Hi all,

I'm currently chasing a long-running issue where my bike (1989 CBR400RR-K) struggles to rev beyond 9-10k rpm. Feeling a fresh motivation to get to the bottom of it this summer, I've taken my carbs of the bike for the umpteenth time!

I noticed one of the needles was quite worn, the tip had a much bigger surface area when looking at it head on compared to the rest of the needles. So I ordered some used needles from eBay, however now notice that these replacement needles are much shorter. There isn't any guarantee that my originals are the correct item, so I wanted to ask the forum.

My original needle set are marked '8GH', and the eBay set are '7MJ'. Could anyone shed some light on if this is an NC23/NC29 or maybe an RRJ/RRK thing? Or possibly from a different bike altogether? Just wanting to be sure I use the correct set for my '89 RRK.

As a side note, I've verified from engine number, frame and carbs that the bike is definitely K (I suspected a while ago it had J carbs, but that was debunked, I have the VG04B's).

Cheers fellas

Has it reved beyond 9-10k in the past. I would have thought that reving beyond 9-10k rpm was more a main jet parameter than needle. At such high rpm only the tip of the needle would remain in the needle jet. Have the main jets been changed for a smaller size. A standard airbox fitted. For what its worth a google search for 8GH and 7MJ needles gives an ebay result that says they are from different models of CBR600.

hey mag, thanks for reply. It will rev slowly to redline at around 1/2 throttle, but WOT it only goes 9-10k before it stops accelerating (and I kick up a gear). The needles I found in the bike are notably longer than the 7MJ needles I bought, and you're right, I think both may be shared parts with the 600 too.

its all standard air filter, 105 mains and 35 pilot stamped with star so genuine Keihin originals. I tried up jetting to 115's to solve this issue, but it was exactly the same. I wonder if the needles were too long, (I. e. for different carbs/bike) they could obscure the main jet even when diapraghm slides fully open?

My dimension diagram of an NC30 needle, has its overall length as 43mm. That maybe a guide as to the approximate oem length for your CBR400. 1/2 throttle redline is in neutral, no load and WOT 9-10k is in all gears? Have you checked ignition timing setting and used a strobe light to check ignition timing advances with increasing rpm.

Thanks, that reference length is really helpful. That's right, in neutral it redlines as normal, but in the road in any gear it hits this 10k limit WOT. And if I let off the throttle a little, it will slowly climb to redline.

I don't have a timing light, I think the ignition timing is non adjustable, but I've checked the resistance of the ignition pickups at red connector, which was within the range Haynes manual gives (around 420 I think).

I realised yesterday that the CDI appears to be one of these £35 Chinese units on ebay, as it looks fairly new and has "Lot no. 8910" printed on it which matches the Chinese ones. It's also stamped "Made in ZXMT" rather than made in Japan. I suppose it's an easy thing to swap out, if not that any genuine Honda CDI is priced over £100 on Ebay :/

I believe the NC-23 has a fuel pump. Perhaps the fuel pump has failed or its performance has deteriorated and the carby fuel level is falling as the engine is accelerated to max revs. My understanding is that engine will run via gavity feed from the tank which may explain why the problem does not appear until extracting max performance.

You might be bang on the money there. Just had a long day of testing...

Currently it's running with 115 jets and the shorter 7MJ needles I bought (stock airbox and exhaust), and I have to say the mid-upper range has never been better, it pulls awesomely. It still has this issue though, coming in at usual spot ~10k.

For my test run I bridged the fuel pump relay black/blue wires hoping that perhaps the pump wasn't being triggered at high enough frequency (as with the relay, I think it's pulsed piggybacked from coils. The pump now clicks away with ignition on, and it just doesn't sound like it's actually pulling enough fuel through (a dry click, similar to when I've tested it dry). Putting my thumb over the hose end which connects to petcock, there's almost negligible vacuum, it seems to struggle to pull any pressure at all.

Also, bypassing the relay and getting more pump clicking erm... drains the battery rather quickly! I think for my sake I best replace too... Measuring voltage across battery when holding revs at 5000rpm.. it can drop as low as 9V! So I'm not sure if the battery is goosed, as closer to idle it does in fact show around 14V. I've tested the 3-phase output from alternator, and it's 50V at 5000rpm so nothing wrong there.

In summary, my best guess right now is a weak fuel pump (which has healthy flowrate, but maybe not enough pressure to supply carbs through tank and inline filter) or something funny going on due to weak battery. It's worth adding that the battery is a Haiju SPS Lithium Ion unit, and the fuel pump looks non-OEM, it seems relatively new so may very well be a cheapie part that's not up to task.

I'll price up new bits, thanks for your input magg

Rect/reg could be an issue if alternator is outputting correct voltage. If battery voltage drops as low as 9v then ignition system and pump performance could be compromised. My reading of the wiring diagram suggests that the pump may only run whilever the engine is running, remaining
closed when ignition signals exist or provide a pressure control dependent of engine speed. If pump rate is dependent on engine rpm and the control by the relay is faulty or the pump is faulty this may cause your high rpm issue.