Limited to ~12K RPM
Posted: Fri Sep 01, 2023 10:53 pm
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
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