Chain should be at just the right tension for the machine to run smoothly. Too loose and the chain will drag on other pieces and marbles might fall out of the buckets. Too tight and the motor will have to work needlessly harder than it would otherwise and might break down much faster.Solution is to either scale up or down the chain links.You should scale a few chain links up by 102% or down 98% in X direction (length wise). Then print them and replace a few and you will see immediate improvements. And just repeat this step until it runs smoothly.Also I suggest buying Silicone Spray. It is similar to WD-40, but that is not silicone spray, so you shouldnt use that. You should buy silicone spray instead, and spray it on the gears and chain. It acts as lubricant for plastic.