Honda NSR 125 Rev limiter?

I have a Honda NSR 125 1998 year and it has had a problem since i bought it. The problem is that the revs only go to ~6,500-7,000 around that time on 5th gear, i'm going ~90-100km/h after that it just starts going down (doesn't go up) and it just starts sort of slowing down, until i go to ~6,000 revs then it starts acting normal again. So i was wondering what could be wrong here? Is it the black box? Is it a restriction? What is it? And please give me advice (or the solution) on how to fix it, because it was always a dream of mine to ride NSR on it's full potential.

There's a few things you could try first.

1, Check the spark plug is clean and gapped correctly. Air-filter is clean and dry.

2, If both of those are done, but problem still happens - Try getting to that RPM in that gear then turning the choke ON. The choke will add a little more fuel into the carburettor, and if it runs better like this and there's no problems, then you need more fuel in the mixture. At those RPM's, I'd say it's more than likely a slightly larger main jet.

If the choke doesn't help or makes it worse, then try increasing the air-flow a little. A simple thing to do is to remove the snorkel from the air-box. This will allow more air-flow in. It could however bog, from running to lean, but it's worth a try. Run the bike up to those RPM's again in that gear, with snorkel off, and if it runs better then you need more air in the mixture. You can run it with the air-box snorkel off if it doesn't bog or run to lean, I did it with my old NSR for 2 years or so.

Possibly, although for a bike that age the limiter is likely to be in the carb than anything electronic, but if the bike revs freely in every other gear then probably not. I don't know enough about the electronic bike limiters circuits to help you then. If it's in the carb there will be either a block on top of the slide, which is a little fiddly to remove,. Or on the bottom of the carb top which is a lot of problems to remove.

If this fixed it then you have to tell your insurance company, or otherwise your insurance is invalid, which is six points and a fine. And if you're not licenced fpor a full powered bike, that's another 3 points, and a fine.

The 125 cc and 11 kW limit should give you a top speed of about 60 - 65 MPH. If it goes faster it does not comply to the legal description.

Andy C