Honda NSR 125 rev problem

[Kasmaas]

Hey everyone 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.

Thank you in advance, Justas.

[-]

Sounds like it's got the wrong gearing. At 100khm (62mph) 7,000 revs in 5th gear sounds way too low for me when it has a 6 speed gearbox. If you hit the redline at 11,000 in 5th, you'd be doing 97mph (155kmh) with another gear to go.

Standard sprocket sizes should be 14 front 36 tooth rear.
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[Kasmaas]

Oh, i'm sorry, i meant it's top speed is ~~60mph (~~100kmh) and the revs don't go past 6,500 at every gear.

[skatefreak]

First port of call:
- Power valve.
This should be open from 0-2,000 rpm then close from 2,000 - 7,000 rpm and then open again from 7,000 rpm >.

Square box with the round bit on it in this pic:https://i136.photobucket.com/albums/q193/ri_powell/NSR125/IMAG0103.jpg

The engine itself is ported to run at its strongest above 7,000rom so the power valve closes to effectively change the porting of the engine to give you usable mid range power.

If this valve is not working and stays closed then the idle will be a little lumpy but the top end will not have much power.

To test. Just turn the ignition on and you should hear it move (sounds like an R/C servo).

Have a check and let us know.

Most restrictions will reduce the power however the bike should still rev to 11,000 rpm.

Is the bike restricted?
The most common restriction on the NSR's is a restriction plate in the intake (between the carb and the engine) also one in the exhaust if it is.

Best regards

-Jvr

[Kasmaas]

[skatefreak]

Right.
Where abouts are you?
Sounds like this bike could do with a looking over
First issue to tackle, you will need to get the power valve adjusted and working right.
I have PM'd you a link to a manual, this will show you how to setup the power valve. Its a fiddly job but will make a world of difference once its sorted.
Secondly it sounds like you may have a problem with the servo, check the wiring is all secure/connected and take off and clean the connections. I am not sure if you can take the unit apart if its still not working but you don't seem to have to much to lose.
I do have a spare servo knocking around somewhere however if you send me yours I can have a good look at it/connect it up and see what's occurring.

Once the power valve is working correctly then if the air intake is de-restricted then the exhaust needs to be checked (easily done) and restricter removed if its present.

The car will then need to be checked to make sure the jets are correct, de-restriction = more power = more fuel = larger jets.
Skipping this bit can cause the engine to run lean and go pop.

It may be worth checking the frame number with this post here incase there may be further restrictions.

Getting there

-Jvr