My major ***** about the two stroke frame mounted engines is the vibration at high speed. Of course, everyone wants to go fast.

Has anyone done a ballance job on one of these engines? If so, did it improve the engine?
Guess not. The reason I asked the question is I have had V8 engines ballanced at a machine shop, and it really helped, but you have to have the crank, rods, pistons, and rings seperated for them to do it.

Has anyone had one of these engine apart to that extent, and if so, how hard is it, and do you need any special tools?
Interesting question....seems like balancing would be a good idea but I wonder where you would take the engine to do that.....I am thinking maybe the parts are too small for a normal auto shop to handle? (as an example, I wanted a custom pipe bent at Tuffy's but the smallest they could bend was 1 1/2 inches....maybe the same idea would apply when trying to get other parts worked on?.....Maybe a hi perf motorcycle shop?
As I understand it, with a multi-cylinder engine, most of the work is in making all the pistons, and rods the same weight, both ends. That would not be required on a single cylinder engine. Then they take a percentage of the weight of the piston, rings, and rod, install it on the rod journal of the crank, and then ballance the crank. I would think the ballancing of the crank with the appropriate weight of the other parts could be done by any machine shop that does ballancing, which is really all that would be needed on one of these little engines.
,it seems that without a flywheel it can not run smooth even if you had perfect balance ,,also allot of new owners work on the chain to try get the chain to run smooth ,and again i think its because there is no flywheel ,, and after 500 miles the motor does smooth out ,,somewhat ,,
my own belief is that these motors 2 cycle will be collectors items in a couple years ,,the 4 stroke will be the king and it going to happen fast,,

would i go to the trouble of trying to balance one of these motors ,,,not a chance ,, only for pass time if nothing else to do ,,
I agree. The 4-strokes are coming. Even if they cost a bit more, they should last a lot longer.
Not to be argumentative but, if you're talking about the little Chinese two strokes, they do indeed have a flywheel. The flywheel is incorporated into the crankshaft and is comprised of the two large, round things either side of the connecting rod.
the 2 weights are the counter balance ,, the extra weight of the flywheel is more to keep the motor turning at a even pace , , you will notice how much your chain jumps that to me is caused by the lack of a flywheel ,so even if you could have a perfect counter balance weights it would not smooth out the chain , for sure the counter balance and the flywheel works together to reduce vibration and to smooth out the rpm , a power saw turns 10 or 12 thousand rpm smooth as silk ,,good example counter balance and flywheel working together

,someone commented on a earlier post that if the happy time motor had a flywheel they would not get up to speed as fast ,, the difference would be very little as long as the flywheel matched the size of the motor ..

""vibration "" interesting topic and a very important topic when talking about happy time motors ,,as the motor breaks in it get better but a long way from perfect and until counter balance ,and a added flywheel working together its not going to get much better

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Again, not to be argumentative but the 2 weights act as both the counterbalance and flywheel. Single cylinder engines cannot function without some type of flywheel. Without a flywheel there would be no kinetic energy available to return the piston to the top of the cylinder between power strokes. Locating the flywheel/counterbalance on either side of the connecting rod is actually better than locating the flywheel at the end of the crankshaft. With the flywheel located at the end of the crankshaft, there is torsional flexing of the crankshaft as energy is imparted to (and to a lesser extent harvested from) the flywheel by the piston. The torsional flexing of the crankshaft causes additional vibration. This is the reason there is a harmonic balancer located at the front end of your car engine's crankshaft. It's designed to dampen the vibrations caused by torsional flexing of the crankshaft.

The reason the little Chinese engines vibrate so badly is simple. They're made cheaply. The crankshaft/rod/piston assemblies are not individually dynamically balanced. They couldn't sell them as cheaply as they do if they took the time to accurately balance the internal components.

Granted, a heavier flywheel would serve to smooth out the power pulses a bit but at the expense of added weight, slower throttle response, and increased gyroscopic precession effects. The major source of vibration in these engines at high RPM is the lack of proper balancing and compensation for the reciprocating components.

Your chain saw example is an apples and oranges comparison.

1. A proportionally heavier flywheel can be used on a chain saw because throttle response is not much of a consideration.

2. The chain saw is driving a load that has a much smaller moment of inertia so the power pulses can more easily accelerate that relatively small mass. Our engines are driving a load with at least several hundred times the moment of inertia the chain saw has to deal with.

3. The chain saw engine is probably manufactured to more precise tolerances and more attention is given to dynamic balancing since the engine must run at high RPM to perform the task at hand.

4. The chain saw engine is intended to do its job at one power level; wide open throttle. Operation at partial throttle is not a concern.

You mentioned "matching the flywheel to the size of the motor". A properly designed flywheel considers not only the size of the engine, but more importantly, the type of work the engine is designed to perform.

In a way we're just debating semantics. A flywheel can take many forms. For example; a piston aircraft engine depends largely on the propeller to act as a flywheel but we still call it a propeller.

Every engineering endeavor is simply a matter of choosing the best compromises to get the job done. The little Chinese engines seem to be a fair compromise. Granted, they are not optimum mostly due to the fact that they are manufactured cheaply and therefore can be marketed at a low price. A good dynamic balancing would go a long way to reduce the vibrations at high RPM, but are we going to spend an amount equal to the cost of the engine for a proper dynamic balancing? Probably not. We accept it as it is and just enjoy the ride!
That is a clear and concise description of the situation. In the future if anyone asks me about CHT vibration, I can just direct them here. Thanks Pilot.