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only 3 cylinders firing

  • Thread starter Thread starter Leadlegs
  • Start date Start date
L

Leadlegs

Guest
Hey all could really use some help.
my 1981 gs550l is only firing on 3 cylinders when riding. the #2 cylinder fires when idling but as soon as I start riding it stops. I noticed this happening off and on occasionally but now it's as soon as I go from idle
to riding. She starts up no problem and idles fine. I've also pulled that plug to check for spark and all was good I even tried a new plug which seemed to make the problem worse. I can still ride it however it definitely has a loss of hp and backfires when I let off the throttle.
Currently it has a half tank of gas with a whole bottle of sea foam run through it and sitting to see if that changes anything (only did seafoam because the guy at advanced auto said he used it to revive his old lawn equipment with it).
 
You may have to pull the carbs and see if the float needle is opening up all the way. You might check to make sure your no. 2 plug wire isn't arcing before pulling the carbs.
 
Any easy way to check for arching? Or freeing a stuck float without pulling the carbs?
thank you for the reply!
 
First of all, welcome to the site.

Usually, it's the other way around, a cylinder will not work at idle, but works well at higher throttle settings, which is a blocked pilot jet. If your case, it's more likely in the needle setup. Either way, your best bet will be a full carb rebuild.

It would also help a bit to ensure that you are numbering the cylinders correctly. They are numbered from left to right, as you are sitting on the bike. #1 is under your clutch hand, #4 is under your throttle hand.

If that is the way you are numbering it, there is another thing to inspect. The vacuum line that opens the petcock connects to carb #2. Check it for proper integrity and routing.

Although there is not much problem with Seafoam, I would really think twice about recommendations "because it worked on my lawn equipment". There are so many other considerations for a motorcycle application, mainly because of the wet clutch.

Here are a few links for you to start reading on your journey to get a really nice motorcycle:

Our Library, where you will find virtually anything you will need.

Carb Rebuild Guide, which details how to rebuild your carbs. If you choose to not do them yourelf, there are a few of us here that offer that service.

Valve Adjustment Procedure.

Service Manual

Last, but certainly not least, Top 10 Newbie Mistakes Read this list carefully, it hits every point RIGHT ON.

Also, pictures are MANDATORY. We threaten to withhold any tech help until we see pictures, so click HERE to see how to post pictures.

.
 
Thank you for the welcome and advice! If it is a bad needle set up do you think it would have been running fine for about a thousand or so miles and then all of sudden stop? Also could you direct me to the said members that would rebuild the carbs for me and the cost?
 
There are three that I know of. Two of them are in the GS members and services forum, I don't think the third one advertises.

Chuck Hahn (real name and username) does not advertise, but does good work. He is in Oklahoma.

Chef1366 (username) does good work. He is in southern California. Click HERE to see his thread offering his services.

Steve (yes, that's me) does good work. I am in oHIo. Click HERE to see my thread and what I have to offer.

Probably won't go wrong with any of us, but we all offer slightly different 'extras' for prices that are quite similar. Look over the threads, make your own decision.

.
 
That sir is some extremely nice looking work! Any idea what tools I will need to pull the carbs? What's the turn around time?
 
How do you know that it isn't firing when riding?

Before you take anything apart, pull the float bowl screw on the carb in question and see if a steady stream of gas comes out.

As for what tools you need: A screwdriver to loosen the clamps, and 10 and 12mm sockets/wrenches. Sidecovers off, remove the rear tank mount bolt and prop it up, remove vacuum and fuel lines, airbox bolts, loosen clamps, slide (wrestle) the airbox out, pull carbs. Take a bunch of pictures so you remember how all the hoses are routed.
 
When it's idling all four exhaust pipes get searing hot but when I get back from a short ride the number 2 line is cool enough to grab without melting your skin off. I'm going to pull the plug on float bowl #2. Should it drain empty and then stop or should the flow continue?
 
With petcock on `On' it should drain empty and stop. Once you do that, put the petcock on `Prime' and more fuel should start flowing. (This is all with the bike OFF).


Leadlegs said:
When it's idling all four exhaust pipes get searing hot but when I get back from a short ride the number 2 line is cool enough to grab without melting your skin off. I'm going to pull the plug on float bowl #2. Should it drain empty and then stop or should the flow continue?
Click to expand...

I'm not sure how to explain that maybe somebody else will chime in. My guess is a fuel thing and that at idle the carb. gets enough fuel to stay hot but when riding isn't getting a fast enough supply of gas and is also cooling down from the wind.
 
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Great thanks I'll let you know what I find. Any thoughts on blasting inside the bowl with carb cleaner while I have the plug off?
If there is fuel do you think it's a fuel air problem? Should I adjust the air fuel screw
 
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Leadlegs said:
Great thanks I'll let you know what I find. Any thoughts on blasting inside the bowl with carb cleaner while I have the plug off?
If there is fuel do you think it's a fuel air problem? Should I adjust the air fuel screw
Click to expand...


You can hit the bowl with carb cleaner if you like but it's definitely no replacement for a full carb clean. It will maybe clean the float/bowl but won't do anything for any of the internal passages which would likely already be clogged to some degree if there was any gunk in your bowl. I say leave the adjustment screws alone until you determine if that carb is getting fuel at all.
 
Take a close look at the #2 plug, plug cap (just go ahead and change them all with new NGKs caps) and plug wire. Something may be shorting out while you're moving.
 
So here's what I found. I didn't check the float bowl but it got me thinking enough to do some more digging in the forums. I just set the petcock and set it to prime and went for a short rainy ride. The ride wasn't great the throttle was jumpy but when I got back #2 exhaust was skin searing hot (glad I only touched my finger tips to it). I'm now thinking carbs are ok but must be the petcock or vacuum line? Any thoughts? I'm guessing the jumpy throttle is from all the dam sea foam I put in it.
 
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