1. If you don't find some way to hold the collar, you're not actually adjusting the clearance. The nut is just rotating against the valve and not adjusting its position. You need to find some way to hold it. I either wedge it with a screwdriver or use channel locks since I refuse to waste money on the wrenches.
2. If the clearances are too tight, correct me if I'm wrong, then the valve doesn't seat all the way and can lead to lower compression and possible burning of the valve. It could be the other way around but I think that's how it goes. Also I think it does something to the amount of time the valve is open, decreasing the amount of time and thereby allowing less fresh air in and less exhaust out.
You need to make sure you recheck all the clearances before you button everything back up. Also make sure to check that you tightened all the adjustment nuts back down. Once I left one completely loose and I noticed a loud rhythmic thumping noise coming from the rear of the engine. Tightening the adjustment nuts on cylinder number 5's exhaust valve made the noise go away. Go figure.
