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The specific problem and solution I discuss in this post pertains to CV carbs on a 1980 GS1000G, but I believe it is potentially a universal problem for all GS bikes with VM or CV carbs. In hindsight, the problem was ridiculously simple to evaluate and resolve, if I only knew what to look for. The replacement part costs only 98 cents on Bike Bandit!!! That?s why I?ve posted this, to save some other poor bastard from the frustration that I have gone through.
I have noted that a few GSers have posted that they have been frustrated trying to resolve a problem, the symptoms of which are that one cylinder runs rich as indicated by the spark plug turning dark brown, then black (dry and sooty, not wet and oily) and perhaps eventually fouling out. GSers have adjusted mixture screws, pulled and cleaned carbs, checked/adjusted float levels, changed float valves and seats, etc. without solving the problem.
I too have had this problem, and it has been driving me crazy for a few months. I have had the carbs off my bike three times in the last three months, and have carefully worked on them myself and also worked with mechanically-experienced friends with GS bikes. I have partially disassembled the carbs, dipped all the brass parts and the bowls in carb dip overnight, ran guitar strings through all the tiny holes in the emulsion tubes, etc., shot aerosol carb cleaner and compressed air though all the holes big and small, cleaned the inside of the bowls almost to a shine, bought and installed new valve seats and needles, bowl gaskets, slide springs, checked float levels, etc. Then, I put the whole shebang back together, synchronized with a Morgan Carb Tune, screwed around with the mixture screws, and still the #*#*ing number 4 plug was getting dark and eventually fouling out. It was fouling with the #4 mixture screw turned all the way in. I was so frustrated, I felt like jumping off a cliff!
To make a long story short, it turns out that my problem was in carb #4 and was simply a slightly bent float pin that holds the float bowl assembly in place between the two turrets or towers. This slightly bent spring increased the friction on the float operation. However, the problem was so slight that when I held the carbs in my hands and turned the carbs upside down and right side up during my inspections and repairs, the momentum of turning the carbs over was enough to overcome the slightly increased friction and the floats on all four carbs appeared to operate properly. However, I have concluded that the slight increased friction was just enough to hang the #4 float bowl open just a bit when the bike was underway, either never letting the valve needle seat all the way closed or was hanging the valve needle open often enough to let too much fuel too often though the float valve, and thereby run the cylinder rich and foul the plug.
I learned a trick to replace float pins. First, assuming you?re doing carb work and the carbs are off the bike, inspect the float bowls and manipulate them with slight finger pressure and repeatedly turn the carbs right side up and upside down and each time sight along the line of the four float bowl assemblies, looking from the left or right sides of the carb bank . They should all move at the same time and all line up the same, when full open and full closed. Check the float height with the carbs upside down and just resting in place by gravity alone. If you suspect a bent or otherwise compromised float pin, just buy a new one, it?ll cost about a buck. DO NOT attempt to drive float pins in or out with an impact approach, such as using a small awl and hammer. Rather, take a pair of diagonal side cutter pliers, and carefully place one half behind the tower and the other half with the point of the end centered on the narrow end of the pin. Carefully close the pliers and drive the pin about 1/16-inch with gentle pressure, rather than brute force, and then carefully pull it out from the other side of the other tower. Press the new pin in using the same technique, or perhaps using the flat edge of a screwdriver. Again, DO NOT use an impact (tapping) approach to install the new pins.
After the pin replacement, I put the carbs back on the bike, adjusted the mixture screws, and now I've got four nice tan plugs! I'll get around to checking carb synch, next time I meet with my friend with the Morgan Carb Tune.
So anyway, that?s my fix-it story, and I hope I helped somebody else on the GSR.
I have noted that a few GSers have posted that they have been frustrated trying to resolve a problem, the symptoms of which are that one cylinder runs rich as indicated by the spark plug turning dark brown, then black (dry and sooty, not wet and oily) and perhaps eventually fouling out. GSers have adjusted mixture screws, pulled and cleaned carbs, checked/adjusted float levels, changed float valves and seats, etc. without solving the problem.
I too have had this problem, and it has been driving me crazy for a few months. I have had the carbs off my bike three times in the last three months, and have carefully worked on them myself and also worked with mechanically-experienced friends with GS bikes. I have partially disassembled the carbs, dipped all the brass parts and the bowls in carb dip overnight, ran guitar strings through all the tiny holes in the emulsion tubes, etc., shot aerosol carb cleaner and compressed air though all the holes big and small, cleaned the inside of the bowls almost to a shine, bought and installed new valve seats and needles, bowl gaskets, slide springs, checked float levels, etc. Then, I put the whole shebang back together, synchronized with a Morgan Carb Tune, screwed around with the mixture screws, and still the #*#*ing number 4 plug was getting dark and eventually fouling out. It was fouling with the #4 mixture screw turned all the way in. I was so frustrated, I felt like jumping off a cliff!
To make a long story short, it turns out that my problem was in carb #4 and was simply a slightly bent float pin that holds the float bowl assembly in place between the two turrets or towers. This slightly bent spring increased the friction on the float operation. However, the problem was so slight that when I held the carbs in my hands and turned the carbs upside down and right side up during my inspections and repairs, the momentum of turning the carbs over was enough to overcome the slightly increased friction and the floats on all four carbs appeared to operate properly. However, I have concluded that the slight increased friction was just enough to hang the #4 float bowl open just a bit when the bike was underway, either never letting the valve needle seat all the way closed or was hanging the valve needle open often enough to let too much fuel too often though the float valve, and thereby run the cylinder rich and foul the plug.
I learned a trick to replace float pins. First, assuming you?re doing carb work and the carbs are off the bike, inspect the float bowls and manipulate them with slight finger pressure and repeatedly turn the carbs right side up and upside down and each time sight along the line of the four float bowl assemblies, looking from the left or right sides of the carb bank . They should all move at the same time and all line up the same, when full open and full closed. Check the float height with the carbs upside down and just resting in place by gravity alone. If you suspect a bent or otherwise compromised float pin, just buy a new one, it?ll cost about a buck. DO NOT attempt to drive float pins in or out with an impact approach, such as using a small awl and hammer. Rather, take a pair of diagonal side cutter pliers, and carefully place one half behind the tower and the other half with the point of the end centered on the narrow end of the pin. Carefully close the pliers and drive the pin about 1/16-inch with gentle pressure, rather than brute force, and then carefully pull it out from the other side of the other tower. Press the new pin in using the same technique, or perhaps using the flat edge of a screwdriver. Again, DO NOT use an impact (tapping) approach to install the new pins.
After the pin replacement, I put the carbs back on the bike, adjusted the mixture screws, and now I've got four nice tan plugs! I'll get around to checking carb synch, next time I meet with my friend with the Morgan Carb Tune.
So anyway, that?s my fix-it story, and I hope I helped somebody else on the GSR.
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