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Carb issue! I'm going coo-coo!

  • Thread starter Thread starter nicholascott
  • Start date Start date
N

nicholascott

Guest
I cleaned the carbs on my GS850l and was riding it around. When I came up to a stop light my rpm's went up to 3. I turned down the idle screw to lower the rpm's and then the bike would stall. This happened a couple of times to me that day. I called a local motorcycle mechanic who said that I must have missed something when I cleaned the carbs out. He told me to clean through them one more time. Well, after cleaning them now for the second time I can guarantee that everything is as clean as it will ever be! After putting them on my bike the exact same thing happened. I started the bike up and was trying to adjust the idle when all of a suppen the throttle stuck again. Then just like before, when I turned out the idle screw the rpms went down and it wouldn't stick. It will ONLY stick when I turn the screw in. Someone told me it could be a faulty throttle cable. Maybe it's sticking. I really don't know what to check. Or, what it could be. Any help is always appreciated.
 
Check your throttle cable tension. If that's not it, you probably have an intake air leak and need new carb boots and o-rings.
 
To clarify, I'm guessing that you mean 3000 RPMs.

The infamous issue with these bikes is vacuum leaks. If you have them anywhere from the airbox to the engine, you could see symptoms such as you've described.

However, if you suspect the that the throttle valve or throttle cable is actually becoming stuck, check those things first. When you observe the run up on RPMs, have you verified that the throttle grip/cable/valve are all returning to "zero" as they should? If the grip or cable is stuck, it's probably as simple as lubricating the culprit. If the cable is adjusted with no slack, adjust in some slack (just enough to relax tension on the cable in idle position, or when "zeroed"). If the throttle valve itself is stuck, the indicate a mechanical issue with the carbs. Correct by cleaning. If that doesn't do it, then figure out what is mechanically sticking the throttle valve.

If all that pans out, then it's possible that you have a vacuum leak as mentioned at the top of this post. You'll want to inspect all connections from the air box up to the engine block. If there is anything that needs to be tightened, do so. If there's anything that doesn't seem to be getting a good seal, figure out why and clean, repair or replace the offending piece(s). Of special note are the o-rings that create the seal between the boots from the carbs to the engine block. These are notorious for becoming brittle and providing an imperfect seal. If they are supple and the rings still have a round profile, you MAY be OK, but if there's ANY question, replacing them could save you a lot of time and trouble.

On tip that many folks use to "find" leaks is to spray WD-40 on the various connections while the bike is running. The theory is that if there is a leak, when the WD-40 "fills" it the RPM's will fluctuate until the WD-40 is pulled through all the way. Bear in mind that if the leak is not significant enough or your not familiar with the process, you may not be able to detect the difference.

That's my best guess based on what you've posted.

Best of luck!
 
Wow! Thanks for the very detailed responses. I will try out checking for air leaks and go from there.
 
Does this happen when you turn the steering all the way one way or the other?
 
Carbs really aren't my area, but I can tell you that I ended up cleaning my carbs 5 times for a similar problem (minus the sticky cable) before I found this web site and read in one of the threads that if those boots and o-rings are more than 20 years old, I could clean and sync the carbs until doomsday and I still wouldn't get the bike to run right. Replaced the boots on both sides of the carbs and the o-rings, and it made all the difference.
 
If backing off the idle screw brings the revs back down then I doubt it's a stuck cable or too tight cable, doesn't make any sense as the idle screw is only a stop for the part that the cable pulls & would have zero effect if the cable was too tight or stuck.

Vacuum leak or maladjusted mixture screws?

Dan :)
 
Would the vacum leak make the bike rev high? And would it make the throttle stick like it is?
 
Last edited:
nicholascott said:
Would the vacum leak make the bike rev high? And would it make the throttle stick like it is?
Click to expand...

Vacuum leaks are the number one issue on these old GS engines. They will cause the idle to shoot up to about 3k rpm and make the idle inconsistent.

The carb boot O-rings are almost certain to leak if you have not replaced them. You should also replace the carb O-rings if you didn't do this already when you cleaned the carbs.

Check the following link for the parts you will need.
http://www.cycleorings.com/
 
nicholascott said:
Would the vacum leak make the bike rev high? And would it make the throttle stick like it is?
Click to expand...

Yes, and nobody's sure about your 'sticking throttle'. Odds are that you have an intake leak and/or badly unbalanced (= unsynchronized) carbs.

If your intake o-rings are older than 5 years, they need changing. If you have no idea how old they are, that explains a lot. You're pulling your hair over a $5 fix.

Fix the intakes and sync the carbs.
 
robertbarr said:
Yes, and nobody's sure about your 'sticking throttle'. Odds are that you have an intake leak and/or badly unbalanced (= unsynchronized) carbs.

If your intake o-rings are older than 5 years, they need changing. If you have no idea how old they are, that explains a lot. You're pulling your hair over a $5 fix.

Fix the intakes and sync the carbs.
Click to expand...

Robert, you are too modest. :oops: Please don't be afraid to post a link to your GREAT web page (which provides O-rings to the masses). If I had it my way, your page would be a sticky on the top of this forum. http://www.cycleorings.com/
 
I'm not familiar with the carbs on the 850, but my 750 has the Mikuni VM carbs. They have a rod that runs through the tops of them that the bell cranks on the top of each of the carbs screws into, and the lever that the throttle cable connects to also screws into. At one point, I hadn't tightened those screws down tight enough, and that caused the throttle to appear to stick, very similar to what you are describing. If the idle adjuster was backed off, there was a enough slack to positively close each carburetor. If the idle adjuster was turned in, the carbs wouldn't seat evenly or consistently, and the idle was all over the place. Not sure if your's has something similar. Mention this because you seemed to start experiencing the problem right after the cleaning. (I assume you dis-assembled the carbs and then re-synced after the cleaning.)
 
Success

Success

I took off my gas tank and sprayed around the carb boots with carb cleaner. Nothing happened so I decided to check my mixture screws. I took off the gas tank and checked the air mixture screws. They were almost all the way in! I turned them out a few turns, put the tank back on and started the bike. SUCCESS! I rode it around the block a few times and reved it at every stop sign. NO STUCK IDLE!! WOOO HOOO!! Well, with all your help we have solved another issue. Thank you everybody. As always, your help is always appreciated.
 
nicholascott said:
I turned them out a few turns, put the tank back on and started the bike. SUCCESS!
Click to expand...
Now that you have the bike running, get it running right. :shock:

Get the bike up to full operating temperature.
Take the tank off, but find a way to supply gas to the bike while it's off.
Turn the idle mixture screws in to seat lightly, counting the turns as you go.
Record the number of turns for each carb, set them back where they were.
Start the engine, adjust master idle screw for 1200 rpm.
Turn the idle mixture screw on carb #1 slowly in both directions, listening for highest rpm. Reset master to 1200 rpm.
Repeat on the other three carbs.
Turn the idle mixture screws in to seat lightly, counting the turns as you go.
Record the number of turns for each carb, set them back where they were.
Yes, you just counted the turns again, but this is the setting you want to remember for reference.


.
 
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