0,10 mm + or - 0,04 mm of course metric system measurements. This shouldbe on any feller gauge under 20 years old overhead cams use very little lash tighten lock nut set screw is going to change setting by pulling threads backwards on set screw a few trys will get right. stick something plastic in sparkplug hole find the top dead center both valves closed and on low side cam lobe start working with all valve very loose lash piston bend face on valve. Turn engine by hand are small wrench with plugs removed. Trun engine 360 degrees for other lash adjusting. just check my feller gauge it has ,102 mm blade in english thats .004 thousands which is ,002 mm over the 0,10mm.

Here is a link that might be useful: honda engine spec.

Part #12 should be the lock nut and #11 the actual adjuster. It should have a slotted tip.
Loosen lock nut and turn adjuster to get the specified lash.
Tighten lock nut.
Feeler gauge should fit between the adjuster and valve stem tip.

Often the adjuster screw will turn with the lock nut, so you have to set the lash greater and then have it "screw in" to the proper adjustment. This may take several tries to get it right, but is NOT uncommon. GET IT RIGHT!. It gets easier with "practice".
There will be no time that the lash is greater than what is specified, so as a test, rotate the engine through several rotations and insure the feeler gauge WILL fit at the maximum lash with a SLIGHT drag. This will be with the piston at its closest point to the head.

At TDC, BOTH valves should have the specified clearance/lash. "True" TDC only occurs every other rotation.

Intake .004"
Exhaust .006"
+/- .001"

Forgot to add-
Don't remove the lock nut! Just back it off 1/2 turn or so. Just enough so the adjuster turns.
When tightening the lock nut, it may want to turn the adjuster with it, so my advice about possibly having to set the adjuster out more than the specified amount. You'll get the idea when you do it.

The valve train is different because it's Overhead CAM.

bill kapaun and gator rider2:

Thank you very much. I tried doing it once, but after putting it back together I am hearing a little valve clicking. Also, there seems to be excess smoke from the exhaust, so I will try this again tomorrow or the day after.
As you guys told me, and as I now know, this takes practice. The tolerance is insanely small (.001") so I think I may be off because of several things: I may have screwed the pooch when I tightened the retainer nut, or I made the valve clearance measurement when it was not at TDC. I also failed to note that TDC occurs every other rotation, but now that you educated me on these issues, I will try to get it done right.
Thank you again. I am pretty handy, sometimes I just need a boss to get things done right.

Just use a little patience.
It's the kind of job that goes MUCH faster the second time.
You can use a pencil in the spark plug hole to gauge when you are getting TDC. Mark the pencil. Remove it when rotating the engine. You don't want to have to remove the head to pull out the broken off piece!:-)

"At TDC, BOTH valves should have the specified clearance/lash. "True" TDC only occurs every other rotation."
--------------------

So for the first cylinder, I rotate until the piston is fully up and then I adjust the exhaust and intake valves on that cylinder. Then I turn the motor until the other piston has come up to it maximum, and adjust the intake and exhaust valves on that cylinder while the piston is up at its highest point?

When you say the "True" TDC occurs every other rotation, I am still unsure of myself on that. Was I wrong to assume that every time the piston reaches the top of its stroke, it is TDC for that particular cylinder?

Also, I notice that on the cylinder to my right (as I stand in front of the tractor), that the piston is fully extended when the sensor on the flywheel is lined up exactly with the ignition module for that cylinder. On the cylinder to my left, the sensor on the flywheel has travelled an inch pastthe ignition module for that cylinder when the piston is at the top of its travel. For that cylinder, should I continue to consider the full upward extension of the piston as its TDC, or use the point where the sensor on the flywheel is centered with respect to its ignition module?

Maximum piston extension.
On "true TDC (compression/ignition), both valves are closed AT THE SAME TIME, thus you have clearance on BOTH valves.
On the "alternate" TDC, only 1 valve is closed. (Exhaust stroke/start of intake stroke) It will quickly transition from Exhaust valve being open, to it closing and the intake valve opening as it starts the intake stroke.
http://www.small-engines.com/4cycleth.html

Bill: I found an abbreviated shop manual at the Honda site gator rider2 recommended (for the GCV 530 which is almost identical to the GXV 530). I am not seeing the marks on the cam pully that they are talking about so, as you suggest, I will check that both valves are closed so that I adjust it on the compression stroke. Thanks again Bill and Gatorrider2.
{{gwi:350744}}

Report back!

I did it using all the tricks of the trade I could borrow here. I had to purchase a new set of feeler gauges because the set I inherited from my dad did not have thin enough blades.
I temporarily took the two sizes (roughly .004" for Intake and .006" for Exhaust) off to make it a little easier to get them into the gaps.
The nut loosened with a wrench and the adjusting screw with a pliers. At TDC on the compression stroke I tightened the adjustment down so the respective feeler gauge fit, but would kind of scrape indicating that it was right on.
I then mounted the valve covers using Permatex Ultra black gasket maker. (Honda recommended their own brand, but it has to be similar).
As for the follow up, the thing started immediately, and it runs great without any valve noise or other problems. For a while there, I thought I was in over my head. I had the tractor parts on the floor of the garage while I was coming in and out of the house to ask questions here on how to put it back together and printing pages from Honda Europe.
Anyway, it worked out so thanks again. I will mow today with a smile now that she's all tuned up.

Glad it's working good. Listen for noisy valves the first couple times you use it, just to make sure every thing's back together correctly.
"I temporarily took the two sizes (roughly .004" for Intake and .006" for Exhaust) off to make it a little easier"
I'll usually do that + and - 1 size. The next smaller size should "fall" in the "gap", and the next larger should obviously take too much force to fit it in. Kind of a "Three Bears" approach (from the famous Goldilocks and the....)