• Required reading for all forum users!!!

    Welcome!
    Register to access the full functionality of the GSResources forum. Until you register and activate your account you will not have full forum access, nor will you be able to post or reply to messages.

    A note to new registrants...
    All new forum registrations must be activated via email before you have full access to the forum.

    A Special Note about Email accounts!
    DO NOT SIGN UP USING hotmail, outlook, gmx, sbcglobal, att, bellsouth or email.com. They delete our forum signup emails.

    A note to old forum members...
    I receive numerous requests from people who can no longer log in because their accounts were deleted. As mentioned in the forum FAQ, user accounts are deleted if you haven't logged in for the past 6 months. If you can't log in, then create a new forum account. If you don't get an error message, then check your email account for an activation message. If you get a message stating that the email address is already in use, then your account still exists so follow the instructions in the forum FAQ for resetting your password.

    Have you forgotten your password or have a new email address? Then read the forum FAQ for details on how to reset it.

    Any email requests for "can't log in anymore" problems or "lost my password" problems will be deleted. Read the forum FAQ and follow the instructions there - that's what we have one for...

  • Returning Visitors

    If you are a returning visitor who never received your confirmation email, then odds are your email provider is blockinig emails from our server. The only thing that can be done to get around this is you will have to try creating another forum account using an email address from another domain.

    If you are a returning visitor to the forum and can't log in using your old forum name and password but used to be able to then chances are your account is deleted. Purges of the databases are done regularly. You will have to create a new forum account and you should be all set.

Sudden loss of #2 cylinder....puzzling

  • Thread starter Thread starter Anonymous
  • Start date Start date
A

Anonymous

Guest
Hello all,

Finally took the bike out for it's first little trip. Met some friends in Dubuque, IA and headed south out of town, eventually riding along the Mississippi for a while. She rode pretty good, tooling along at about 5,000 - 5,500 RPM for most of the time. Made it 41.1 miles out and she starts backfiring....popping through the carbs.....losing power.....not a good thing. Out of nowhere....flat grade at 5,000 rpm. Ending up trucking her back. The guys we were with thought it was a plug problem, but I though carbs, since they have given me the most trouble of anything on the bike. So, I took the carbs off the bike, dropped the float bowls, found no evidence of dirt or junk in them, and sprayed them down w/ carb cleaner.....sprayed through fuel passages, jets, so on and so forth. Anything I could hit with them still on the rack. Put them back together and back on the bike, fired her up, and she ran fine. Hmm. Ok.

So we head out into town that night. Made it about 10 miles or so, and it resumes doing the same thing it did before. Really pissed me off now. Left her in a grocery store parking lot til the next night. Came back that night, and just for the hell of it started it up before pursuing the possible dead plug theory, and lo and behold, it fired right up. Ok. Makes no sense. Rode it maybe 2-3 miles down the road to the local Wal-Mart to try and buy a plug just in case it resumed it's problem on the way home. Park it, shut down and run inside. We're in there for 5 mins at most, come back out, fire it up, and it immediately runs like crap. Argh!

Determined all the plugs were firing just fine, and after more deduction figured out that the number 2 cylinder is not getting gas. This was the same cylinder, if I remember correctly, that was not getting gas when I was prompted to completely disassemble and clean out the carbs last winter. It's been running absolutely fine since then. Until now.

My current theory is that there might be some sort of junk from the tank that is clogging some internal passage in the #2 carb. I don't think it's in the main fuel passage, as the number #1 carb is downstream and wouldn't get any go juice either. If there's crap floating in the tank, that might possibly explain why it will go 40 miles and lose cylinder #2, then after being cleaned out go another 10 miles before another piece gets stuck, and then after sitting all night run for a little bit before failing again. Why the #2 carb would be the victim every time is puzzling, since the blockage could just as easily go to the right side, and into #3.


If that's not the issue, then I don't understand why the carb would just all of a sudden fail like that. Just makes no sense. If anyone has any idea or suggestions I would be most appreciative.

