A spacer makes the pipe longer, which in turn makes the return wave take longer to return to the cylinder. This tunes the pipe for a lower RPM.
However, a shorty silencer will typically help low end and a longer silencer will favor overrev.
A spacer in between cylinder and head pipe will complement the low-mid range, while no spacer will complement mid-upper ranger. If it's a 125, I'd take the spacer out, as 125s work best in the upper revs anyway.
However, a shorty silencer will typically help low end and a longer silencer will favor overrev.
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