A stock '21 engine/carbs etc. attached to a PGMIV on the std card goes pretty well. Mine (a std '28 with a stock '21r engine/carbs, opened up std '28 pipes, modded airbox) shocked me, I was expecting it to be really flat like an unspliced '21 is and it's just not.

General consensus tends to be that the std carded PGMIV itself is worse for outright grunt than a PGMIII and just swapping flywheels to a '21 does not really fix this.
The problem is of course that very few people have done it and the '28 is restricted in other places (pipes, airbox, lower ported barrels) so the whole thing is just a pain.
Andy always says they're worse but that's just based on Leigh's bike making 54(?)bhp, however Leigh's has a std airbox and that kills anything stone dead anyway.
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