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Discussion Starter · #1 ·
Hi!
Newbie here, Swede :)
Just bought a CBR 400R Aero-87, NC23 engine. Vacuum carbs. It won't rev over 8-9k. The vacuum throttles sticks vibrating on 50% open. If I lift them manually i get a smooth 14krpm. Cleaned the carbs, checked the diaphragms and all is fine.
Suggestions on next step? Fuel pump issue? Other ideas?
Grateful for all input!!

Regards,
Fredrik
 

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Hi Fredrik
Is the engine breaking down (missing) at 9K or just stop revving? There could be several issues. Does it do it when at a stand still, or as you are riding? Sounds like ignition, but need more info!!
 

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Hi!
Newbie here, Swede :)
Just bought a CBR 400R Aero-87, NC23 engine. Vacuum carbs. It won't rev over 8-9k. The vacuum throttles sticks vibrating on 50% open. If I lift them manually i get a smooth 14krpm. Cleaned the carbs, checked the diaphragms and all is fine.
Suggestions on next step? Fuel pump issue? Other ideas?
Grateful for all input!!

Regards,
Fredrik

What do you mean by "lift manually"?

More info if possible.... cheers
 

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Discussion Starter · #4 ·
Ok. Here's what I have so far: It's the same w/wo load. It just revs on at 9000, like it would at top revs normally. The vacuumthrottles does not open more than approx 50% where they stop, vibrating, like there isn't enough vacuum to lift them. If I assist them with my hands while carefully increasing the gas it revs up to 14.500 (red).Sounds fine all the way, no misfiring. First I thought it was a vacuumproblem but all membranes are good, carburettors clean.
It's like it is restricted at 9000... Could be an ignitionissue, I've found an instruction to "de-restrict" the CDI/revsensor but I haven't had the time to look into it yet. Yesterday I found out (correct me if I'm wrong here) that it should give me like 55kW but my swedish regpapers say 41... Anything else that can help..?
I'm also going to overlook the fuelpump and filter...
Cheers/ Fredrik
 

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Yeah fuel pressure and filter checks.

Have you had the air filter out to give it a knock?

When you cleaned carbs did you drain bowls?

You can de-restrict the NC23s by removing the disc behind the clocks in the speedo. Literally a 5minute job as opposed to switching blue/gold CDI.
 

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Discussion Starter · #6 ·
Yeah fuel pressure and filter checks.

Have you had the air filter out to give it a knock?

When you cleaned carbs did you drain bowls?

You can de-restrict the NC23s by removing the disc behind the clocks in the speedo. Literally a 5minute job as opposed to switching blue/gold CDI.
Thanks!
Drained bowls, jets rinsed.
Derestriction sounds easy enough...
As for now no air filter at all (box lifted for working...) but a new one in the house :thumb:
Gonna be a greasy weekend... :):D
 

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Has this problem been occuring when you have been riding with no air filter?

Have you bought a new one? if so please dont say its K&N :nono:

As for the de restriction.... how are you going to go about it - CDI or the speedo?
 

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Discussion Starter · #8 ·
I'll check the speedo first, anyways...
I rode it when I bought it, filter on, noticed the problem and though "how bad can it be..?", good price on the bike so...:idunno:
Bought it and took it apart and there we are now, all testriding w/o the airfilterbox. Not a K&N, new original :phew
 

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I'll check the speedo first, anyways...
I rode it when I bought it, filter on, noticed the problem and though "how bad can it be..?", good price on the bike so...:idunno:
Bought it and took it apart and there we are now, all testriding w/o the airfilterbox. Not a K&N, new original :phew
Report back after you install the new filter, this could sort it out for you :thumb:

Also, each time you remove the tank to get to the filter/sparks/carbs do you remember to plug the tube back into the back of the air box before you screw it down?

For the de-restriction - do you know what you are looking for behind the speedo?
 

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Discussion Starter · #10 ·
Report back after you install the new filter, this could sort it out for you :thumb:

Also, each time you remove the tank to get to the filter/sparks/carbs do you remember to plug the tube back into the back of the air box before you screw it down?

For the de-restriction - do you know what you are looking for behind the speedo?
Will do! Going at it on saturday...
As for the restriction, not really. Can you explain it a bit more or is it obvious when I see it..?
 

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It wont take you long but it can be 'fidly.....

Behind the back of the speedo (clock face) there is a very small metal disc. It will be sittuated on or around the back of the 180kph mark (top speed currently)

Some people add a kph to mph converter which means that the restrictor will kick in when the bike hits 180mph, which will never happen obviously.

So instead of buying a convertor or new CDI the cheepest (free) way is to simply locate and remove the metal disc.

Use a pair of tweezers or a very thin knife to simply pop it out without damaging the speedo itself. It may need a wiggle, a bend or a pull but if you are patient it will come out :)
 

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Discussion Starter · #12 ·
Allright, thanks! What does it do, exactly..? It can't be just a mechanical "needlestop", must be somekind ov "switchfunction" to it, or..?
 

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It sends a carrier pidgeon to the CDI to say " hey, this guy is trying to exceed the 180kph limit, please cut the power off"

Removing the metal disc kills this pidgeon ;)
 

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Discussion Starter · #15 ·
Allright!
Cleaned the last bits, checked the fuelpump/relay, new fuel/airfilter mounted and off we went! Hesitates a bit around 9000rpm and then revs all the way up to 15.500 (seriously red) and goes like stink:thumb:
Thanks for all input. Next step is carbsync.
Anyone knows why the red light on the revclock saying "speed" lights up? Is that the restrictiondisc in the speedo doing that? Haven't checked yet :rolleyes:
 

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That is the carrier pidgeon i told you about ;)

Get it out!
 

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Discussion Starter · #19 ·
She rides ok, have to fill the tires, fork air and stuff before I know. Engines running now so... BUT: how do I disassemble the speedo? I get it off the dashdoard but i can't get inside...
 
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