I've never ridden a 28 in any form on the std card and flywheel, with or without the airbox modded but it's no real shock to hear it's pretty underwhelming. All you've done is the pipes?
My racebike has the 030 card, Tyga pipes and open carbs, freshly ringed it makes between 62 and 63 bhp and pulls really nicely right round. It's a good match for any truely stock barreled mc18/mc21 (Andy's optimized bike does not count).
With the stock (but derestricted) pipes it lacks a tad of top end zip (past 10 k or so), 56-57 bhp? Covering the airbox holes up kills it stone dead though it was silly rich, low 50's at the most, 10,000 rpm in 5th up Schwantz Curve was all it would pull.
My roadbike isn't really that much of a '28 at all. It's an entire 21 motor inc. carbs and modded airbox dropped out of a 21R and put into my '28.
The pipes are stock '28 with the washers ground out and it runs stock 21 cans. It'd be around 60bhp on a PGM3, it's not making that but it goes quite nicely really, I was pleasantly surprised.
I've also ridden Leigh's roadbike, stock mc28 engine, mc21 flywheel, JHA pipes, stock airbox, makes 54bhp (I think) and that makes nice progress but I found it a little disappointing, it just doesn't rip onto the pipe like, well, like a 2 stroke should I guess.
Looking on the Tyga site there is a graph of a '28 with just their pipes on (vs stock restricted) which as you say doesn't really zip at the top, fitting a '21 flywheel helps quite a bit - graph here and modding the airbox graph here gives another small increase, but nothing like what you get with a '21 just by modding the airbox (graph here) which probably shows the real restriction is still in the PGM4.
The problem with the '28 is nothing is designed for power, low geared, more spaced ratio gear box, lower ported barrels (anyone know how much power this looses? It's not much anyway, mine made 60.x on an open airbox - no lid - and both '28 barrels), restricted pipes, restricted airbox and worst of all, restricted PGM.
Leigh has back to backed mc21 and mc28 flywheels I believe but to show how far from a HRC card the mc21 flywheel is Tom found 11 bhp on his 300 swapping from PGM4, stock card and mc21 flywheel to PGM4, 030 card and mc28 flywheel. 11 bhp!!
I don't know what to tell you really, if I were you I'd take the airbox lid off (means you don't take a drill to your spotless lid, wire the filter in place), go up a couple of jet sizes and see if that makes a difference (obviously chop it, go down again if it's rich etc), then I'd get hold of an MC21 flywheel, a flywheel puller and try that.
Without the card (or a PGM2 or 3 instead) or internal engine mods that's about it.
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