(Carbed) CBR600 flat spot/hesitation

[Mr. Dazzle]

My day-to-day bike appears to have a massive power hole (or whatever you call the opposite of a power band ) at certain rpm / throttle combos.

Specifically, it really struggles to get 'through' 7-8kprm in a lot of situations. Riding home today it wouldn't actually get past 8krpm at all in top gear. I could sit at ~85mph with the throttle wide open and it would just hit a wall. Changing down to bring the revs up solved it immediately and it pulled strongly after that. Tucking in behind the fairing (and hence reducing the power needed to go faster) had the same effect. If you're accelerating and 'pass through' that rev range it's OK, but if you're already at 7kprm it won't pull. Which is really annoying, 'cause that's motorway speed!

The bike is pretty much stock 1998 CBR600 except for a K&N filter and dyno jet kit fitted by a previous owner. TBH due to work and various other things I've not ridden it much at all lately so I'm not sure if it has been doing this forever or it's a new thing.

It feels like a carb thing though surely? No other engine problems, will pull to the redline strongly above/below that speed and otherwise runs just fine.

Ideas?

[weeksy]

Fuel pump. Fuel filter in tank or inline?

[Mr. Dazzle]

Fuel pump. Fuel filter in tank or inline?

It wouldn't get to full power? The problem is just 'passing through' IYSWIM? I'm not sure where the filter is.

I just say carbs 'cause the fact it occurs mid-revs implies it's on the transition between jets?

[KungFooBob]

Does sound like fuel starvation.

[weeksy]

Fuel pump. Fuel filter in tank or inline?

It wouldn't get to full power? The problem is just 'passing through' IYSWIM? I'm not sure where the filter is.

I just say carbs 'cause the fact it occurs mid-revs implies it's on the transition between jets?

That's exactly what a fuel filter issue feels like, honest

Ducatis, gsxrs, etc. Not sure about Honda's, not owned any

[Mr. Dazzle]

It's an inline jobby by the looks of it (#4)...probably not too hard to get at. Touch wood Actually thinking back, I think I might have seen it last time I took the fairing off....

EDIT: Oh and I recently ran out of fuel (within rolling distance of a petrol station!) so maybe that made me suck some crap up?

[weeksy]

It's a cheap and simple one as a first port of call. It'll have an arrow on it, make sure the arrow is the same way (assuming that one is correct of course)

[Mr. Dazzle]

I doubt I'd have got the last few thousand miles on it if the current filter was in backwards.

[Taipan]

If you're not sure it was ever set up right, then you need to check the settings are right. You should be able to find the advised settings for the DJ kit online. You're on the needle/main jet circuits there, so you're easiest and first port of call should be to check the needle height is where it should be and next would be the PITA of checking what main jets are in there. K&Ns don't flow any more, and some times less, than OEM filters, but people used to think they did and set up DJ kits wrongly based on that belief.

[Mr. Dazzle]

You summed it up nicely with PITA. I've got enough of those ATM

[KungFooBob]

For £500 plus expenses, I'll come set it on fire for you.

[Mr. Dazzle]

But the fuelling doesn't work properly. You'll need more than £500 to get it ablaze!

[Taipan]

If you do take the carbs off, remove the float valve as they have a little mesh filter behind there which never gest cleaned and often gets gummed up.

[Le_Fromage_Grande]

Has it ever run as it should?

[Mr. Dazzle]

Has it ever run as it should?

Honestly not sure....

This bike came to me with the Bonneville in late 2019 and its been my "day bike" ever since following some garage consolidation. I've not ridden it much over the last few months.

Previously I didnt do much "trolling along at 7000rpm in top on part throttle" 'cause that's basically motorway riding. I have only just started commuting again and hence using motorways again.

[Tricky]

If you're not sure it was ever set up right, then you need to check the settings are right. You should be able to find the advised settings for the DJ kit online. You're on the needle/main jet circuits there, so you're easiest and first port of call should be to check the needle height is where it should be and next would be the PITA of checking what main jets are in there. K&Ns don't flow any more, and some times less, than OEM filters, but people used to think they did and set up DJ kits wrongly based on that belief.

This would be my approach after draining the float bowls in case there is a bit of water in one and then changing the plugs if that doesn't change anything- I had exactly this sort of scenario twice with my (FI) ZX10R twice after it had been lying around unused for 6 months or so- ended up being one very low milage (NGK iridium) plug gone bad each time.

And if you do end up with the carbs off, alongside what Tiepin says, also have a good look at the emulsion tubes (the brass tubes that the slide needles travel up and down in)- dependent upon how many miles they've been in there, and who they are made by, they wear- basically eventually go a bit oval- which manifests as exactly these symptoms
" Factory" (a direct competitor to Dyno-Jet- produced the same sort of carb-correction kits) emulsion tubes were notorious for doing this after next to no miles at all in some bikes

[Skub]

Carbs at an angle tend to wear the emulsion tubes,as Tricky said.
ZZR1100s used to get a shiny wear mark on one side of the needles.
Incidentally on the ZZR I had a similar problems as the CBR which was caused by a loose needle on one carb. Man it took many stripdowns to find that bastard. The wear marks flagged it up.

[Mr. Dazzle]

I remember when EFI first started becoming common, everyone hated the fact you 'couldn't fix it at the side of the road'.

'tis true. But you don't have to strip down a load of fiddly bloody screws either!

[Le_Fromage_Grande]

I'd start with the easy and gradually get to the harder stuff

1. Give it a good thrashing
2. Change fuel filter
3. Check pump is pumping
4. Take the airbox off and squirt carb cleaner everywhere
5. Take the carbs off strip, inspect and clean

I suspect it's worn needles though, Dynojet needles aren't very good

[Mr. Dazzle]

Already done number 1, as far as is sensible on the road.

P.S. I'll probably just get OE Honda bits if the needles etc. need replacing. It's not like I need that extra (claimed) 4% of power in this bike.