1976 Honda Goldwing. Makes popping sound through the # 2 carburetor. I can feel the pressure with my hand as it is in the plenum. Definitely comming from # 2 cylinder. I also hear a tweeting noise around the left side of the muffler. I removed the exhaust system and checkled for any restriction. Found none. I have run the engine without the muffler installed and still had the popping noise. Reinstalled the muffler with new gaskets. Carbs were rebuilt by Pistol Pete. New timing belts installed. Ignition and cam timing is spot on. Valves adjusted to .004". Compression on all cylinders 165-170. Changed ignition system over to electronic (Dynatek) coils, wires, plug caps, plugs. Have rotated plugs. Plug reading, # 2 plug always is darker. It popped back through with the points system, too. Tried a different set of cleaned and rebuilt carburetors. Same results. Checked crankshaft markings and verified that TDC marks were actually TDC at the piston. Removed both cylinder heads and had valve job done on both. New valve seals, head gaskets, o-rings (all genuine Honda parts). No change. Found another left cylinder head complete with camshaft. Did valve job on it and installed. Same results, still pops back through the # 2 carburetor. What else can it be? I'm out of ideas. Help.