Friends bike needs mechanics suggestions

I am working on a friends bike an 1980 Suzuki GL 550 I think (rode bike triple) that he had $1200 of work done to it and rode it 6 times and parked it because of this problem. The work he had done was rebuilt the carbs and other things.He has been really good about draining the gas and running it empty ect.(Since thats why the carb rebuild in the first place) The problem is it runs well/nice at half thottle and above but real crappy below half thottle. It is real dead, hesitant and sloggish and barely runs and dies when it drops to an idle. I was thinking carbs but the spark seems weak on on cylinders 2 and 3 witch are on the same coil. It has 2 coils. When I pull the number 2 or 3 plug wire while running at an high idle it makes no difference, if I pull a 1 or 4 it dies. Question #1 Can a coil put out low spark at an idle and work fine in higher rpms? #2 Is there a way to test the coil? #3 Should we buy a new coil? #4 Could it be the voltage going to the coil? #5What else do you guys think it is? I cleaned all the wire connections to the coil. By the way all the plugs look nice after running it at half thottle or above for 5 minutes but dont seem to be loading up at the idle even though I turned the idle way up to keep it from staling. Im stumped and about to find him a new coil.

Thanks a lot for your help.


Is there somthing I can test before the coil?

I worked on one of those for a friend, nice bike with the exact same problem. I changed out the coils and it was a much happier bike.

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