New bad behaviour - misfiring, bogging down and only firing on one cylinder...

[Toado]

Hi all, sorry for the long post and potentially confusing descriptions but riding to work yesterday my bike was performing well but on the way home it started playing up - hesitating then surging and finally the revs starting to drop to about 500 or so when I was stationary and it stalled a couple of times at traffic lights although it always started on the button. I figured out that it was only firing on the right hand cylinder (removing the left hand ht lead made no difference) but I am not sure why this has happened now. I started it up this afternoon and the idle was unsteady again - ranging from normal to low. On the sidestand, when I held the throttle steadily open at revs of about 1.5k it would intermitently seem to misfire, which I could feel almost like a hiccup or catch through the throttle and then sometimes the revs would rise, which felt like it had been held back from the level it should have been, before dropping again. Firstly, can anyone guage if this is likely to be electrical or fuel related? Secondly, any pointers about how to approach fixing it? Thanks in advance! 

[Joolstacho]

First some simple easy things:
Empty the tank and put in fresh fuel
Clean fuel tap filter
Run it with the petrol cap off to see if the hesitation goes (ensure the cap can breathe)
Is it worse at higher revs?
Experiment with the choke, if it's a fuelling (lean/starvation) issue it may run better with a little choke held on
(Remembering that most fuel problems tend to be electrical issues) Thats called irony!

[Toado]

Thanks, Joolstacho, I don't think it's an issue with the tank though - the inside of the tank looks spotless - so replacing the petrol is unlikely to help. Would a fuel/carb issue cause it to run on one cylinder and make it misfire or could it be another part of the electronics breaking down?

[Joolstacho]

Certainly a fuel starvation issue could cause the symptoms you're describing.

[herennow]

Check for a loose vacuum hose or vacuum plug, but that would normally cause revs to go up...

[Bluesmudge]

When you pulled the plug, did you check for spark by grounding it to the engine? You need to do that to determine if it's an electrical or fuel issue. Could be clogged carb or a bad coil or something else, no way to know until you narrow it down.

[Toado]

Thank you all for your suggestions! Bluesmudge, I will attempt to check the spark on the left cylinder's plug to see if that helps me locate the issue. If the spark is a good one then I suppose my next step would be to check the fuel, which would mean cleaning out the left hand float bowl on the carbs.

[Bluesmudge]

Yeah, if you have a good strong spark, I would start by opening the float bowl drain screw for that carb and see if gas comes out to confirm that fuel is getting into the carb at all. If it passes that test then you can start breaking down the carb for cleaning.

[Toado]

Bluesmudge, I drained the float on the left hand carb, the one that was not firing last week, and there were black floating particles in it. I ran the bike for a while and drained it again with the same result but I am unable to see anything floating in the petrol tank. Could the particles be an indication that the fuel lines are breaking down and need replacing? I am otherwise not sure how they are getting into the system but I have never had a fuel line in any vehicle break down like this - or not that I have been aware of, anyway...

[Bluesmudge]

Without seeing the particles its hard to know. Does/did your bike spend a lot of time sitting? The float bowl is past the screen in the tank and in float valve, so it would be hard for large particles can get down in there. Most likely its gasoline that's been sitting there a long time and turning to varnish and sludge.

Either way, it sounds like its not a spark issue, its a fuel issue. Those black particles have clogged the carb jets. I would take the carbs apart to clean them and refresh all rubber parts. While you have the carbs off, might as well replace the fuel and vacuum lines and the carb to intake manifold boots/o-rings. Better to just get it all back to stock than try and pinpoint the exact problem, or you will spend forever taking the carbs on and off and ordering parts one at a time.

[Toado]

Just to finish this thread off with an update - I swapped all the fuel lines with a new set purchased from GS500 world, which came complete with clips and fuel filter and fitted with no issues. I also fitted a new coil, which may not have been necessary but I thought that I might as well as I had purchased it anyway, and also took the opportunity to replace the valve cover gasket, which was weeping a bit of oil, while I had the tank off. I'm not sure whether it was just replacing the lines or if the coil was failing but the bike performed very well on the test ride this afternoon. Thank you to all that responded to my question!

[herennow]