Right guys should be a quick one. 1985 ish bike stutters when accelerating rapidly, i assume it's fuel relegate but unsure if it's normal. It only happen when only have say 4 litres or less left in the tank, I lived with it for years since I just fill up again but never asked the question. Is this normal? Could there be water in the tank or do I need to reroute hoses etc. I replaced the fuel filter some years ago but it didn't make a difference.

It's normal. The fuel pump is near the front of the tank, and the fuel surges to the back of the tank under hard acceleration. When the fuel level is low the pump picks up air, causing the bike to stall.

Just like Avenger says. 

Need to be a little cautious when low on gas and accelerating hard, particularly if going uphill - it's caught me out a few times and that sudden loss of power is not what you want as you are in the middle of trying to overtake slower moving traffic!

Great thanks for your time replying., I suspected as much but it's good to know for sure, I always thought I might investigate one day so it's saved me some needless tinkering.. Cheers.