Limited to ~12K RPM

[callum4hunner]

Hello everyone,

I have a 1989 CBR400RR NC23 (K) which refuses to rev past 12,000rpm when under load, no matter what gear. It'll spin up to 14,000 on the sidestand (bad I know, but done once for testing purposes) however on the road it can't.

It pulls absolutely superb right up till this point, then it suddenly hits a wall and stop accelerating. I can pin full throttle in any gear and it'll hit this 'soft' limit, where it fluctuates a little between 12-13K. It sounds like it's breaking up, and I thought ignition issues, more details below.

It's puzzling as performance is top notch everywhere below that point in the rev range, no issues whatsoever. I've owned it around a year and it's always done this - only modification is a stainless slipon exhaust, though I've tested with standard can and no difference.

- Valve clearances checked 6 months ago, needed shims all around
- Yesterday carbs cleaned, blasted with compressed air, balanced within 1cmHg - all standard jets according to Haynes, 105 main and 35 pilot. Float heights and mixtures screws set to stock
- New ignition coils and leads, plugs replaced 6 months ago
- Tested fuel pump, great flow rate (but maybe not enough pressure for high RPM?)

It had an intermittent misfire I've recently solved, which was bad spark plug caps (the 90 degree candlestick type, intermittent continuity) and the pulse generator pickups were also losing continuity when hot. Pulse gen coils just replaced with used ones off of eBay, around 420 ohms each coil when cold. Plug caps are £40 ebay specials, closest I could get to OEM after trying another ebay used set which were DOA.

I don't know what type of derestriction has been done to it, but I haven't had a chance to beat the 112mph limit so maybe an issue there? Or am I overthinking this, just needing bigger main jets?

My current thoughts - where is the tank breather on the RRK bikes, is it the little rubber seal on inside of fuel cap?

Thanks in advance, forum advice invaluable this past year

[CMSMJ1]

Welcome :)

It does sound electrical from your post - how's the charging system and all that? Plugs in a good condition?
105 mains does sound small - though I am not as clued up on the CBR - but as an NC30 runs 115/118s standard (and enjoys 120s all round) then it might also be a simple case of getting in onto a dyno for a run to see the AFR and go from there.

[callum4hunner]

I'm pretty sure the charging system is good, as I've installed an aftermarket reg/rec with heat dissipation fins for peace of mind. To be fair, the plugs are a little sooty and around a year old, I've already fouled a set of plugs last year by letting it sit on a high idle for too long to sync carbs and check the charging system (with old reg/reg) after I did the valve clearance job.

I should mention that the only non standard component on the bike other than regrec are a set of 4 Chinese spark plug caps... They were the only rubber stick type caps I could possibly find new and I wanted to ensure the plug wells couldn't fill with water on a wet ride. I tested them at 5kohms each which is what the OEM plugs read, and they make good contact with the plugs.

Maybe next course of action to get a set of proper NGK spark plug caps and new plugs? I'd be concerned about waterproofing though, as I'd lose the rubber flange/seal that sits in the valve cover.

What's everyone's thoughts?

[callum4hunner]

Charging system sending a steady 14V~ to the Li battery at a high idle, so not sure if there's anything wrong there.
Plugs are in good condition, though I guess it's maybe worth throwing a fresh set in. I don't really have access to a dyno near me, so if it's a fuelling issue I'll have to figure it out at home.

Thanks for the suggestions, keep em coming