Hello everyone,

I’ve been in a huge process to bring my recently acquired 98 CBR 600 Smokin Joe back to life if you will. Long story short, the bike did / does run fine, however, since it had sat for 2+ years the o-rings in the fuel system at the carbs were shot and causing gas to leak. Well, I took apart pretty much everything including the carbs. I replaced the o-rings and cleaned the carbs up, however, the synchronization got all screwed up (one of the springs popped off and that’s where it started going south). Well, I got the springs back on and carb installed and was about to sync the carbs but the idle is very HIGH, like 4000-5000 rpm high. I thought it was a vacuum leak but haven’t found anything. I did notice that if I force the throttle back I can get the bike to idle down, however, I have adjusted the cables at the carbs with the help of a fellow biker to no resolution.

I noticed on the carbs that the Throttle Stop Screw does not make contract with the throttle linkage on the carbs without a few turns in. In other words, if I were to remove the throttle stop screw completely the throttle linkage would not rest on the plate that the throttle stop screw screws into. Wow, that was a mouthful. Also, looking into the carbs at the butterflies I can see 3 little holes in the carbs. Right now, the butterflies are adjusted that you cannot see those holes without opening up the throttle a little bit. Is this a good place to start?

From my research on the site using the “search” button I have come up with some likely causes of this problem.

1. Throttle position sensor is whacked – It’s been suggested in this thread that if I tap the TPS sensor a little bit in one way or the other, I can adjust the idle speed. It’s possible that the TPS was banged around a bit when my buddy was giving the carb a slight wack with a rubber mallet to get the carbs back onto the Throttle body insulators. I suspect that this is my most likely culprit and can’t wait to get home and try it (I work the overnight shifts.. )

2. Throttle / chole cables are wrong – When I originally took off the throttle cables in order to replace the o-rings and clean her up, I paid close attention to how they were mounted and which went were, however, during some troubleshoot my buddy removed the cables without detailing their locations. We hooked them up one way and the throttle was reversed, cool but not cool . I then reversed the cables to match what was still vaguely in my memory. Now my throttle works in the correct direction but I just wanted to confirm that there are only two ways these cables can go on and I’ve tried them both? I would hate to think that while the cables operate in the correct direction, they could still be on wrong. Any insight? Is the choke working correctly? I believe so. At least on my bike, the choke doesn’t operate until about the 75% mark of the possible travel of the choke lever. It looks as if the choke is fully disengaged and I can’t force it back anymore with my fingers.

3. Throttle cables / Throttle snap back – The cable function of the throttle seems to work pretty decently. Before removing everything the bike idled fairly well even though I was only able to run it 15 seconds or due to it pouring fuel through the bad o-rings. The throttle snap back is a different story, and while I believe I have mastered it’s adjustments I am a little concerned about its correct functioning. Like I said earlier, I am able to force the throttle back a wee bit and drop the throttle, however, it requires enough back force for me to question if this is a problem or not. I really don’t think the design was to require 4ft/lbs of throttle snap back, let me know if I’m wrong. 

Well, I guess I’ll sit back and wait for valuable responses to this thread in hopes to finally one day being able to RIDE her as hard and I’ve worked on her!

Thanks in advance!

 

Sounds more like the springs got installed wrong. There not putting the correct pressure to close the butterflies. I always hated breaking the carbs apart for that reason. Its easy to get it just slightly wrong and it wont act right.

 

TPS is a sensor that is used onFuel injected motorcylces. Your bike being carbed, it should not have one.

You should never have to force the throttle back. I believe you do not have the cables adjusted properly. You should have slack in the cable. You should be able to turn your throttle a little without it engaging the cables, this is the freeplay that the throttle should have. If there is none, then you need to adjust more slack into the cables. If there is slack in the cables, then something else is wrong somewhere else. You may just have one carb that is adjusted out of sync with the others and it will cause all sorts of issues until you get all carbs the same. It is possible that you have them all adjusted to far.

The cables are a push pull type and they can be installed backwards. If it is working, then its installed correctly. Take a close look at the mounting points for your cables and make sure they are seated properly and at the proper location. Not seated properly can lead to a throttle that will not return fully.

Your carb sync will also include syncing the air fuel mixture of each carb, not just the getting them all the same throttle position. You will need a vaccum gauge to mount to all 4 carbs to get it correct.

Ive done this in the past and it works ok. Im sure there is a better way to do it but here goes, take a small drill bit, Twist your throttle enough so the drill bit ( unfluted side)goes just under the butterly of the carb of the number one carb.Lock your throttle in that position. You can lock it by using the cables or just laying something between the throttle stop to keep the carbs locked. Once locked, re check to make sure it hasnt moved. If it hasnt, adjust all of the others to the same height. this should sync the throttle position fairly close. Again, its not perfect, but it does work to get them close.

good luck, if I can think of anything else, I'll let ya know.

 

Thanks for all of your helpful input! I am confused a bit about the "carbureators don't have TPS" comments, as you can see below.



I have removed the carbs, so I will take everyone's suggestions and just go through the damn thing AGAIN! Nothing like being 100% sure!

Thanks in advance for any other insight!

 

Hmmm. I guess I was mistaken. I really didnt think they had TPS sensors.

Sorry about that.

 

It isnt fuel injected but it does have electronic timing advance. That's what the TPS is for.

 

some carbed bikes DO have a TPS. chances are since you were messing with the carb synch, you've got a throttle valve out of whack and you need to synchronize your carburetors properly before it'll EVER idle right again.

 

Well, I looks like it may have been a combination of two things. After removing the carbs, I adjusted the sync screws so that all the butterflies completely covered the 3 pin holes down the throttle bodies by about the same amount. I also twisted the TPS sensor by hand and it definitey could be moved. When I reinstalled, the bike idled high still, but not as bad as before. I hand twisted the TPS again and bam! that did it! I then threw the carb sync on and got everything nice and clean. Today was her maiden voyage and she was a dream!

Thanks for all of your helpful comments and tips, I couldn't have done it without you!

After I finish getting her just like I want her, I'll post of up some pics and maybe some video.

I just got home from a 20 minute ride and I'm thinking I need to hit up Ace hardware for some things (and I've got a grin from ear to ear)