Originally posted by never lift
the backplate is under your pullstarter but on a non pullstart engine that's where they put some boost bottles at . you will get
a very slight increase in the power range because there is more presure in the crank case .. retuning will need to take place ..
The backplate boost chambers work exactly the same as the MIP bottles. There is no opening in the back plate, it's just a hollow chamber. The pressure comes from the intake fitting below the carb.
Again, there is no aperture from the crankcase to the backplate boost chamber so it doesn't get any pressure through the engine's crankcase except at the intake fitting.
You will get very little to no power gain from this and often plugging the fitting will allow the engine to run better. Just MHO based on owning several .21 engines so equiped.