On virtually all Over-Head Valve (OHV) small engines including the GX390, 420/440/460cc clones, Predator Engines, Vanguard V-Twins, and our 625cc engines...the valves will have to be kept in adjustment to keep the engine running smoothly.
What is valve lash?
Valve lash is the space between the rocker arm tip and the valve tip.
What is the purpose of valve lash?
Since metal expands at higher temperatures, the engine has to be able to run cold and also hot, therefore, there needs to be an air gap between the rocker arm and valve tip so both temperature conditions are possible.
Unlike cars, these engines do not have hydraulic lifters which are self adjusting and lower maintenance. When I say that these engines are using the technology of the 1950s, 1960s, and 1970s...this is partially what Iam referring to.
What happens if they are out of adjustment?
If you set the valve lash too tight (not enough space for expansion), the engine will not want to start and will spit fuel out of the carburetor. Conversely, if they are set too loose, the engine will be sluggish and there will be additional & un-necessary wear and tear of the valvetrain.
How do I adjust my valves?
Not to fear, its a very simple process since there are only 2 valves to adjust.
Step 1: Remove your valve cover. (625s have 4 bolts on the perimeter, 460s have 1 in the center)
Step 2: Rotate your engine such that the intake valve is fully open.
Step 3: Insert a .004" or .005" feeler gauge between the exhaust valve tip and rocker arm.
If the feeler gauge does not fit, loosen the locking nut and turn the adjuster until the feeler gauge fits snug. (Do not put torque on it so that you cannot remove the feeler gauge, just enough so you can slide it in with some tension.)
Step 4: Ensure that a .006" feeler gauge DOES NOT fit between the valve tip and rocker arm.
Step 5: Tighten the lock nut down while holding the adjuster so it does not move.
Step 6: Double check that the .004" or .005" gauge fits snug and the .006" does not fit.
Step 7: Repeat the above steps from 2-6 with the opposite valve (rotate engine until exhaust valve is fully open and set the intake valve)
Step 8: Re-install your valve cover