Knocking under load

[Chitownjohnny]

81 Gl 1100 has what sounds like a rod knock under load. 1st and 2nd no noise but once in 3 and I hit 4500 rpm it starts making a low knocking noise. It was running fine now this. Carbs were rebuilt in August and bike was winterized and was running well. I have been told the carbs can be out of sync is there anything else that it may be.

[DenverWinger]

Doubtful it is Carburetor sync, that mainly affects the cylinder balance at idle and very light throttle. Once the throttles are opened to any degree the sync adjustment is insignificant.

[Lucky07]

Pull the plugs and see what they look like. If it's a combustion issue you should see it there, might have a cylinder running lean/heavy or with weak spark. If all plugs are the exact same I'd be leaning towards something mechanical (not so good) but easiest first would be checking the plugs.