CBR NC23 Misfire over 5k revs

[Snazzy]

Hi everyone. Long time reader, first time poster as it were.
I searched the forums but couldn't come up with anything quite the same as my problem so thought i'd make a post of my own.
I have a 1989 Cbr400rr, and have been having some problems with it.
When i pull off, it pulls clearly untill just under 5k revs, at whih point i can feel a drop in power and the exhaust note changes, sounding like a constant misfire.
I took the bike into a mechanics a couple of weeks ago to get some wiring problems sorted out, getting the spark plugs replaced at the same time. This fixed the problem for a couple of weeks, but it started up again the other day. It was running fine, but it was left outside when I was at work in near-torrential rain. I took it home that night, noticing the problem again.
Any suggestions? Could it be dirty fuel? Gunked carbs? Could the rain have done it some damage? Been having this problem for a while now so any suggestions are very welcome :)

Thanks in advance!

[micklewright]

Hi there!

With my experience of these bikes, I would head straight for the plugs and see what condition they are in. Did you put new ones in yourself?

Whilst they are out check for spark on each lead and plug.

You MAY have a fuel or air restriction, make sure the fuel pump is working and there is nothing in the airbox that shouldnt be. (The fuel pump will vibrate when its powered if working)

I had a whole host of problems with an NC23 so have had various misfires and failures. I would start with plugs and post the results on here.

[CMSMJ1]

Sounds very much like the 5.5k revs restriction that is triggered when the CDI cannot see the speedo.

Wiring problems you say??

I'll bet 20p that it is this....

[vr46]

^^ I have had this problem before.

Motor runs on 2 cyls as soon as RPM reaches ~ 5k, a butchered speedo was the problem.

Graeme France was very helpful and told me what was wrong

[micklewright]

^^ Ahh very good point! Seems a bit too intermittent to be much else

[kiwi racer]

Hi everyone. Long time reader, first time poster as it were.
I searched the forums but couldn't come up with anything quite the same as my problem so thought i'd make a post of my own.
I have a 1989 Cbr400rr, and have been having some problems with it.
When i pull off, it pulls clearly untill just under 5k revs, at whih point i can feel a drop in power and the exhaust note changes, sounding like a constant misfire.
I took the bike into a mechanics a couple of weeks ago to get some wiring problems sorted out, getting the spark plugs replaced at the same time. This fixed the problem for a couple of weeks, but it started up again the other day. It was running fine, but it was left outside when I was at work in near-torrential rain. I took it home that night, noticing the problem again.
Any suggestions? Could it be dirty fuel? Gunked carbs? Could the rain have done it some damage? Been having this problem for a while now so any suggestions are very welcome :)

Thanks in advance!

If as you say the mechanic fixed the misfire for a few weeks and it then started again , maybe it was because the mechanic cleaned and drained the carbs which removed the water you didnt know you had in the bottom of the carbs bowl.
Now its been out in the rain again and the misfire started up just like majic ..... i suggest old chap that you have a fuel cap that lets water get into the fueltank. Try down that line to see what happens. Get a bit of hose , connect to the nipple of each carb one at a time, drain and catch in a container and check for water to see if my theory is plausable. If it is then look for paint damage or knackered seal which should be easy to fix.


Cheers , Kiwi.

[kiwi racer]

Sounds very much like the 5.5k revs restriction that is triggered when the CDI cannot see the speedo.

Wiring problems you say??

I'll bet 20p that it is this....

Thats a serious bet there mister , sure you wanna risk that much ????

[Snazzy]

Hi all and thanks so much for your help. Haven't been around a computer for a few days, hence the lack of replies.
First of all, the plugs weren't replaced by me but I know the mechanics in the garage I use so I can't see it being a problem with the sparks. Maybe I should pop down and double check with them anyway
As for the speedo, I have a converter on it that's supposed to derestrict it. Could it be a problem with this, or the speedo itself? This is something I know absolutely nothing about unfortunately
As for everything else, i'll check the fuel/carbs/etc. as soon as I get a dry day. Not owning a garage I don't wanna get caught out in the rain with my pants down.
I'll post back to this thread when I next get a chance.

Thanks so much to everyone. This is by and away the most helpful and friendly forum i've found so far!

[CMSMJ1]

I'll take your 20p Kiwi...and raise it to ten bob (or 50p)

@ Snazzy - make sure the speedo is plugged in - simple check and hopefully save me some brass..

[Snazzy]

I think i've sorted it. Took the fuel tank off and drained some of the fuel and it was indeed (very) dirty. Swilled out and flushed the tank a few times, then took it out for a ride. Problem persisted for about 15 minutes untill I got onto some twistys, allowing me to blast it out. It coughed a bit for a few minutes but once i'd revved it some it all started working fine. This was christmas eve and I haven't ridden it too much since (due to my families obsession with horrendous amounts of alcohol during the festivities), but work started up again today and it's been treating me fine. Thanks very much everyone for your help Really is appreciated!!

Oh, and I also made sure there were no loose connections etc. on the speedo, couldn't find anything but the kph/mph converter was slightly loose. Tightened that as well, so it may or may not have been the problem. Good luck trying to sort out who gets those coppers!