One other quickie. It's a 77 GS750. What should she cruise at, roughly. I think at about 65-70 mph (speedo's a little off) it was running at roughly 5,000 rpm. Sometimes 5,500. Redline is 9,000. Is that about where it should be? Thanks for the help as always,

Ryan
mueller@xta.com
 
Are you sure it's out of gas on #2 when it screws up? Run it till it fails and then open the drain plug on #2 carb to see if you've got gas. You're right, it doesn't sound as if the fuel lines could be bad or you would lose #1 also. Have you checked the float level and needle valve for that carb? Just thoughts.
 
A key indicator is that it was running fine when you pulled into wall*mart but when you came out it ran very poorly, that normaly doesnt indicate a clogged carburator but a loss of ignition fire.
what did, or does the number 2 plug look like?
if it is sooty black or normal greyish brown, you are getting fuel.
try swapping plug wires between #2 and #3 and see if the problem moves to the number 3 cylinder.
or with it running (but not riding down the road) take a stick and wiggle the plugwire and tap at the plug boot.
 
Since a single line feeds fuel to all four carbs, I dont think the problem is the fuel line. The next time it happens, I would shut it down and open the drain plug on the number two carb bowl to see if there is fuel in the bowl. There is a difference between fuel being available and it not drawing fuel. Since the bike runs fine for a short time, then acts up, then runs fine again, I do not think it is a clogged carb jet problem. If it was a clogging problem, it would likely stay clogged rather than return to running fine. If when it acts up, you check and find no fuel in the carb bowl, then it has to be the float sticking and not allowing the bowl to fill, If there is fuel, I think it is a vacuum problem. I would replace the "O" ring on the intake carb boot where it mates to the cylinderhead. The problem appears to be heat controlled. You run the bike for 40 miles, it gets hot.........intake leak develops, you loose intake vacuum and consequently fuel draw. Bike pops and misfires signifying running lean. Let bike cool down and everything is once again ok.

The good news is if it is the "O" ring, they only cost about a buck fifty. :-)

Earl
 
Well, I'm pretty sure it's not the ignition. We chased that devil for a while in the Wal-Mart lot. We determined that 1, 3, and 4 were firing, and running just fine. You could pull the plug wire off #2 spark plug and it wouldn't change how the bike ran. Pull the plug and it looked dry, and more whitish/grey than it ought to be, as if running lean. Lay the plug on the head and crank it over and it fires just fine. Gapped properly and everything. Swap it with a known good plug and the problem remains. Swap the wire from the #4 cylinder and it doesn't change anything (when swapping on this bike you have to go 1 to 3, or 2 to 4 otherwise it fires at the wrong time. One coil fires 1+3, the other fires 2+4, but I digress). So it gets spark, and enough of it, at the appropriate time.

As far as the induction o-rings, I replaced those about 2 months ago, and it only maybe has 1 hour total run time since. Those should be just fine. I inspected the induction boots, and although there was some cracking on 1 or 2, it was all external in nature, and not deep at all. Those should be ok as well. I could see the intake leak being feasible the first time it happened, after getting plenty warm. But the second time we only went 10 miles at most, at night as opposed to 12 noon, so it's less run time, cooler ambient air temp, etc. It should take even longer than 40 or so miles to cause it to happen again. The third time we went 2 or 3 miles, and then it sat for a bit before exhibiting the problem. When I loaded it up 2 days later to haul it back home I started it up, and it ran poorly immediately.

I will check the sticking float idea tomorrow, and will let you know what I find out. Thank you very much,

Ryan
mueller@xta.com
 
NO, the left coil must fire cylinders 1 and 4 through the left pointset (located at 9 oclock on the breaker plate). The right coil fires 2 and 3 through the right pointset located at the 3 oclock position. All GS750 ignitions are this way.

Earl

mueller said:
One coil fires 1+3, the other fires 2+4, but I digress). So it gets spark, and enough of it, at the appropriate time.
Click to expand...
 
Earl is right. 1 & 4 and 2 &3 are the cylinder pairs off the coils
 
It also might be worth your while to do a compression check. I know that this is not as good as a leak down check but it will quickly tell you if you have a burned piston (from running lean) or and valve problem.

Mike
 
You must log in or register to reply here.
Back
Top