Engine stumble after engine braking
 

Hi all. Got a problem that i cant nail down. I think the answer is just that i need bigger pilot jets but wanted to get more opinions from you fine people.

CBR600 1989, fightered. It idles great, throttle response is great, and it beat a modern r6 from 0-illegal so it runs well on that front.. but the issue i have is that after a hard pull then, engine braking for a few seconds, if i go to open her up again its like im running out of fuel for a few seconds. I have a working fuel pump and i have adjusted the floats so they are as "rich" as can go while still shutting off the fuel. Has anyone else had this issue, on any bike really as the principal is the same.

Cheers!

Is it running a standard pipe, airbox and air filter?

All standard apart from no fairings and a chopped original exhaust to make it a stubby.

That’s possibly the problem, cutting it down will effect the jetting

I've been into the carbs, no lie, 50 times now playing with jets. Running 110 mains and 35 pilots. Just been tuning the screws a bit and I got it a bit smoother. I've synced them with mercury gauges as well and they are pretty good. From cruise if you open it up it rips your arms off (55t rear sprocket) and it cruises pretty smooth, its just after a long engine brake it acts like it's running out of fuel for a few seconds..

Go do a run and then a long engine brake, then immediately kill the engine. Coast to a stop with the clutch in and yank the plugs right on the side of the road. Don't let it idle or try and rev it up first, it has to be an accurate, at the time, reading on the plugs.
I think you will see exactly what the problem is then. They will either be lean and clean, or fouled and rich. Either will tell you what the carbs need to have done.

That's actually a pretty good idea lol no idea why I didnt think of that.. Do you think the valves could be the cause of this? I have checked them and they were all in spec but maybe I checked wrong.. It pulls so well and smooth I don't think its them... Also at idle I've checked the header temps and all of them were within 10f of each other which is fine and shows they are all firing.. I've just put the air screws to 3turns out and its a bit better.. I'm gonna check for a vac leak as well while I'm at it.

Vac leak could do it.
Differential in air pressure between the air box and the carb float bowl could do it by pushing fuel out or not letting it flow in.
Have you checked voltage as well? Mine did this intermittently until I upgraded my RR and stator.

Problem solved lads... Fuel pump was ****e.. Put a temporary one on and now its a damn bullet haha

Glad you got it solved and thanks for sharing the solution.