Ok, here it goes. I'm gonna' git tired of huntin' and peckin' on this new 'lectric typewriter but I'm sure others will help out where I miss something. Try to follow this sequence for adjusting the valves on a Generac twin cylinder.

We would like this to be on a cold engine or a least 4 - 6 hours of cooldown if it has been running.

Turn the unit to 'OFF', turn the gas cock off, for more safety disconnect the negative battery cable, remove the upper front and lower front sheet metal at the left end of the unit - about 12 - 14 10mm hex screws, this will expose the large plastic cooling fan enough for you to get a paw in there to turn it by hand, remove both spark plug wires and both spark plugs (5/8 deep socket on most twins).  Remove 8 10mm hex bolts holding the valve covers, leave the rear cover on to keep debris out of the engine and remove the front valve cover.  If you're gentle you wont have to replace the gaskets.

Now we're going to find Top Dead Center on the compression stroke of the front piston, you need a drinking straw or something that will fit down the sparkplug hole and that won't fall apart while in the cylinder.  A pencil will work.  While turning the big cooling fan with your left hand in a counterclockwise direction and with the pencil inserted in the spark plug hole with your right hand, you want to "feel" the piston moving up and down the cylinder until it reaches the very top of it's stroke. Now is the spot where folks can get into a little trouble.  You see, there are TWO Top Dead Center strokes on this engine, one is when the piston is pushing the exhaust out of the cylinder through the exhaust valve, the other TDC is on the compression stroke and both valves will be closed at that time.  We want TDC on the compression stroke and the way to figure out which one you are on is by looking at the rocker arm and valve action - both valves and rockers will be in a relaxed position with the valve stem tips at their highest when on the compression stroke.  If you continue to rotate the fan and watch the valves on the next TDC stroke you will see the exhaust valve being pushed down (open) by the rocker and the intake valve closed - we don't want to try to adjust the valves on that stroke!  So with the piston at the very top of it's stroke you can turn the fan a little each direction and the piston won't move up or down - that is TDC. The specs for clearance between the rocker arm and the valve stem are .002" - .004", I like to set them on the wide .004 end because the exhaust valve always gets tighter with use and we don't want it to lose the free play in the valve train.  With your 10mm hex key (Allen wrench) inserted in the rocker stud and your 1/2" or 13mm open-end wrench on the lock nut underneath the center of the rocker you want to loosen the lock nut just enough to allow you to turn the Allen wrench, the rocker stud is threaded and will raise or lower as you turn the Allen wrench - this is how we change the gap between the rocker and the valve stem. Slide a .002" feeler gauge between the rocker and valve stem, if it won't go then you need to turn the stud counterclockwise to gain more clearance, if .004" is too loose then turn the stud clockwise. It may take some small adjustments to get the right "feel' of the feeler blade, when you're happy with the gap then snug down the lock nut, you will find as you tighten the lock the gap will get bigger so you have to re-adjust a little to make sure you're still in spec., then tighten the lock nut. Do both valves on this cylinder., put the valve cover back on, don't over-tighten the bolts as the valve cover OR head could crack. Now the rear (#1) cylinder, same procedure as the front but you can't adjust the valves UNTIL you find TDC on the compression stroke for that cylinder - so go ahead and stick the pencil in the hole and turn the fan while watching the rockers and valves, you want both valves at rest - neither one rocking and adjust as you did the first cylinder. That's it, simple, take your time.