I seem to remember in the dim and distant past a set of intructions that came with a M- Max box I bought for my MC21. The instructions were for set up of the M-Max box but they also had info regarding the installation and set up of the HRC jet kit. I'm pretty sure the small jet in question is to replace the jet in the black 'Y' piece, as Andy has mentioned. I do have the instructions somewhere and actually had the intructions translated into english.