thought i'd share, did a tune-up on lawn mower, it was in bad shape from neglect. gave it a good cleaning, new air filter, needed 2 oil changes, and thorough gas tank and carb cleaning. Engine would start right up, but had a slight miss and exhaust sounded funny almost like a bad exhaust valve. There's was also a lot of vibration through the whole mower. The blade had been put on upside down and was spun on the shaft, so i had corrected that and also sharpened it. but from the vibration i figured the blade had to be quite off balance, I know it's hit quite a few things. So I bought a new blade, a heavier heavy-duty one. Engine runs smooth with no vibration now, basically fixed everything. From the the way it had run with the old blade, if it weren't for the vibration i would have went back into the carb or spark plug. So if you ever have troubles, don't overlook the blade as it also serves as a flywheel and if it's out of balance it will cause the engine to run poorly.