John Deere L-120 starts good , doesn't smoke , idles ok engine pops at higher rpm, or under load such as w/blades are ingauged, has new plugs, fuel filter, put fresh gas in never dies, no rod knocking etc. poping at times is off and on ???
All righty! The first thing you need to do is remove the valve covers. These are the sheet metal things that probably have "OHV" stamped on them. There's usually a couple bolts to take them off. You may have to also remove some plastic shrouding, depending on the engine. Be careful with the gasket that's in between the cover and the cylinder head. Once you've got the covers off, you can see the rocker arms and springs which operate the valves. Before you can set the clearance, you need to spin the engine until you see both of the rocker arms are not pressing on the valve stems. This is usually about 1/4 of a revolution past TDC. You may have to do this independently for each cylinder. Once you've got the valves and rocker arms in the right spot, you slide a flat feeler gauge (available at an auto parts store) in between the rocker arm and the end of the valve stem. You can shoot for between .004 and .006, so .005 might be the ticket. The correct feeler gauge should slide in with a little resistance. If the clearance is too large or too small, you put a wrench on the bolt mounted on top of the pushrod and loosen it. Then you use a Torx driver to tighten or loosen the screw inside of the bolt and then retighten the bolt. This procedure may vary depending on the engine; if you get stuck, post a picture and we'll try to help out.
By the way, there are lots of videos on YouTube on how to do this.