Hi all, picked up a 1998 CBR 600 F3 for a bargain £400 last week!

Bike revs up nicely, the idle wanders a little bit but nothing terrible. The issue I have is after revving the bike, when it goes back to idle, the rpms temporarily drop down almost making the engine cut out and then a few seconds later it's back to normal again.

I'm assuming it could be a possible vaccum leak causing this? Or is there anything else I should be looking out for?
I didn't notice any cracking in the inlet rubbers and the vacuum nipples are fine. I know my way around carbs relatively well, I've just not had an issue like this before.

Carbs have been ultrasonic cleaned, fuel pump has been cleaned, tank is spotless & has a new OEM fuel filter.
Carbs balanced with CarbTune pro.

New battery & air filter. Haven't changed the plugs as they don't look too bad.

Any advice is great appreciated! 👍

 

i've heard it's an issue with running rich, but for me it was lean.

You can test for a vacuum leak easily by spraying some starter fluid on the underside of the carb around the carb boots to see if any of them are leaking. Any leaks upstream are technically still metered air and would only marginally impact the mixture i think

I had this problem, and I solved it by syncing the carbs and going up to the correct size stock pilot jet (keihin 40) from my undersized one (keihin 38... i had been chasing a running rich issue all last summer and decided to step down to be sure haha), and turning out mixture screws about half a turn.

 

Hi clippy!
I'll get the sprays out and get to work I guess! If it's not a vac leak I'm clueless. Jets are all stock sizes, stock airbox, stock exhaust, new OEM filter, pilot screws on stock settings so I highly doubt the mixture is wrong on this old hog! Glad you got your issue sorted