Sorry for the long post but I need your help!

Brand new on here but not to CBR's. Had a 91' Red/white F2 almost 13 years ago, loved it. Moved to a 93' (red/white/blue) 900RR and loved it even more, CBR's are great bikes!

Just picked up a 92' F2 with 25K miles. Vance & Hines slip on and K&N air filter (not overly oiled). Pulled carbs and had them cleaned by a mechanic, plugs/wires are good, has fresh gas in tank. Bike was put on dyno by previous owner and it shows it going lean around 7K rpm. Engine idles perfect, runs up perfect till around 6-7K rpm where it stumbles really, really bad. In first and second gear it'll pull through it and run smoothly to redline. (after 7K rpm) In third gear on up it won't even pull through the 7K flatspot and just stumbles really bad. It pops really bad on deceleration indicating a lean mixture from what I've been told.

We're not sure what jet kit is in the bike as there are no markings on the needles. Some people have said to get a Factory 1.0 jet kit and simply follow the directions, all will be well. What is your advice to fix this annoying flatspot? If you need more information just let me know.

I appreciate your advice and help!

 

Here are a few pics of the bike if you're curious.

 

With 25k on there, did you do a complete breakdown, inspection and cleaning of the carbs, including soaking the needles, checking float valves and diaphragms etc etc etc etc etc and perform a sync? That's far and away step one.

 

If someone put in a K&N and no adjustable jets, then that is your problem. Make sure 100% that your jet needles don't have multiple clip slots. if they are stock jets, just get a stock air filter. If you want the K&N (trust me, doesn't make a difference other than it is reusable) then you will need a jet kit to richen the mids up.

 

I think he's saying it IS an aftermarket jet kit, he just can't find any manufacturer markings to tell WHICH kit it is. I wouldn't go putting new jetkits in the thing until we've ruled out that

A) it doesn't already have one

B) The problem isn't just from use and abuse.


BTW the exterior of that bike is in AMAZING condition for 25k. Nice find!

 

Thanks guys for the feedback!

Johnny: The carbs were cleaned thoroughly by a local bike mechanic. He's very good and has been doing this for over 20 years so I'm guessing he went through them properly. He did say they were VERY dirty and hoped that would improve the flat spot. Unfortunately it didn't. He didn't mention synching them, not sure on that one. Thanks for the compliment on the exterior, I detail cars on the side and try to keep the F2 clean as well. A neat trick is to use Pledge furniture polish. It really richens up anything plastic and adds a nice gloss to the paint. It's very similar to the old Honda Pro Cleaner Honda used to sell. (I loved that stuff) I voted for your F3 btw, very nice! We used to call that the Miguel Duhamel scheme, the bikes he raced looked very similar.

Disserved: The jet kit is adjustable. My mechanic said he had previously shimmed it as much as he could, he simply couldn't shim it any further evidently. There were no markings on the needles so he couldn't tell what jet kit was in it. The K&N filter came with the bike, I may pick up a stock filter from Honda just to rule out that variable.

 

Oh, regarding the bike being abused in the past, while I'm not 100% certain I think it may have been taken care of. It seems most of these CBR's lose 2nd gear if they are ridden hard, this one seems to be intact. Bike made 85 hp on the dyno so that seems fairly healthy as well. Having said that it's been down on both sides in the past, some things you can't hide

 

Syncing is a massive issue, and a lot of people don't do it as part of a carb cleaning. Your engine cylinders wear unevenly over time, and the vacuum changes, thus requiring a sync to be done. Call and ask him if he did that.

As for the jets: I don't quite understand why he shimmed it that far. The fact that he has the jets all the way out (or in???) could def affect your midrange.

And don't worry about the filter, it's not going to fix anything.

 

Oh, If I understand correctly syncing is done while the bike is running correct? If so, then no it wasn't done as I pulled the carbs myself and gave them to him to clean.

I believe he said he had shimmed the needles as far as they could go indicating he was trying to richen the mixture but I'm not 100% positive about this. I'll give him a call and try to get a better understanding. He has the dyno graph as well, I'll try to scan and upload that as well.

Thanks again.

 

You are correct. unfortunately, Syncing is going to require the carbs to be removed again to get to the vacuum plugs on the cylinder head. I know that sucks (pun intended).

Woah. That would be bad ***.