Hello, I am riding a 98 CBR900RR(carburated) and have a question about lower RPM performance (idle - 4K). I haven't riden many other bike models so I don't have a foundation of reference to base this question... but here it goes...


So I can hit 70+ in first gear, but I don't always wind it out through the power band. Sometimes I like to cruise at 35-45 MPH and my bike doesnt seem to like it. I can ride at 2K to around 4K and in any geareventually get to a spot where I hold the throttle still...to settle into a low RPM cruise, the bike doesnt slow drastic or speed up much, but there is a slight but annoying forward and back jerky feel to the performance, and when beginning to excellerate from a lower RPM there is a slight hesitance like it bogs down for a split second. like the carbs are sputtering saying "WHAT???? OH! OK, you want mego faster, here you GOOO" and I'm off like afrakkin bullet!!

Is this normal?

I am mechanically apt, but would it be easier to get a professional tune up. I bet its been a while, I bought the bike with 26000 and now it is at 29000. Two oil changes w/ new filter. I've run marvels mystery and sea foam through the fuel system. The chain is adjusted properly with 3-4 cm play.

Do I need to adjust the carb pilot screws? The mixture? Sync?

I don't know the history on the bike accept for it has down 1 on the drive sprocket and a 520 conversion. Everything else seems to be factory but may have been tinkered with. Maybe it's my turn.


When I hit 4K in any gear it takes off smooth and as fast or as slowas I want, purrs smooth and easy.
When I excellerate from Idle the hesitance doesnt last through more than a few hundred RPM, like I said it very quickly settles and smooths out, but its the reaction time that gets me, maybe a half second of bogging down.

Should the bike run as smooth at 3K as it does at 7K?

Any info or suggestions? I will have a few months (DEC-Feb) to do maint on it, but I don't know if I need to if everyone has the same experience.

I'll post moreif I think of it, if you need info from me or if I have left out something crutial just ask.

Thanks again y'all







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I Think I've found out what I need to do...

I don't think it will be too hard to give the 900RR a tune up myself.All I need for special tools is a micrometer. a home made manometer, and an auxillery fuel supply because the tank will be off the bike.. I'l probably wait until the cold weather sets in before I tear my bike down...but I really want to be able to work on it myself instead of paying an arm and leg for someone else to do it for me, and then not even know if they did a good job.

Hopefully this is correct:

1- Check the timing
2 Check valve clearance
3 Adjust the valves that need it using new shims under the lifters
4 Recheck the valve clearance
5 Replace the spark plugs
6 Clean or replace air filter


This is the part I'm having trouble wrapping my mind around...

7 Sync carb #4 to match #3...makes 1st pair??
8 Sync Carb #1 to match #2...makes second pair???
9 Sync Pair two to pair one


A concern would be how to sync all four carbs with a manometer that only supports two carbs at a time!


Do I just keep one side hooked up to Cyl #3 through the whole procedure or do I sync pairs?

Is there even an adjustment screw that adjusts #1 and #2 as a unit to match #3 and #4?

 

Your 900 looks brand, spanking new!

I don't know what could be the problem, does it use carbs or fuel injection?

If it's carbs maybe its' a fuel issue. Nice bike!

 

Thanks Mt Wallet...It's carburated and my first sport bike. I've always wanted a sportbike after a few years of riding cruisers. Sorta late getting into it.

Yeah I consider myself lucky to have found the bike, after ten+ years of drooling over the 900s, Found it in Nashville for $4K cash. Hooray for tax returns eh?

I'm pretty sure it needs the carbs synced, after investigating further it shows all the symptoms of needing a tune up. It has been layed down, but for some reason (maybe color) it doesnt stick out at ya.

It had a busted D&D exhaust, It had a horrible high pitched buzz, had to wear earplugs it hurt so bad. I found a used Honda CBR muffler for ten buck on ebay, I don't think its the same year cause I had to drill a new hole in the mounting flange, but it works great and I'm not a fan of loud bikes anyway. Plus it looks great to me. I noticed a slight drop in power after I put it on but I'm used to it now and have had a good time getting there, had to adjust my style to get the front off the ground again.

I don't know if it's been rejetted. I do plan on going back to the 525 chain and steel sprocket with +2 on the rear, as its already -1 on the front sprocket.

Sooooooooo, I'm still stuck on this carb sync issue...

I'm pretty sure I will still need the 5mm adapters to hook vacuum up to the carbs?. I want to save money if I can, and learn this sh$t, planning on having another bike in the family by December, so I have a need to be self sufficient.

There are two types of home made manometers. One has just a tube. The other uses two bottles for air chambers that allow for safer operation.

I'm also worried about overheating, There's tell of using a fan to cool the engine during syncing, but wouldn't the fan and radiator work just fine?

