I own a 1990 Suzuki GSXR 750, I, bought the bike about a month ago. The first time I took the bike out for a ride I noticed that when you accelerate hard and the engine is at high revs it feels as if the bike has got a dead spot and as if it's losing power. I didn't think much of it and rode the bike everyday to work the bike did not give any trouble. a Few weeks later on my way to work the bike suddenly lost power and started misfiring. I rode the bike home while it was misfiring. I thought that the bike probably had a faulty spark plug because i have experienced a faulty spark plug before. I replaced all four spark plugs and then took the bike for a test drive and it was running smooth after a while of riding the engine started to misfire again when I accelerate hard then it misfires all the way whether you accelerate or decelerate till you reach a stopping point in the road then the engine cuts out. I switched the engine off and smoked a cigarette then I started it up again and then the engine was fine and running smooth on my way home the engine started to misfire again. When i got home i went to a motorcycle dealer and explained him the problem he advised me that the problem sounds to him like a faulty coil. I then bought a coil and replaced it taking turns in the two coils to check which coil is faulty no matter which coil i replaced the new coil with the bike kept on misfiring. The bike misfires so bad now that it misfires at start up point where it just used to misfire when the engine is hot. Another question, should the bike run on LRP fuel or unleaded? i was running the bike on unleaded fuel for a month because i bought the bike with unleaded fuel in the tank.
If the bike has been sitting in a garage for an extended time, the membranes in the carbs might gotten hard. If that's is the case the mix is too poor, when the engine has to deliver power. The plugs is probably light grey in that case. If the RPM limiter sets in too early, the sparks would be wet and black, because of the unburned fuel.
My first thought was that the coil was faulty... It can be a CDI problem too... although what you described sounds like a malfunction... Try cleaning the carbs with pressurized air (take them out, dismantle them and clean them, or ask somebody to do it. Beware, you risk damaging them if you make any mistake), then you can try some carb cleaner spray. My 2 stroke does that too when the engine is cold, but it gets smooth again after 200 meters... i think mine has some sort of rpm limiter that only works when the engine is cold. When it goes up to 7000 rpm, when the power valve is supposed to kick in, it will just start lagging and the valve won t open. I know 4 strokes are different, but maybe yours has a rpm limiter too... How could you know? It s kinda time consuming but it s worth trying: Download your bike's user manual from the internet (make sure you get the exact model and YEAR), and search for the electrical diagram. Then go to the bike and compare what is in the diagram with what is on the bike... it is going to be a bit confusing, but that is the only thing i can think of.
About the fuel question: check the manual. It has to be written somewhere what kind of fuel you are supposed to use. Every owner's manual has this specification.
I would suspect an electrical problem with the coils or CDI. Analogue CDI systems of this era used components that can degrade over the decades and act intermittently as you describe particularly over different temperatures. I am less convinced this is a coil problem since a GSXR750 has 4 coils and it would be unlikely for multiple coils to begin to fail together.
I would look for a replacement CDI after you have verified that this is not a fuelling problem ( clean carbs / jets . check for vac leaks )
The bike should be run on unleaded fuel