Internal dimensions?

[Ward P]

So I've got this little bike (GB400). . . the previous owner reckons it has been bored and new crank (650cc), but doesn't have any documentation on it. I've no reason to not believe him, but I thought it should have a bit more go - or have I just been riding a hayabusa for toooo long?

Same motor as the XL/XR600 - and from memory those things pulled monos off every set of lights whether you wanted to or not, and would pull 160kmph.
This thing is pretty docile in comparison, wont lift the front wheel, and won't go near 160.

I can tell that the head and barrels have been off, new looking gaskets everywhere, but is there an easy way to work out the internal dimensions of an engine without opening it up and measuring things?

Also the exhaust is pretty "open" could a lack of back pressure be hindering it? It accelerates and runs smoothly, but farts and backfires heaps on decel.

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Just checked - RHS base of barrel is stamped 591cm3
so the barrel has definitely been changed.