Anyway...I'm pretty deep in this, and determined to figure it out.

My major questions are...do you sync carb pair 1/2 to pair 3/4 with a single adjustment screw, and where would I hook the vacuum hose to measure a "pair" of carbs relative to another "pair"

Anyway, I'm looking forward to digging in, but I cant bring myself to take her apart until the weather gets uncomfortable to ride again.

thanks again.

 

If you cannot find the answers here, since it sounds extremely technical...you might wanna try sportbikes.net in the "help me fix it" sub-forum.

They have actual motorcycle mechanics who respond.

Can anyone here help?

 

Thanks MT Wallet, I'll look into the sportbikes website. Still gathering lots of data. Next step is to find out if my local dealer supplies individual valve adjustment shims before I tear into the bike. I think I can get the whole job done in one days worth of work, providing I have:

manometer built beforehand,
a micrometer handy,
shims available down the street
anauxilery fuel supplyhandy
feeler gauges
new plugs on hand
5mm adapters for the vacuum
timing light


Darn!!! Maybe I should print this off so I don't forget anything...LOL

 

 

Just by a manometer from motion-pro for 50 bucks. It comes with the 5mm adapters that you will need. It's not worth the hassle to make your own when a tool is so affordable. Then just sync them all to carb #3 and you're all set. You don't even need an aux fuel supply. just undo the tank and raise it up with something. I did mine and it took all of about 15 minutes.

 

Hi Silver, nice looking bike. I have a 97 model with a 99 engine; yeah the engine do feel like sputtering when she runs in a low rpm but that is pretty normal unless you feel there is a bogging when you hit the throttle say when you are idling at 2 - 4k rpm, I will say your bike is performing neatly. I don't really suggest to sync the carbs if there is really no issue. Those 4 carbs can be sensitive.
If I'm not mistaken only the 96-97 model was with the 525 chain. The 520 conversion can be really nippy when pulling the bike but on safety aspect I don't really feel comfortable. Imagine with a huge powerband and you are using chain size actually made for smaller cc's bikes. Currently I'm running a 530 with stock in the front and +2 at the rear and she pulls like what so many people described here, raped ape!

What makes you to change your ride from a cruiser anyway?

 

Whats up stynkmonkey, I see your point, the unfortunate side is that I have way more time than money and I'm trying to save where I can, even if it is a little. On a positive note I have found a way to make one that simultaniously measures all four carbs, will cost $4 in parts and take 15 min to make. Your right about the tank too, but I plan on a valve adjustment prior to sync so it will already be off. I can afbricat the 5mm adapters in my woodshop using a drill press to make holes in M5-8tpi bolts from the local hardware store.I just enjoy a challenge in theshop. I'll let ya know how it goes and report the results...the pros/cons if you are interested.

Hi dinez74, thanks man, I definately feel a bogging when I get on it from 2-4K rpm.....and...It doesnt seem to want to start right back up when its warmed up and I stop at the store for a few minutes. Like it is flooded slightly, points to a sync issue from what I've read.

I don't feel comfortable either with the 520, or the aluminum sprocket. Yeah they are lighter, but shoot, I'll never, never, never be able to ride this monster to its fullest potential.

I readthe followingon Bikesetup.com and have to agree wholeheartedly, even if I am 100cc less than a liter......

Quote: Eddie Lawson, 4 time world 500cc Grand Prix champion, AMA superbike champion,2 time daytona winner, and suzuka 8 hour winner said "There may be someone out there that can ride a modern literbike to the limit on the street, but it isn't me!"


Therefore my interest wains when it comes to making my 900RR lighter and faster by 1/2 pound and 3 mph . But I do want the +2 on the rear for torque purposes, Seems like a good trade-off, and I think I've read somewhere that the -1 +2 almost completely fixes the speedo inacuracies on the 900RR.


So why did I change to the sport bikes from the cruisers????? IMHO I started riding the cruiser because I was conditioned to fear the sportbikes and the cruisers were for the more mature and "safer" riders.But I always wanted to have a sprotbike. I'm also more of a Lamborghini guy than a rolls royce guy. Deed down I prefer performance over comfort, and I have grown to think that a great number of men on cruisers act like they dont approve of the sportbikes simply because they are afraid of what their buddies would think or say. It's the classic bandwagon mentallity I had to fight within myself to make the change, and I couldn't be more snug and happy. I truelly believe that some of the guys on cruisers went that way because of peer pressure, or that was the easiest way to get the spouse to allow such a purchase. I could go on forever, but I just always new that I prefer performance over approval and image. Sometimes it just takes a while beforepeople allowthier outside self to be whothey really are on the inside. Thanks for asking.