[phoenix]

I think your carbs are too small to work well on such a large engine at both high rpm and in the low-mid range. Don't forget that each cylinder is about 700 cc and at 6000 rpm the engine needs about 680 cubic feet per minute of air - and with those cams maybe more. Many people, including me, have used 45s on engines as small as 400 cc per cylinder very successfully, and they 2 litre four pot boys use 48s to feed 500 cc pots if they have revvy engines don't they? So I feel 45s are too small.

This is because ideally air speed through the carb should not fall below about 14 metres per second nor go above 75 metres per second.

On your engine at 5000 rpm through a 45 mm carburettor without a choke the air speed would be already be 70 metres per second. Obviously you run chokes, so the airspeed would be higher and almost certainly exceeds 75 m/s.

With large chokes you can tune the engine for higher rpm - as it can breath more easily - but the large chokes will give poor mixing of the fuel, so the low and mid range will suffer.

Larger bodied carbs with relatively smaller chokes would give better fuel mixing and therefore improved low and mid range and flow more air altogether to improve the top end.

48s may give you 6000 rpm but if you want to run at say 6500 rpm or beyond I think you will need to run 50 DCOEs. If that involves buying new carbs, then I would seriously consider going for throttle bodies, or even a single 80mm throttle body, plenum chamber and an injector per cylinder setup which may work out cheaper than one throttle body per cylinder but could perform as well - definitely better than the 45mm carbs.

Martin

p.s. when comparing throttle body sizes and carb sizes, the choke size on the carb is what you need to look at, as gas speed through the throttle body depends on it's given diameter whereas in carbs max gas speed is achieved in the choke.