pls help.1991 gs500  my bike has just started running on and off the left cylinder. there is times when its just been started its fine for about the first 10min then starts going off a cylinder. then other times when started its off a cylinder straight away. ive changed the carbs and set them with a balancer. changed the pickups, cdi, reg/rec,coils,new plug and caps put a direct feed to the carbs from the tank and blocked off vacuum port on carb,removed the tank cap so no tank vacuum. compression tested cylinders and both are same. any ideas pls helpppppppppp.

Did you check the float height.

http://www.gstwin.com/float_height_check.htm

hi sorry i left that out. yeah i have and its spot on. mores replies pls

I think there's a ground(secondary ground?) that can cause the problem you're talking about. Someone else should know the details.

Quote from: mbbikes2005 on July 30, 2006, 02:39:29 PM
pls help.1991 gs500  my bike has just started running on and off the left cylinder. there is times when its just been started its fine for about the first 10min then starts going off a cylinder. then other times when started its off a cylinder straight away. ive changed the carbs and set them with a balancer. changed the pickups, cdi, reg/rec,coils,new plug and caps put a direct feed to the carbs from the tank and blocked off vacuum port on carb,removed the tank cap so no tank vacuum. compression tested cylinders and both are same. any ideas pls helpppppppppp.

I had the exact same problem on the 02 GS earlier this year.  I have a very damp garage and moisture gathers everywhere at times during rainy periods.

It was same #1 cyl and I changed the plug and fixed it briefly and then it came right back.  It turned out to be a loose connection shorted out by the dampness where the spark plug wire went into the coil.  I just turned the plug wires to tighten them on the screw posts and it's been fine for thousands of miles since thru rain and shine.

The parts fiches show a boot covering this connection but there is no such boots on the 02, don't know if they're on older models.  Gotta take a look at the 97 one of these days.   

Quote from: runsilent on July 31, 2006, 07:21:48 AM

Quote from: mbbikes2005 on July 30, 2006, 02:39:29 PM
pls help.1991 gs500  my bike has just started running on and off the left cylinder. there is times when its just been started its fine for about the first 10min then starts going off a cylinder. then other times when started its off a cylinder straight away. ive changed the carbs and set them with a balancer. changed the pickups, cdi, reg/rec,coils,new plug and caps put a direct feed to the carbs from the tank and blocked off vacuum port on carb,removed the tank cap so no tank vacuum. compression tested cylinders and both are same. any ideas pls helpppppppppp.

I had the exact same problem on the 02 GS earlier this year.  I have a very damp garage and moisture gathers everywhere at times during rainy periods.

It was same #1 cyl and I changed the plug and fixed it briefly and then it came right back.  It turned out to be a loose connection shorted out by the dampness where the spark plug wire went into the coil.  I just turned the plug wires to tighten them on the screw posts and it's been fine for thousands of miles since thru rain and shine.

The parts fiches show a boot covering this connection but there is no such boots on the 02, don't know if they're on older models.  Gotta take a look at the 97 one of these days.   

That's a good point.  You might also try dielectric grease to prevent moisture from getting inside the boot.

thats a definite coil problem, id say either the coil is stuffed and when it gets hot it stuffs up(quite common) or that the connections are corroided, (also quitre common) swap the coils from the left to the right side, and visa versa, clean all the contact points and put some grease on them to stop waster(by grease i mean petroleum jelly, vasilene) like what you would do to your battery.
if the problem shifts sides then chances are you need a new coil, if it still on the left side then its the wiring from the cdi
good luck

I agree with the above posts, it's most likely an ignition problem. In addition to the things sugested above, make sure that connection where the plug wire goes into the spark plug cap is good too. I believe that the spark plug caps are resistor caps too, so check the resistance to make sure that they are good.

hi. checked all thats been suggested but no faults. took the cam cover off last nite and checked clearences. both inlets were .04 so they were fine but both exhausts were .15. dont know if this big enough gap to cause such probs. took out the exhaust valve shims and measured them so that i could get thickness of them as there was no size on them. got new shims today and just got everything built up when the rain started pouring down so now gotta wait till it stops as it was just rideable in the dry the way it was running. thanx for the replies so far guys.