Are you asking about the bone stock engines?
But when we start with a 5 1/2 pound flywheel and swap in a 3 pound flywheel more vibration should be expected at low rpm like at idle. These engines were not designed to sit around at idle. They were designed to start up and run at 3600 RPM.
I have an early 70's push leaf blower that I repowered with a clone. This thing has a steel impeller (fan) bolted onto the PTO that weighs at least 5 pounds. This is the smoothest running of all my clones. Even at a low idle speed, very smooth. All that weight covers engine imbalance.
But when we start with a 5 1/2 pound flywheel and swap in a 3 pound flywheel more vibration should be expected at low rpm like at idle. These engines were not designed to sit around at idle. They were designed to start up and run at 3600 RPM.
I have an early 70's push leaf blower that I repowered with a clone. This thing has a steel impeller (fan) bolted onto the PTO that weighs at least 5 pounds. This is the smoothest running of all my clones. Even at a low idle speed, very smooth. All that weight covers engine imbalance.
There is a lot of mis understanding about single cylinder engine balance. a single cylinder engine has both rotating and reciprocating motion. The rotating component can be balanced out but not the reciprocating. These engines are balanced through the crankshaft counterweights. There is a balance factor that is applied to these engines to average the vibrations between the horizontal and vertical planes (relative to the cylinder axis). It is not the engine alone that is balanced but the entire assembly that the engine is mounted in as well. If you look at motorcycles engines that are mounted in a vertical direction have a different factor than the same engine horizontally. The engine manufacturer has no idea of what the engine is being put into so they generally shoot for a balance factor in the 50 to 60% range. The reciprocating imbalance in a single cylinder engine is comprised of two types primary vibration that occurs every revolution and a secondary vibration that happens twice a cycle caused by the piston travelling a greater distance from TDC to 90 degrees ATDC than form 90 Degrees to BDC. You cannot balance out the reciprocating imbalance unless you add a balance shaft or other device. To reduce vibration you can reduce the recriprocating mass. (IE: piston, rings, clips and small end of the connecting rod.