Belgium, it sounds as if it's now time to do some serious, step-by-step diagnostics. Step one is to find out if you are getting spark to that cylinder. Use your timing light to confirm spark- just clamp it around the suspect cylinder's wire and see if the light flashes. No flash=no spark. If you have spark, it's now time to check cylinder contribution. Pull the spark plug wires off of each cylinder, one at a time and replacing them before moving on; before moving to the next cylinder, allow the engine to recover normal idle speed. Sometimes, the engine needs to be run at a speed above idle (usually 1,500 rpm), in order to arrive at meaningful results. If the rpm do not drop when you kill spark to the cylinder, that cylinder is not contributing. You have located the problem. If you get an rpm increase when you remove a wire, keep going with the test and see if any other cylinder also does this. This usually denotes a crossfiring between spark plug wires. Usually, this is the result of "induced voltage" from an active wire to one that should not be receiving spark. This is usually because the 2 wires run next to each other, and in close proximity. If you determine that a cylinder is only along for the ride, and it is receiving spark, move the spark plug from the bad cylinder to another; if the problem moves to the new home of the spark plug, the plug is the culprit. If not, then it is time to move on with our diagnosis. Check with an aerosol that is compatible with your running engine, for vacuum leaks around the intake manifold/cylinder head junction. I like to use starting ether, as it always yields a result for even the smallest leak. It is extremely flammable, and I recommend that you use WD-40, or carburetor cleaner, or brake cleaner. Spray a shot into the airhorn of the carb first, in order to determine what result you are looking for from the aerosol's introduction to the engine's intake path. After you know what to listen for, it is time to spray around the intake manifold mounting, and check for leaks. Be careful with the spray, as the running engine may suck it in the airhorn as easily as the intake gasket. I use a shield between the carb and where I am spraying, in order to keep false positives to a minimum. If you have spark, no intake leaks, and fuel, the only other part of the equation is compression. It is time to check leakdown on the offending cylinder. We have already visited that, so the instructions aren't necessary again. It would also be wise to perform a compression test of the offending cylinder, warm engine conditions. Block the throttle wide open, and disable the ignition, so it won't start. Record the 1st and 4th compression strokes, and the ultimate compression- the highest it will crank up pressure. Also do a couple of good cylinders, for reference. The answer should present itself in this testing sequence.
