The engine rarely has even fuel distribution, and the lean cylinders will have the worst detonation. Having the engine go lean from fuel delivery problems induces detonation because of the lean mixture and takes out the rod bearings just as well as too much timing or low octane fuel.
Did you do some playing on the highway recently? If your fuel pump and fuel lines can't keep the carb filled during sustained full throttle (top end runs for instance) when you slow down you may find that you've cooked some rod bearings. I learned the hard way on this!
Have a look at the underside of the pistons. You may be able to see a difference in the colour. Light brown usually is okay but darker indicates excess heat in the cylinder.
