check compression before spending money
Often, you may have a leak in the gasket over the thinnest part of the wall between two cylinders. When that happens, you'll get a loss of compression in either of the two during the power and compression strokes, with a resultant blow-by in the adjacent cylinder at inappropriate times (e.g., during intake). So the compression test is the best way to determine that there is a problem.
However, it will not tell you what the problem is or where it's located exactly. You could have a bad spring on a valve, a stuck or burned valve, bad rings on a piston, etc.
Here's a quick and simple method to help identify and locate the problem: take the plug wires off the plugs, one at a time (best to have the engine off when you do that so your long hair and necktie don't get sucked into the fan, and your pacemaker doesn't get zapped by the current; at least use something with some insulation on it to do the r&r) until you find a difference in how things sound (i.e., smoother, but consistent with one cylinder not firing). Then take out the plug that made the difference and squirt a little gear oil, lucas oil additive, stp, or other thick gooey oil stuff (to use the technical terms) into that hole, put the plug in, and reconnect the wire - if that helps, you've got bad rings.
If it is a problem between two adjacent cylinders, then removing a single plug wire will ease the problem, but it won't tell you which side of that cylinder the hole in the gasket is on.
But my strong suggestion is that you check the compression before spending any more money.
