I just bought a 1999 F4 which I thought to be in mint condition. It's seen two owners, 20k miles and is stock apart from a Vance and Hines slip on and jet kit. When I test rode it, the idle was smooth but I noticed it only ran on three cylinders at slow speeds but picked up the fourth at speed, so I assumed it was just a clogged slow jet in the cold cylinder. However, it had sat for a while so I thought nothing of it (and it was a bargain).
Onto the issue... So I took out the carbs and to my surprise they were very clean, except for one slow jet which was plugged from debris (just as I had thought). I threw the carbs back on and started her, it fired twice, (popped?) and died. I then remembered that I forgot to plug the TPS back in, as well as the fact that the bowls were empty so it wouldn't have even made sense for it to run immediately. It then took two full batteries worth to fire, but once it did, it idled smoothly and when I revved it slightly it ran on all four. Success then...? Well no...
I went for a test ride and all seemed great until I got up to speed. In higher gears (4,5,6) it won't rev past about 5k rpms and the bike seems hard capped at 60mph. Revs just wont increase and WOT makes the problem more pronounced. In first gear it makes it up to about 7k rpm, then it stutters the same way except it keeps climbing slowly and sometimes surges forwards once the revs get closer to 10k.
I shorted the fuel pump and compared the volume to the manual specifications and it exceeds them by far so fuel delivery to the carbs shouldn't be the issue (unless the pump doesn't turn on while riding and I broke my TPS by leaving it unconnected?). The carbs themselves looked very clean and since it was able to rev up when I test rode it, I don't think they are the issue either. I changed the spark plugs and air filter, and the previous owner had just changed the oil and filter. I also made sure the airbox and carbs were seated properly. I don't know what to look at next, can the gurus at the CBR forum share some of their vast knowledge with me?
PS. I love the colour of this bike.

 

Is the intake air restricted in any manner?

 

What could cause a restriction in the air intake? I don't think I do unless I have squished one of the air lines underneath the air box when reinstalling it? But wouldnt that typically cause low speed issues because the ram air is used more at high speeds?
I am not at the bike currently but I will be able to check tomorrow so keeps the suggestions comin if you get ideas.

 

If it's getting enough air to create vacuum at lower throttle positions but not a 1/2 or more that might cause the issue. I'm not saying that is the issue, just a possibility.

 

Since it wasn't doing this before, I'd retrace my steps. Make sure the hoses are all on and routed correctly. Same with the connectors. Sometimes the connectors are the same and can get switched, so verify the colors on a diagram.

 

OK, thanks for the tips guys! I'll post updates as I go along and inspect the bike

 

Problem solved! Thanks for your help, guys! Either I had forgotten to connect it, or the carb vacuum hose had worked its way loose. Either way, she's back to her ripping ways. Picture of the culprit...

 

Good deal. Glad she’s back the she should be.