ENGINE BALANCE AND BALANCE WEIGHTS
In the balance of two-cycle engines, it is important to consider disturbances due to the reciprocating action of
the piston masses. These disturbances are of two kinds: unbalanced forces and unbalanced couples. These
forces and couples are considered as primary or secondary according to whether their frequency is equal to
engine speed or twice engine speed. Although it is possible to have unbalanced forces or couples at
frequencies higher than the second order, they are of small consequence in comparison to the primary forces
and couples. Even the secondary forces and couples are usually of little practical significance.
The reciprocating masses (the piston and upper end of the rod) produce an unbalanced couple due to their
arrangement on the crankshaft. On an in-line engine, this unbalanced couple tends to rock the engine from end
to end in a vertical plane. This couple is cancelled by incorporating an integral crankshaft balance component
and by placing balance weights at the outer ends of the balance shaft and camshaft. This balance arrangement
produces a couple that is equal and opposite in magnitude and direction to the primary couple.
On the balance shaft and camshaft (in-line engine), each set of weights (weights on the outer ends of each
shaft comprise a set) rotates in an opposite direction with respect to the other. When the weights on either end
of the engine are in a vertical plane, their centrifugal forces are in the same direction and oppose the primary
couple. When they are in a horizontal plane, the centrifugal forces of these balance weights oppose each other
and are, therefore, canceled. The front balance weights act in a direction opposite to the rear balance weights;
therefore, rotation will result in a couple effective only in a vertical plane. This couple, along with that built into
the crankshaft, forms an elliptical couple which completely balances the primary couple.
The balance weights are integral with the gears and the circular balance weights (pulleys) on the shafts.
Additional weights are attached to the camshaft and balance shaft. gears.
Both the rotating and primary reciprocating forces and couples are completely balanced in the engines.
Consequently, the engines will operate smoothly and in balance throughout their entire speed range.
Remove Front Balance Weights
Remove the nut at each end of both shafts as outlined in Camshaft and Bearings.
Force the balance weight off the end of each shaft, using two screw-drivers or pry bars between the
balance weight and the upper front cover as shown in Fig. 1.
Install Front Balance Weights
Reinstall the Woodruff keys in the shafts, if they were removed.
Align the keyway in the balance weight with the key in the shaft, then slide the weight on the shaft.
If the weight does not slide easily onto the shaft, loosen the thrust washer retaining bolts at the
opposite end of the shaft. Then, to prevent possible damage to the thrust washer, support the rear
end of the shaft while tapping the weight into place with a hammer and a sleeve. Retighten the
thrust washer retaining bolts to 30-35 lb ft (41-47 Nm) torque. Install the other weight in the same