What would be the effect on valve opening if the clearance was set too large or too small, please help
We have to assume the camshaft is the standard one that came from the factory. #1. If the valve clearance between the rocker arm and end of the valve stem is set too close you risk burning the exhaust valves. As the coolant travels through the cylinder head it cools the valve seats. The valve seats cool the valves. If the valve clearance between the rocker arms and valves are too loose damage may result to the end of the valve and rocker arm. You'll also loose cam duration and valve lift.
If the clearance was too big, the valve will not open or only open slightly and your engine will suffer. Too small and the valve will make a ticking noise everytime and will open your valve too soon. You have to set the clearance according to the manufacturer recommendations.
If you had .25mm too much clearance then the valve would open .025mms less. If the valve is set too having too little clearance then as the engine heats up it will not open enough either. All very minor items. The noise is another matter if you have too much clearance. Always only adjust valves with the engine stone cold.
Increased valve clearance has the effect of increasing low end torque by decreasing valve overlap (the amount of time both intake and exhaust valve are open at the same time at TDC exhaust). This simulates what a tractor camshaft might yield. My father modified cars for econo-rallies back when they were popular and used this technique to improve low end performance by running as much as .020 valve clearance. He ran these engines for as much as 300,000 miles with no ill effects. Later in life I met some mechanics from a foreign specialty repair shop who used the same technique to achieve the opposite effect; running less valve clearance than is stock to maximize high end performance. This can be tricky because the valve uses the time closed to transfer heat from the valve to the head, thus cooling the valve face. I met a Honda motorcycle mechanic with decades of experience who would experiment with stock clearances to determine best overall performance for a given model (usually increased valve lash slightly).
