Bad flat spots at 5000 - 6000 rpm. When I accelerate hard the engine seems to die and then catch. Engine runs great at the higher revs.

SOURCE: MY YZ250 WONT REV PAST 3/8 THROTTLE. I NEED HELP!!

It does sound like an ignition timing problem, but could also be a breathing issue, air flow in or exhaust flow out could be restricted i.e.:dirty air filter, excessive carbon build up in exhaust pipe from burning too much oil or rich fuel mixture.
possibly and more likely some junk got into your fuel system and partially plugged up the main jet in the carb, -or has the needle on the slide been moved? The little circlip at the top of the needle, if it has one.
1.Replace the spark plug, check gap, check spark, it should be bright blue and fat.
2. check the fuel filter, if no filter check the carb for dirt.

SOURCE: No power at low revs but works a treat at higher revs.

Hi mate it seems your powervalve is turned up fully, It can give u power on top end ,middle,and bottom end of revs.With it being a 87 a manual valve you will have to take off your exghust off.Now take a look into your engine through your exghust manifold and turn the power valve round ,you will see the hole in which the burnt gases escapre gets bigger or smaller by adjusting this gap you will find the perfect riding style for you (agressave or plesant ) remember to tighten all nuts up ,as you dont want your valve spinning round cus it will shread your piston and rings to death. hope this works
scotty

SOURCE: Problem when cold

Sounds more like a problem with the carburetor's enrichment circuits (1 for each carb; linked by a shaft across the tops of the carbs). These carbs have a separate "well" in the float bowls that fill up via a very small jet pressed into the bottom of the bowls. A brass tube goes down into the "well" to syphon fuel. This circuit easily gets gummed up if not used too often. Try putting some Seafoam, Yamaha CARB-CLEA-N-ER or Techron in the fuel and see if this cleans things up.

SOURCE: 94 yamaha xj 600 want rev over 6000 rpm

hi m8,,1st,, would u no if the bike has been resricted ,,to 33bhp,,if it hasent2nd,, have a look at the spark plugs,, start her up rev until it coffs or wont rev then hit the kill switch,,see wot colour the plugs are ,,they should a biscuit brown,,if not clean them up or change them,, 3rd test,start her up again throttle it a few times ,,untill shez warm,, then ,this sounds not nice, but either spit on your hand or dip hand in sum water, then quickly touch each exhaust down pipe, and make sure there isnt a pipe cooler than the other 3,, if u do find there is that would narrow the test down to that cylinder thats playin up,,try that 1st m8,, then come back if any probs still, and il take u threw next steps,,

SOURCE: 2003 YZ250 engine coughs and sputters at high RPM

fuel carburetor problem! it causes most of the spluttering problems i have encountered with mine so far!
it could have blockage and insufficient gas enters on it during high revs!