when changing down (only under stress) or letting off the throttle when revving high, the back wheel jumps all over the place and theres a noise which sounds like the chain is jumping the cogs.
however the chain is tentioned properly and the cogs are fine!
theres no problem changing gears and the bike only does this when under load(theres no probs at all if i drive like a puff cumin from the hair dressers).
i bought the bike as a non runner and was told about a 3rd gear problem, but ive found its in all - but only when under load!
must be the box, so the engines cumin out next week. can any one give me an idea on what to look for, apart from the obvious worn teeth
and or  does anyone know of a decent place (north east ish)where i could take the full gear assembly for repair if need be.
cheers

have u checked that a correct type chain is fitted for the sprockets and that the "pitch" is right?  ive heard of this happening before when someones fitted the wrong chain, so its worth checking ( should be stamped on the side plates somewhere)

its a difficult one to diagnose tho. could be engine on the overrun.. slipping clutch, gearbox probs.. etc etc..

If the chain is correct then gearbox is gonna be next. apart from worn teeth check for play on splined shafts. wear and play in any bearings... check for worn selectors etc etc.. if u can get a service manual it should help you out.

When you say the back wheel is jumping all over the place what exactly do youmean?  Is is skipping on the road as though locked up?  You say changin down under load- does this mean changing down when gunning hard?  Maybe you're changing too early with too many revs on or letting the clutch out too quickly when you have too many revs on - this could lock the wheel and cause it to skip or skid.
Not sure this fits with it skipping when you shut the throttle though.

I have probably been of no help so sorry but im a tad confused by the symptoms.

essex mate you may be right!
the bike was in a bump once,i know the back wheel isnt original, neither is the swing arm so the sprockets may be a miss match and also the chain.
ill check theyre correct before i start out major!!
ive no contact with the guy who had the bike on the road last so i dont know if there was a problem before the bump.
the bike was bought by a relation on the cheap (he only had it 2weeks and never got it running properly). it needed various minor things for m.o.t. but he cudnt afford to sort any of it out, so i took it off his hands.
when my uncle bought it he was told it had a gear box problem. thats why im suspecting it! but he was also told about a serious electrical problem which had the bike cutting out after 5 miles. i knew before looking at it that it was a fuel vaccum, you could hear the tank hissing for air.
so the guy who had it obviously dosnt know much about bikes and his diagnosis may have me barking up the wrong tree.

zombie mate! it feels and sounds just like a chain jumping cogs!!!! thats what i would first diagnose if i hadnt been told about a dodgy box.

its when you let the clutch out and it starts to engage(less than quarter gunning it). its as if the bike is going too fast for the drive and therefor the box or the chain is jumping because of the force. but which ? is the question.
the same when you let off the throttle.
you can horse it up the box nee bother, its only cumin down....
it does i guess feel like and act like youve come down too soon but ive got to (unless driving like miss daisy) hideously slow right up before releasing the clutch. theres definatley a problem some where.
even when driving gingerly i cant rely on the box to slow me down at all, ive got to rely on brakes alone to stop me, then select a gear and start again.
im definatley gona look into the chain and sprockets.
thanks fellas

Clutching at straws here mate but if the wheel and swingarm are not standard have you checked the alignment between front and rear sprockets?

You should be able to get a fair idea if they're out of line by just looking along the chain from about 10 feet behind the bike.

Are the cush rubbers ok?

rubber cush ok mate, i went to check it and when i took off the wheel it was blatently obvious the rear sprocket was shot.
it looked ok on the bike, but once off and cleaned it was a different story, there were chips out here and there, half the gaps were well worn and the front one sprocket is second hand(its got breakers writing all over it).
ive got a new chain and sprocks to gan on and touch wood, problem shud be solved.
i asked me uncle to ask the guy who had it if he ever seen him if there was any gear box problems before the bump. ive got word back that there was NEVER any trouble with the box, the guy had said that you couldnt change down into 3rd, you had to slow down, then drop the gears and accelorate again.
(which i know).
so there was a bit of a missunderstanding.
my uncle assumed on hearing this that he meant the box was goosed and passed the info on to me.
so ive got decent a zxr750h for 400 squid which i thought had a knackered box.
its got new tyres and a brand new full micron exhaust.
i only put pads in, a front brake switch, a number plate and it got its m.o.t. wooo hooo

That's a fecking bargain.  :buttrock:

You'll have trouble picking up a kids field bike for £400 up here.

thats what i thought, even with a dodgy box.
a frame alone is 400 from most breakers.
initialy i bought it half thinking i would split it up, but once i had a good look at it and knew there wasnt an expensive leccy problem, i sorted a few things out which were obvious m.o.t. failures and took it up to see what else needed done - nothing!
the guy who me uncle got it off was 400 short off a trident he wanted and needed the cash quick, he insured the trident 3rd party only and come a croppper within a fortnight. over 2grand damage.
needless to say hes a bit miffed.
wonder if he wants to buy a zxr?

sorted!!!!!!!!
twas the chain and or sprockets.
cheers for replies fellas

well within a week the brand new chain started jumping again and is now stretched. i stripped the back end down to check the swing arm bearings and they were ok.
when i went to put the suspension back in, the upper bolts which hold the pivot arms in place were loose and i knew i hadnt loosened them before hand.
tightened these up and put it all back together.
there was always a clonk when going over a bump, but i never noticed it due to the chain jumping.
when first i heard it id just assumed it was the battery or summit.
but when a brand new chain jumps a brand new sprocket and has tight spots, i knew there had to be summit else causing it all!
now its sweet as a nut, no noises, no jumping and i think ive just caught the chain in time!

yip, that's a hell of a quick for a chain to wear. Hope you've caught it this time.  :buttrock: