I had my MC21 sp at the track last weekend and something funny was happening. 1st and 2nd gear revved all the way to red line but every other gear struggled, nay refused to pass 9,000 rpm.

What could cause this? I was thinking it could be RC valves or spark plugs? But I’m just a weekend mechanic that tinkers on my own bikes. Any help would be greatly appreciated.

Thank you in advance,

Markus

Bad jetting is what springs to mind first. Have you carried out any mods to it?

Oh, and where in the world are you?
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Dave Ett wrote:Bad jetting is what springs to mind first. Have you carried out any mods to it?

Oh, and where in the world are you?

Thanks you Dave for your response,

ahhh I should have mentioned that the bike was capable of reaching just under 200kmph but this weekend did not pass 140!, if it was jetting, wouldn’t I also have problems with 1st and 2nd gear? Aftermarket bits air filter, expansion chambers and exhausts but nothing has been changed on the bike since the above speed was reached.


I am in Sydney Australia.


Thanks for your help in this matter.


Markus

well i personally dont have any experiance with nsrs but juding from those mods u mentioned ALL would require the jets the be matched to suit (ie re jet the carbs).

Any modification to the way the bike breathes and exhales would be serious in terms of new jetting.

So id sugggest giving jetting a try...

I would check the fuel tap for flow and fuel tap filter first.

After that strip and clean the carbs.

If the bike was running well with the current setting there is no reason why it shouldn't again.

Quite often a bike lacking fule will rev out in the first couple of gears only to give up once the fuel demand is greater.
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If I have to take the carbs off once more...

2-Strokes Rule wrote:well i personally dont have any experiance with nsrs but juding from those mods u mentioned ALL would require the jets the be matched to suit (ie re jet the carbs).

Any modification to the way the bike breathes and exhales would be serious in terms of new jetting.

So id sugggest giving jetting a try...

Can the jetting change so much as to cause 60km difference with no changes to air input and exhaust? As I said earlier I have made NO changes to the bike since this restriction in revs started.

If it is jetting why would 1st and 2nd not be affected?


thanks,
Markus

StephenRC45 wrote:I would check the fuel tap for flow and fuel tap filter first.

After that strip and clean the carbs.

If the bike was running well with the current setting there is no reason why it shouldn't again.

Quite often a bike lacking fule will rev out in the first couple of gears only to give up once the fuel demand is greater.

very interesting, thank you for your advice. makes allot of sence as the carbs would have had time to fill up again before 1st and 2nd gear are used.

I will check all this and keep everyone updated.


Thanks guys,

Markus

quote:
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Originally posted by StephenRC45:
I would check the fuel tap for flow and fuel tap filter first.

After that strip and clean the carbs.

If the bike was running well with the current setting there is no reason why it shouldn't again.

Quite often a bike lacking fule will rev out in the first couple of gears only to give up once the fuel demand is greater.
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Sounds right on if the bike is running fine other that at top revs and load....
I've seen once the fuel tank vent get cloged on a dirt bike cause the same type symptoms. Same lack of fuel at top end....
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MC21
04 NSR50R
93 CBR F2
98 CBR F3 track bike
95 VFR750
89 FL400R Pilot
90 FZR 4/600

Another possibility is the spark system - it's ok in the first two gears where load isn't so great, but isn't strong enough under more demanding conditions.

Check the connectors are clean and tightly fitting.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Did you disconnect the dash? On the CBR250RR and vfr400, if you don't have the dash plugged in it won't rev right out above 2nd (stops dead on 9000 RPM). check your connections there first.