Why do valves stick

[HondaG100]

What causes valves to stick in small engines? I do not specialize in small engines but due to the nature of my shop I do work on a couple a week. A while back I had a no start Honda GX100 come in. 2 minutes found a stuck intake valve which I loosened up with oil and elbow grease. It worked freely and the engine fired right up and ran well. After cooling down though same thing. I repeated this several times before I ran it with some Marvel Mystery Oil and Sea Foam in the tank and the oil. It was a last resort to keep from taking apart the diagonally split block and it worked. This engine was abused and didn't have its oil changed often enough so I chalked it up as that but the other day I had the same problem on a very well maintained GX620. Bent pushrod, stuck intake valve. Got it loose, straightened the rod and it ran well. I have not heard yet if it ran after cooling down but I am waiting to install a new rod until the original problem is solved. So the simple question is what causes the intake valve on an otherwise normal engine to stick in your experience? I should mention that the GX620 does run in an unavoidably dusty environment but it sees better than average care.

[bgsengine]

9 times out of 10, it will be either bad gas, or dirty air or a combination of both.

Stale gas running through engine - rapid buildup of varnish on valve stems , and just like the varnish you put on your workbench top, when it gets warm it turns into a sticky goo that when it cools, "glues" the valve in place. Dirty air, the reasons are usually obvious - combine the two, and you got dirty varnish build up - even a well maintained engine after enough hours, you'll find the black "coke" deposits on the valve stem from gas that has burned off simply due to high heat and leaves behind the gummy residue and carbon components of the gas.

Combine a sticking valve with overheating, the valve guides can move out of place, and on an OHV if they move the right way, rocker arm can bottom on valve guide and consistently bend the push rods.

[bluemower]

I have experienced this on Kohler command and Briggs V twin engines. After removing the cylinder heads, I used a rubber hammer to drive the intake valves out. The valve stem was coated with a green /yellow / brown residue. One of the engines was less than one year old with only 10 hours of service. I have the impression this was caused by old ethanol fuel. In all cases, the engines were stored over the winter.

[HondaG100]

Thanks guys. That's what I was looking for.

[bobodu]

Hey....how did we end up with so many clowns from Fort Wayne???

[HondaG100]

Hey....how did we end up with so many clowns from Fort Wayne???

Hey someone has to bring the Midwest farm country common sense!