hello this happened to one of my friends bike. it's a CBR250RR MC22 bike which he bought recently.
we decided to do a service and first we pulled out the carb for cleaning, tuning and balancing. but what we noticed was there were some tiny little copper wires inside all 4 main jets as well as all 4 idle jets. it was pretty strange. first of all we checked the jet sizes. they were Genuine Keihin jets with standard sizes which are recommended for this bike. then we removed those copper wires and disassembled the carb as usual. we cleaned all the parts. jet holes were ok too. couldn't see any damage. we tested floats by letting them to float in petrol and there were ok. we polished the float valve seats and tested float valves by pouring some petrol through the petrol hose while the float valves were sitting on the float valve seats. there was a tight seal in all 4 carbs. couldn't see any leaks. all the passages like air jets, idle jet, main jet, bypass holes, choke etc have been cleaned. then we assembled the carb and put the settings to the standard settings. tested the sliders with finger and they were working really well. we bench synced it before the real sync. we cleaned the air filter as well. we fitted the carb to the bike and started.
it started without any issue but we could feel a strong petrol smell. when we rev it to 10-12 RPM there was a black smoke from exhaust can. so we ran it about 2 minutes and pulled the plugs out to check them. they were all pitch black. it was a heavy dry carbon foul. both electrode and insulator were pitch black like the image below.



then we cleaned the plugs and again fitted to the bike. we let it idle for 2 minutes and killed the engine from the kill switch. we pulled the plugs again and still we could see a dry carbon layer on the electrode as well as on the insulator. all 4 plugs were the same and they all had that carbon issue. we ran the bike again without the air filter and same thing happened. so we had no clue. then again we put those tiny copper wires through all jets and ran the bike. it ran well and plugs were fine too. please help us to solve this problem. thank you very much.

 

Strange one, looks like the wires have been put in by PO to cut down jet sizes perhaps, maybe in his attempt to clean the jets he made the holes bigger, would suggest putting new jets in, and checking pilot screw adjustment, just to check Main Jets 85, Slow Jets 35

 

It's running too rich. You need to lean it out. As suggested, the jets have most likely been drilled bigger as a cheap way to make it run richer. Maybe for a different exhaust setup at some point.

Best bet is to replace them and adjust from there. If the plugs look the same, get smaller ones.

 

thank you. yea i'm gong to try brand new jet set

thank you. yea i'll try a set of new jets

 

i have a update we put new idle jets. main jets and pilot screws. then set pilot screws to their standard turns. problem solved thank you very much.

 

Well done