If you look below the carburetor on the side of the engine you will find a sheet metal cover held on by a couple of small screws. That is the cover of the valve chest (it is also the breather valve, but that is irrelevant for this purpose.) Remove the screws and the cover. Behind it you will see the valve springs and the lower part of the valves. Underneath the valves are the tappets: the camshaft followers, which lift and lower the valves. Rotate the engine until the flywheel has rotated an inch or two past top dead center on the compression stroke (i.e, just after it has fired and the piston has moved down the cylinder say, a quarter of an inch). Now, there should be a very small amount of space between the lower end of the valve stem, and the top of the tappet. That space is the tappet clearance. You need a set of feeler gauges to measure it. You select a feeler gauge that is 0.006" and see if it will fit into the clearance space under the intake valve stem. Then you select a 0.010" feeler gauge and see if it will fit into the clearance space under the exhaust valve stem. If it will fit in both cases, without pushing it in so hard that you are lifting the valve against its spring, the tappet clearance is OK. If you were doing an overhaul you would also check that it didn't have too much clearance, but that doesn't matter for our current purpose (so long as it doesn't have a lot too much clearance, which would indicate a disaster to the camshaft or tappet).
If both valves have a reasonable clearance, that isn't your problem.
