0
2007 GS500F. 15k miles. Two cylinder inline air cooled engine. Purchased last winter so I don't know much history on it.
Over the winter I rebuilt carbs and checked valve clearance. Some were on the low side (small gap) but all were within tolerance.
My son got his cycle license a week ago and has been riding it non-stop for a week. He has put 500 miles on it. It was running fine until yesterday, then it started running poorly.
Question 1 is "How much power should I expect from this bike ?" I am an average size guy. When the bike was running well, it had little power. I assumed this was normal, but maybe not. On the highway it was a struggle to keep it at 65 mph. When trying to go up a hill on the highway, it would slow down to about 50mph. I would downshift and try to rev the crap out of the bike, but still couldn't get more than 50 on a hill. It seemed to run fine, just no power. Not enough power to get the RPMs up high enough under load, but on a downhill it would rev up fine. Is this typical for a GS500 ?
Now for the new problem : yesterday the bike started running poorly. It idles fine but when you try to rev it up it backfires and stumbles. I quickly discovered that the issue is with the right side cylinder. The bike runs the same with the right side cylinder spark plug connected or disconnected, so the right side cylinder is not doing much. With the left side spark plug disconnected, the bike barely runs, but does run very poorly. So it seems the left cylinder is strong, and the right is very weak, but not totally dead. It will run on just the right, but just barely.
I pulled the spark plug and the spark appears ok.
Using the unofficial thumb test, compression seems similar on both sides. (strong on both cylinders)
I pulled the carbs again last night. Very clean. I removed the jets and they looked fine. I re-cleaned them. Floats look ok. Float level ok. Float needle valve shows a very small groove visible with magnifying glass, but I would guess it is a normal amount of groove and probably not the problem. (?)
I removed the rubber manifold ring that connects the carbs to the cylinders. They are fine. No cracks or leaks.
There is fuel getting to the right carb. I can loosen the carb bowl drain screw and get gas. Since the left cylinder runs good, I have ruled out the infamous fuel petcock issue. The same line feeds both left and right carbs.
I don't think it is anything major like a bad valve or piston issue. It's too inconsistent. Seems more like an issue with fuel or spark.
Tonight's plan: I plan to try swapping ignition coils to see if the problem follows the coil. (weak coil ?). But the spark seems ok to me, so I keep going back to the carbs. But I have had the carbs apart twice and they are really clean and the jets are good. No blockage. So, I also wonder if the fact the bike never had power is a clue. Maybe it is a major mechanical issue I am overlooking. My last resort will be to remove the valve cover again and check valve clearances again, but I don't think that is the problem.