Weird Fireblade issue

[amancalledrob]

I've got a '99 CBR900rr which is mostly great but lately has developed a weird engine vibration when I use the front brake. Not noticeable while riding along and using the brakes normally, but if I'm at a standstill and pull the brake lever there's a subtle change in engine note and a vibration - I can't figure out what's causing it.

Any thoughts?

[Biker's Nemesis]

That's a new one for me.

[DuraAce]

Hmmm... Could it be the brake light, as it's activated and causing an issue further down the loom that affects the ignition circuit?!

Maybe some corrosion in a multiplug connector or a poor earth connection somewhere?

[graham22]

I think two faults here, vibration when braking either warped disc or tight bobbin (I gave some 'warped' discs off a 98 Blade to someone on here who was able to unwarp it by cleaning/spinning the bobbins).

Change of note at tickover when brake applied, probably something simple like loose RH switch gear/twistgrip clamp being interfered with when the lever is pulled?

On a '99 blade, there's no connection to motor/engine management & brakes other than the additional draw of the rearlight bulbs to the alternator - doubt if that's noticeable!

[Krikkit]

Could be an earthing/voltage regulator fault with the brake light causing the engine to labour? Any other electrical maladies?

[308mate]

Fireblades are famous for eating the Reg/rec, not sure how that could be linked to the brake light though. If it was a current draw issue, surely indicators and headlights would also affect it?

[moto_traxport]

amancalledrob said:

I've got a '99 CBR900rr which is mostly great but lately has developed a weird engine vibration when I use the front brake. Not noticeable while riding along and using the brakes normally, but if I'm at a standstill and pull the brake lever there's a subtle change in engine note and a vibration - I can't figure out what's causing it.

Any thoughts?

Is that the one at the right? The one on the left occasionally causes the symptons you describe.

[amancalledrob]

Krikkit said:

Could be an earthing/voltage regulator fault with the brake light causing the engine to labour? Any other electrical maladies?

I think this is worth investigating, thanks. The reg/rec did go a while back so as a belt and braces approach I replaced the stator as well, since one often damages the other. I think a dodgy earth is a likely culprit as I recently upgraded the front master cylinder to an Accossato radial one and fitted a brake light pressure switch. I wonder if that's where the problem is

[legzr1]

You sure you're not twisting the throttle ever so slightly when grabbing the brake lever?

[amancalledrob]

legzr1 said:

You sure you're not twisting the throttle ever so slightly when grabbing the brake lever?

Absolutely certain, but it's a good shout, thanks. I've remembered since posting that the issue started when I fitted an aftermarket radial m/c, which necessitated replacing the brake light switch with a brake line pressure switch. I think water's got into the connection and is causing a bad earth or a short, so I'll have a look there

[Jazoli]

amancalledrob said:

Absolutely certain, but it's a good shout, thanks. I've remembered since posting that the issue started when I fitted an aftermarket radial m/c, which necessitated replacing the brake light switch with a brake line pressure switch. I think water's got into the connection and is causing a bad earth or a short, so I'll have a look there

I can't see how that would do anything but I'm happy to be proved wrong.

[amancalledrob]

Jazoli said:

I can't see how that would do anything but I'm happy to be proved wrong.

Neither can I can't imagine what else I should look at though

[moto_traxport]

On the VFR of the same era there was a big connector within the wiring loom where all the earths met up. Sort of near the top shock mount on the left hand side. If water gets in there they do all sorts of strange stuff. The blade might have a similar loom.

[LuS1fer]

All bikes I have owned alter vibration on application of the front brakes, it seems to damp the general vibes from the bike.