Suzuki GS550T Running on 3 Cylinders
Hi all, I purchased a 1981ish Suzuki GS550T about 9 months ago, partly to learn to ride and partly to learn to work on a bike. I successfully used it to learn to ride and get my licence. I’ve also done some minor cosmetic changes and repairs since I bought it, but I noticed that at some point over the last few months it has began running on only 3 cylinders. I’m not sure when exactly this issue began, but I noticed a degradation in the bike’s performance over a bit of time, and a couple of backfires, sure enough the header coming out of Cylinder 2 barely heats up and there’s no change in the idle when I remove the plug cap.
I have checked compression across the cylinders, they’re all the same. I have checked spark in cylinder 2, it appears normal. I have cleaned out the carburettors, this had no effect. I have swapped in new spark plugs, this had no effect. The old spark plug in cylinder 2 was very black when removed. After putting in a new spark plug, running the bike for 10 minutes, and removing the plug from cylinder 2 again, it was already starting to blacken.
Hoping to get a bit of advice as to what the most likely cause might be? Seems to me like the cylinder is likely getting too much fuel, but I’ve no clue what’s causing it, I’d appreciate any advice.