My 1990 gsxr 750 goes up the gears ok but i find it very hard to go back down the gears

All other things working properly, that is usually a sign that you're oil is needing to be changed. To help check having your motor during oil change run a magnet over your old oil and see if you pick up any metal shaving or screen the oil from the pan. This will give any tail tail signs or engine wear. My zrx1200r did the same thing but it was a little past due on its oil change. Hope this helps

Shes 20 years old now so she shouldnt be tight anymore assumeing of course that she has a good number of Ks on her now does the clutch engage & disengage properly if it is slipping at all this will cause difficulty changeing slipping is just what it sounds like you increase acceleration the revs go up but the bike is free wheeling this slipping clutch condition is caused by a number of things the most common being maladjustment is there enough freeplay at the lever should be between 1/8 to3/8 of an inch between the lever & the perch next is dirty or burned oil change the engine oil & filter make the neccasary adjustments see how you go worst case scenario you will need to replace your clutch plates in my experience the gearboxes on these old girls are virtualy bullet proof let me know how you get on