It can be due to two reason, firstly bcs of old engine, the rod may be dried up and seized. secondly, if you are using wrong fuel than it is harder to crank the engine. Wrong or very thick fuel reduces the ability to lubricate, thus, more friction, more metal wear and more engine damage. use better oil instead. For a knocking noise however, it might already be a big damage to your engine, like crankshaft bearing or something which needs a big repair job.