A '21 with stock pipes and mod'ed air box should easy see over 58bhp. I know fontyyy doesnt like dynojet dynos for some reason but even my bog stock '18 with an air box mod made just over 60bhp.

I think if your getting low 50's on any dyno there is a whole lot more to come from it as long as it has a good crank, seals and rings in it.

If your air box isnt mod'ed then this is whats holding you back. Fitting a set of swarbrick race cans lifted the '18 from 60 to 62bhp and then the tygas added another 1.5bhp. The biggest jets i have ever had to run in that are 132 and normaly run 130's with 38 pilots.

A whole load of graphs can be found in the "Tuning" section in Ignition mods.
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If I have to take the carbs off once more...