Running on three cylinders..carb related?

[Guest]

So I changed my intake O-rings, swapped out the melted airbox boots and put new clamps. It was like a new bike. It was running awful before which I suspect was from the melted boots.
After a week or so of running great, now, its really hard to get started. Lots of sputtering and backfiring. I got it running and warmed up and it runs semi-decent. Got home, and one pipe is not hot. Can this be dirty carbs? I have yet to strip them down and change the O-rings. It was running great so I thought I could wait until winter.
Can dirty carbs cause intermittent problems? Only three cylinders? Or should I look into the ignition?
Thanks in advance.

[Eli69]

you have to look into all of those things and maybe more. sometimes it's hard to rule out any one symptom. here's a list of things that i've had happen that caused running on less than four cylinders:

- gunk from gas tank stopped up a carb.
- bad plug.
- bad spark. (bad wire, wet wire, bad coil, bad points or ignitor problem)
- intake air leaks.
- carb flooding out.
- combination of above and other unknowns.

a complete carb dip and cleaning will at least lower the percentage of bad carb possibilities. i use inline filters on all my bikes too.

read your plugs, check compression, check spark.

[capnjim]

Yeah, I guess it could be a lot of things. What a bummer, just when I thought I had it running well. Oh well..back to the drawing board. Actually, when I drove it for the first ten minutes it was only working on 2 cylinders. I hope its just gunk in the carbs. I'll do the dip this weekend.

[tom203]

What bike model/year? which cylinder is not getting hot? (#1 is under clutch lever)

[capnjim]

Sorry..its a '78 GS750. The two carbs under the throttle (Right side), then it sputtered and popped and one came back. Now just the far right is dead. I have been using seafoam in the tank, I guess it could have dislodged some crap and now she's jammed.

[tom203]

78 GS750... (put in your signature); If it has stock petcock setup, it has petcock vacuum hose connected to #3 carb- if petcocks fails,gas can leak down this and foul #3 plug. Time to check petcock operation. Put clear tubing on petcock vac hose fitting and suck on it- any gas headed your way?

[capnjim]

If it was a petcock issue, how would it affect only 2 cylinders? I did try running it on prime and there was no difference.
How do I put stuff in my signature? I figured out my location.
Thanks

[tom203]

If it was a petcock issue, how would it affect only 2 cylinders? I did try running it on prime and there was no difference.
How do I put stuff in my signature? I figured out my location.
Thanks

edit your signature in user profile
If fuel is running down vacuum line, it will foul plug #3,even if you are in "prime" spot - might also interfer with #2 since they are on same coil. try the clear tubing test. You could just block off vac fitting on carb #3 and see if it now gets hot.

[capnjim]

I edited my signature...added my bike.

[chef1366]

Check the plug and see if it is fouled.
I'd suspect the points.

[capnjim]

Points can cause intermittent problems? What do I look for?
Thanks

[RobG1970]

If it was a petcock issue, how would it affect only 2 cylinders? I did try running it on prime and there was no difference.
How do I put stuff in my signature? I figured out my location.
Thanks

I it the original >?? vacuum or manual petcock>??

[capnjim]

Its the original vacuum petcock.

[RobG1970]

Its the original vacuum petcock.

check the petcock Diaphragm for a hole or even a pinhole. it will starve #2 and also dump gas through the vacuum line which will also fill your cylinder and then possibly your engine block… you can also spray water on your hot exhaustthe petcock for a hole or even a pinhole, it will starve #2 and also dump gas through the vacuum line which will also fill your cylinder pipe to confirm the cylinder is not firing. If the water steams or rolls off, it working, if it just sits there, it’s not. Also maybe cut back and reinstate you your ignition wire plug caps, you might have some old dirty wires. Have you check the plugs yet>?? Is it wet>?

[azr]

Points can cause intermittent problems? What do I look for?
Thanks

If one of your points has failed then you will not get spark to those two cylinders. Remember, 1 & 4 fire from one point and coil and 2 & 3 from the other. So if you were having problems with 3 and 4 then that doesn't sound like a points issue, necessarily. You can check how things are firing by simply pulling each plug one by one and checking the spark, good spark, ignition system is working. Now could it be something plugging up your carbs? for sure. If you haven't dipped your carbs yet, do. If you were planning on doing it anyways then now's the time. Make sure you put a wire down all the tinny holes, I use a guitar string myself. There's great tutorials on Cliff's web site. You can test your petcock by simply apply vacuum to the vacuum port. Fuel should only flow when vacuum is applied, if it flows any other time your petcock is gone. If your petcock is malfunctioning it usually #3 first, making it rich, hard to fire. Recently my #3 wasn't firing. Checked many, many things. Pulled the carbs out 4 times, dipping them once during all that and still the cylinder would not fire. Redid my valve shims, some were out of spec, and I got it to fire but now well. The thing that seemed to help best was fiddling with the fuel and air mixtures on the carb, go figure.