Dead Cylinder, Spark Issue

[Agent]

The right cylinder on my 1979 Yamaha XS650F has gone dead. I just cleaned the carbs and when I put them back on, the right cylinder wouldn't fire. With the bike running, I pulled the spark plug boot off and nothing different happened. I stuck a screwdriver into the boot and held it up to the plug. The spark jumped about 3/8 inch and the cylinder fired up and ran like nothing was wrong. Moved the screwdriver closer to the plug and the cylinder died. Put it back on the plug and nothing happened.

I changed the boot on the plug wire and got the same result.

I checked the gap on the plug and it is fine.

Left exhaust pipe smells like exhaust. Right pipe smells like straight gas.

I need some help. Can anybody help me figure this out? Has anyone experienced something similar?

Thanks,
Andy

 

[TwoManyXS1Bs]

That right sparkplug likely is bad. Carbon tracking/shorting on the porcelain can do that. You could switch plugs to confirm. Old trick was to hold plug cap a little off the plug, forcing a spark jump to get plug to fire, let it run a little while to burn-off carbon, then reseat plug cap. Later, replace plugs...

 

[Agent]

Thank you so much. I'll try switching the plugs tonight and see if I need to get some new ones.

I guess I just don't understand how holding the cap away from the plug causes it to run and direct contact does not.

 

[TwoManyXS1Bs]

I guess I just don't understand how holding the cap away from the plug causes it to run and direct contact does not.

Haha, another one of the mysteries of the universe.

Forcing the spark to jump the gap causes an immediate hi-voltage to occur at the plug, instead of the slower voltage rise when direct connected, which allows a shorted carbon track the time to conduct and bleed-off the spark energy. Like hitting with a hammer instead of pushing. Years ago, a 'spark gap' gadget was sold by j.c. whitney that utilized this feature, marketed as a 'burns through oil-fouling' add-on for auto ignition systems. We used this trick often on rich-running racing two-strokes...

 

[Agent]

Do you think a good dose of carb cleaner directly on the spark plug would clean it off enough to get rid of the problem, or does the "carbon track" normally ruin the plug beyond repair?

I've been trying to get the carbs sorted out and know I've been running rich. Hopefully I've got them dialed in now.

Thanks,
Andy

 

[TwoManyXS1Bs]

Do you think a good dose of carb cleaner directly on the spark plug would clean it off enough to get rid of the problem, or does the "carbon track" normally ruin the plug beyond repair?

Hard to say. Sometimes it's just unburned carbon and (possibly) metal specs, easily removed with a bead blaster. Other times it may be a crack in the porcelain. Myself, I'm cheap, so I often get away with beadblasting. Sometimes just getting it to fire and burn-off works fine. Your mileage may vary...

 

[5twins]

Electrical contact cleaner does a good job of cleaning plugs. That being said, you should have spares on hand, especially if you're trying to sort carb jetting. I buy my plugs in a 4 pack so I always have extras.

 

[Agent]

Well I put in new plugs last night. Engine fired right up both cylinders. After about 30 seconds, it started to stumble again. Pulled the plugs and the right one was coated in oil. The Haynes manual says either rings or valve guides. Any ideas of which one it might be? I mean, is one more likely than the other?

 

[ridgerunner1965]

do a compression test on the dead cylinder. yu got spark, make sure yu got fuel.

 

[ridgerunner1965]

id vote for rings on the bad cylinder