I have a 1991 Honda Accord and it's a great car. My owner's manual and the service department where I bought the car both recommend adjusting the valves every 15,000 miles. The engine now has 60,000 miles and I'm getting ready for its fourth valve adjustment. I'm wondering if all this money I have been spending is necessary and what happens when the valves aren't adjusted every 15,000 miles?
Let me define a valve adjustment. The purpose is to maintain a specified clearance between the "rocker arm" or "cam follower" and the top of the "valve" which it controls. The cam follower, in your Honda's case, follows the up and down motion of the camshaft "lobe" with one end, and pushes the "valve" up and down with the other. A camshaft lobe is simply a spinning oval. Like laying your finger down on a spinning circle, it stays level. With an oval it goes up and down. This process opens and closes the intake and exhaust valves. The actual change in clearance is made with the use of an adjusting screw on the cam follower.
What happens when you have too much clearance? The noticeable symptom will be the excessive ticking noise from the valvetrain. This may only require an adjustment or there may be a worn out part that requires replacement (camshaft, cam follower etc).
When there is little or no valve clearance, the valve can no longer travel all the way up and fit properly in the "valve seat." The seat is the surface that the valve meets and seals against. Having no clearance means the valve cannot come all the way up and seat correctly. This causes a loss of compression due to the now present leak, and the valve and valve seat will eventually burn, resulting in permanent damage.
I have a lot of Honda experience, and have had many cars in for their 15,000 mile valve adjustment. The mass majority of the time the clearance is within specifications and requires little or no adjusting. I understand the Honda engineers are saying 15K in the owner's manual, but my advice is to double the number to 30K. You'll be maintaining the proper valve clearance and also saving some money.