Got a 93 CBR900rr. While under load, (i.e. not when just idling), between 2.5-4k RPM my bike stutters like it is choking or something. It was worse a couple weeks ago, so I got new sparks, air filter, and had my carb cleaned. Mechanic said it was a little bit dirty.

But its still stuttering, although not as bad as it was.

What could be causing this now? Fuel Pump? Fuel filter? Its got spark and a cleaned carb.

This is a daily rider and hasn't been sitting for more than 2 days, in over a year.

 

what octane are you using"?
have you gone up or changed the octane at all and personally if you own a carburated blade then i personally think you should learn how to clean your own carbs..
i own a few of them and everytime i pull a set it gets easier and easier and better at it..
plus i know for sure exactly how well i clean them, invest in a smalll sonic tub from harbor frieght and save yourself a ton of money on having them so called "professionally done"..
anyway back to your problem, being a 93 its really going to be probably the carbs maybe you need new mains.. do you have a pipe ? or any other upgrades. have you changed anything over the last 12 months that maybe now might just be catching up and changing the characteristics of the bike. is the bike over thirty thousand miles maybe you might need a reshim causing the ports to not breathe right when getting on it..
throw some more info out, theres still alot of us out here who drive that bike...
jay

 

I've always used 89 octane, so I don't think its that. I never fill up if I see a tanker in the gas station either. I work on my vehicles but as its my daily rider and I work full time, sometimes its just easier to drop it off for a couple hours..

Bike has unknown miles (odometer not plugged in) but its atleast 20k, maybe 23k.

No mods recently either. It has a Jardine exhaust, but its been the same since I've had it over a year ago.

My mechanic said he noticed the fuel pump was a bit loose, so he zip tied it. Maybe if it was knocking about for a while, it has caused this stuttering?

Wondering if the carb jets are bad (would a "cleaning" have fixed any bad jets?) or if its just not getting enough gas at the specified RPM range? Going to test the fuel pressure this week and see what it says..

 

Recently I replaced the battery and the stator (as it was burned out, and not charging the battery). I got a used but upgraded CBR stator (off a newer bike) and it has no electrical/charging problems.. Doubt its related to this but thought I'd just throw that out there.

 

honestly, i would buy a jet kit. by doing that you'll replace all the jets at once instead of cleaning them and if you've only owned the bike for about a year and dont know the whole history of the bike, then the pipe could actually be effecting the performance right about now, especially if it was just thrown on the bike before you got it.. people will do that, throw on a pipe, it rides great for a while and bang all of a sudden something like this kreeps up on you..
if you order a jet kit and explain exactly what mods the bike has, they'll send you the right mains and other jets and all you'll have to do is screw them in and take her for a ride and see how she goes.. if you decide to go that road email me and ill help you through it..

 

i wanted to put a couple other thoughts out there..
fuel filters are cheap and should be replaced every year anyway..
honestly just disconnect your fuel hose from the tank , point it into a bucket that gas wont melt and turn your ignition on and try and start it, you'll know immediately if you have a bad fuel pump..
heres the actual manual for your bike, from honda, in case you wanted to do the actual fuel pump test and measure the amount of gas coming out..
RRZone Forums - Downloads

 

seeing you've only owned the bike for a year, that was a good move.. its the r/r"s that go bad but most people just replace both like a set. honestly it cant hurt and now you know you can put some miles on her whith out much worry.

i'm telling you, from my gut its in the carbs.. its too bad your mechanic didnt record the jet sizes when he had the carbs open, that would have told if its ever been jetted..
what kind of air cleaner do you have ? is it stock?
also does the exhaust pop when coming off the throttle??
does it sound mean? do you know what i mean?

 

Thanks for all the help and info..

I will definitely replace the fuel filter too. Can't hurt. Will check the flow while disconnected too.

The air cleaner just got replaced last week (after it was already stuttering). IDR the brand but it was nothing special (not K+N).

The exhaust does pop a little bit.. It's definitely loud and mean sounding. I've yet to ask my mechanic, but the idle is definitely different. It used to idle nicely at 1100+ish, now its lower around 800+ish. I know thats too low cos it fluctuates at idle and not nicely.. I figured this was cos the carb got cleaned and so I have to adjust the idle?

I will take it back to the shop tomorrow - he's cool enough with me to diagnose without a fee and hook me up, thats why I have no problem letting him work on it.

 

no, i had the same problem with my 95 this summmer..
as soon as i jetted her she ran like the animal she was designed to run like..
by having the aftermarket pipe, your bike now has more exhaust flow and less back pressure adding to the idling problem also your bike is like twenty years old. not everything is going to be as tight as it was when new.. especially the air box where the filter is mounted on the carbs, which you can also double check "there" to see if therees any tears in the boots where they mount up, that will also cause havoc with idling ..

another cause maybe maybe that the carbs need to be sinked, meaning one or more of them are pulling more "gas" than the rest of them.. if your mechanic sent this bike back to you with an obvious idling problem, i personally wouldn't be to fast to send it back to him. also solving g the problem yourself is cheaper and way more satisfying.

being the fact your idoling problem is in the 2000 to 4000k range, its definelty going to be in the jets... thats the problem with sending it to someone else, you really dont know how well theyve been cleaned..

this is what i would do, see if you can get them cleaned again but this time ask to haave the carburetors synced. the nice part of owning these older blades is your not involved with all the electronics of "today" and praactially no vacuum lines to worry about, its all about air radio and gas, jet sizes and back pressure.. really its just straight forward.. its allm about how well the work was done the first time around, once you get your baby running right" the smile" on your face will def stay up there for a while...
and the last final bit of info that you want to remind your mechanic is: the main jet needs to be the cleanest, thats the jet that effects the low speed transition to the mid range of the carbs that 2-4000 area, the only other thing that could effect that range is float level which i doubt has changed and fuel flow which your going to check right away..
jay
hey if you want to call me or text to my phone to pick my brain faster 8573334506 i live in mass. just so you know

 

I think you're right about the fact its the jets - simply because of the RPM range. It will be nice to rejet it anyway. Will get that sorted out in the coming weeks. It'll be nice to have her on point again.