Cylinder number 3 is freeloading!!

[Guest]

IE not contributing IE misfiring! I had the plugs out changing them cause they were all fouled. I checked spark on all cylinders with the spark plugs hanging out and all of them showed a good spark. I started the bike up and it sounded good but when I pulled the spark plug wire from cylinder number 3 no change in idle. Also throwing water on cylinder 3 exhaust shows it's ice cold. Any ideas?

[wymple]

I would suspect the #3 carb if you have fire.

[Steve]

If you have fire, you could be missing fuel or compression.

When is the last time you checked valve adjustment?

Missing fuel could be plugged passages in the carbs or lack of carb sync.

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[81GS850]

I checked the clearances and adjusted as necessary when I first got the bike. i haven't put 2000 miles on it so that should be ok. I checked compression when I first got the bike, but maybe I should do it again. If I pull #2 plug it doesn't change rpm as much either. But it is hot. Maybe I have a weak coil or bad spark plug boots? I'll check compression. I just cleaned the carbs because the tank had rust in it.

[Brendan W]

Plugs can spark out of the motor but deliver weak or no spark at cylinder pressures. Were you getting good fat audible sparks?
At this point I would measure the voltage at the coil supply plugs and compare it with the battery voltage. Opinions differ but I would be unhappy with a loss of one volt. Clean them with contact cleaner when you are in there any way.
As all plugs are fouling , presume you mean soot and not fuel, I would measure the primary and secondary resistances of each coil. Should be about 4 ohm and 12 k ohm if you're still running stock. Cleaning and reseating ignitor plugs won't do any harm either.
Sparks may not be the problem but eliminating it is a lot less work than going after the carbs again.

[81GS850]

Plugs can spark out of the motor but deliver weak or no spark at cylinder pressures. Were you getting good fat audible sparks?
At this point I would measure the voltage at the coil supply plugs and compare it with the battery voltage. Opinions differ but I would be unhappy with a loss of one volt. Clean them with contact cleaner when you are in there any way.
As all plugs are fouling , presume you mean soot and not fuel, I would measure the primary and secondary resistances of each coil. Should be about 4 ohm and 12 k ohm if you're still running stock. Cleaning and reseating ignitor plugs won't do any harm either.
Sparks may not be the problem but eliminating it is a lot less work than going after the carbs again.

Yea I cleaned the electrical system and I'll have to verify, but think I have less than .5V drop to the coils. I'll check resistance and connections as well. I'm kinda leading to believe I have some weak coils. I did cut a half inch off the HT leads and rethread the spark plug boots.

[Steve]

I'm still leaning to lack of fuel. Due to a bad carb sync.

If one carb is closed farther than the others, it won't be contributing to power output, especially at idle.

If your bike tends to "come alive" when the throttle is twisted, that one carb is finally open enough to contribute to total power output.

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