This is one of the few engines that still have valves that need periodic adjustment. Noise is not the issue. As normal wear takes place, the valve clearance increases. You are right that you will hear a clicking noise if that gets bad enough, but long before that happens, the clearance causes other things to happen.
As the lobe on the camshaft comes around, it has to rotate further than normal to take up that clearance, then the valve starts to open. To say that a different way, the valve opens too late, and it closes too early. It does not stay open long enough for the cylinder to fill efficiently, and it makes it harder to push the exhaust out. Both of those reduce power, but this develops so gradually that you do not notice the change.
The other issue is valve timing is used to change the "personality" of some engines, mainly race engines and those used in motor homes. If you remember "L" for each of these, excessive clearance results in "loose valves", "late" valve timing, (late valve opening), and increased "low-end" torque. Camshafts are often installed retarded by as much as two degrees so the engine in a motor home will develop increased low-end torque to pull away from a stop sign. Do not be concerned if you can no longer squeal the tires when taking off. You are likely to lose a little low-end torque once the valves are adjusted.
The trade-off is if you remember "T" for each of these, reduced clearance results in "tight" valves. That causes the valves to open sooner. ("T" is for, uhm, "advanced"). And "T" is for increased "top-end" torque.
To color this story a little, a friend has a huge motor home with a Dodge 440 c.I. Engine. He can keep up with city traffic easily, even when pulling a trailer, but once he gets on the highway, he has to really plan ahead if he wants to pull out and pass someone. His engine is tuned for low-end torque. I have the same engine in an old police highway cruiser. That car has the same take-off power as a car with a small V-8 engine, but once I hit sixty mph, and hit the gas, it will tear the seats off the hinges if you aren't hanging on! That engine was designed for increased high-end torque. Same engines; different responses.
What you will likely notice is a slightly increased power when passing people on the highway. But there is one other point to ponder. When a little clearance develops in the valve train, a slight hammering action takes place. At first that is insignificant, but as that clearance increases, the effects of the pounding increase very quickly. Visualize your hand lying flat on a table. Now allow a hammer to drop 1" onto your fingers. That will not hurt. As the pounding continues, the table drops an inch. Now the hammer is dropping two inches onto your fingers. That is uncomfortable. As the table continues to suffer from the pounding, it drops some more, and the clearance increases. Now the hammer is dropping 6" onto your fingers.
It took a long time for the clearance between your fingers and hammer to go from one to two inches, but very little time to go from two to six inches. It will take even less additional time for your fingers to turn into bloody stumps. The valve clearance increases exponentially the same way until it gets so bad, you can hear the clicking.
That sad story refers to wear on the camshaft's lobes, and in the rest of the moving parts, and it causes clearance to increase. Also consider that valve faces and seats wear. That allows a valve to sit deeper, and the clearance decreases. Add to that, valve stems expand as the engine gets up to normal operating temperature. Both of those can cause too little clearance. It is possible for the reduced clearance to prevent a valve from closing fully. This affects exhaust valves more because they get much hotter. They need to close tightly to allow enough time for heat to be dissipated into the seat and into the cooling system. If the valve train holds pressure on the valve, it can overheat, resulting in burned valves. The small amount of leakage that occurs also causes further wear to occur exponentially.
Valve adjustments on your engine should be looked at as normal maintenance, similar to checking tire pressures periodically. That will reduce the chance of developing a burned valve, and will reduce the accumulated wear to valve train parts.
Thursday, April 6th, 2017 AT 6:38 PM