Pull your plug wires one by one at the distributor cap until you find which cylinder is missing (no drop in rpm). Push the distributor end back in and check the wire for spark. If you have spark, check for a cracked plug. If the plug is not cracked, do a compression check. If your compression is good you do not have a burnt valve. If your compression is low, bring the missing cylinder to TDC on the compression stroke and load the cylinder with compressed air. If you hear air rushing out the exhaust, you have a burnt exhaust valve. If you hear air rushing out the carb, you a have burnt intake valve. If you hear air rushing out the valve covers, your rings or pistons are smoked. If the cylinder holds air, you might have a bent pushrod, collapsed lifter or flat lobe on the cam.