Hi there,

I have a 1992 cb400 SF which ive recently got on road after months in my garage.

The issue im experienceing is that I cannot reach anywhere near the top speed of the bike because it revs too high. In 6th gear the engine is doing 10,000 rpm at 100kph! Aside from the over revving the engine runs very smoothly and feels/sounds nice through out the lower gears. This said, i think it should be going faster in the lower gears too.

Although it never really felt like it was slipping, I replaced the friction plates and springs on the clutch to be sure; this has made no difference.

The sprokets are near enough to stock: 16 on front, 45 on back.

Ive looked into possible restictions that might be affecting it but have found nothing: No inlet washers and I have tried riding with the speedo disconnected.

Ive now expended my ideas and dont know what to try next.

Any ideas?

Thanks, Ewan

Are you sure the speedo is reading correctly. You could check it with a smartphone equipped with GPS or maybe ride past one of those speed warning signs, they are usually accurate.
It may have a speedo convertor fitted to convert KPH to MPH. This would give exactly the results you are observing.

Yeah Ive ridden with a mate to make sure its reading correctly, cheers for the suggestion though. I now think it may be a gearbox problem and im not actually getting into 6th, or maybe even 5th.

Could somebody on here let me know what rpm they get when they ride at 60mph in 3rd, 4th or 5th ? if it does match what im getting i think its not able to get into the higher gears.

Thanks

My cb-1 runs around 11k = 100mph

Have a look on gearing commander, that will tell you the 'should be' speeds at certain revs.

Does it have the right size front tyre?

Have you checked it against GPS?

Does it have one of those mph converters on the Speedo cable?

My CB400SFV Revo 16T front 40T rear as follows:-
3rd @ 60MPH 8,000RPM
4th @ 60MPH 6,600RPM
5th @ 60MPH 5,800RPM
6th @ 60MPH 5,200RPM
Suggests you're stuck in 2nd gear?!!! Yet I'm sure you'd be able to detect that....Maybe ask your mate to ride it and give us his verdict

Cheers for all your help and suggestions, especially that gearing commander website which is a really usefull tool.

After all it turned out that the tachometer was massively over reading so I was just never allowing it to accelerate because I was scared I'd blow the engine up. I'd never ridden the bike before putting this new chinese speedo on so didnt know what it felt like at different RPMs.

Was able to diagnose the misreading tacho by using FFT spectrum anlysis to find the actual RPM and compare it with what was indicated. Turns out the motor was doing about 56% of what the tacho was reading. So in the end all that was required was more beans!

The aftermarket speedos generally count sparks and multiply by the cylinders. I guess it thinks it's on a twin? There may be a way to adjust that.