The GY6 line of engines such as the GY6 150cc, were never intended to be used in anything but a scooter design.  As a scooter engine it was optimized for motivating a vehicle of mass between 350 to 400 lbs including driver.  Once these engines were placed in a mini buggy or go kart design the weights that the powerplant is being asked to push is far greater.  With driver and passenger, total vehicle weights of approximately 700 to 850 lbs are not uncommon. 

The CVT (constant variable transmission) built into these engines were designed and tweaked to perform under the normal loads of a scooter and not those of a mini buggy.  Some manufuacturers have made tweaks to the CVT drive system in an effort to make the engine fit the application better, but many manufacturers have not seen fit to do this yet. As a result, a large percentage of  go karts and mini buggies that use the scooter based engines perform more sluggish and less responsive than they should right out of the crate. One fix for this problem is a quick shimming of the CVT's driven clutch torque spring or using a higher strength "performance" torque spring.

 Shimming the clutch spring is one way to improve the performance of the GY6 engine for high weight applications like our mini buggies and go karts require.  The large torque spring inside the clutch unit acts to apply pressure to the pulley plates that hold the drive belt on the driven side.  By applying more pressure to these pulley plates, the plates will hold together longer and thus act to keep the belt in "lower gear" as the engine rpms advance.  This basically helps to keep the engine running where it makes more power and keeps it from becoming bogged down early by the weight and gearing ratio.  Some users also believe that squeezing the driven side of the drive belt harder, acts to keep episodes of belt slippage to a minimum.  In theory, if this is correct, less drive belts should be breaking after shimming or upgrading of the driven pulley torque spring.