strange noise

[bikenutt1980]

I took the bike to the car wash tonight and noticed a strange noise. It sounded like it was coming from the top end and it was like a knocking sound. Seem to only make it when i let off the throttle. Anyone have some ideas? If i can get away with it till winter then i will if its something i need to pull the engine and replace a part ASAP i will do that. The bike has 40,000 miles on it and its a 1980 xs1100 special.

thanks
jason

[prometheus578]

"Knock knock... Who's there?"

Go to your local cheap tool joint and pick up a stethoscope. Noises and vibrations love to travel.... there's nothing up top that can knock.
If a slight knock can be heard at idle... one by one, carefully pull the plug caps. When the knock stops... you've found the offending cylinder.
Time to stress the engine a little.
Bike on centerstand, engine running.
Place it in first gear and let out the clutch... rear tire spins.
Now, slowly apply the rear brake, slowing the tire, straining the engine. Listen for the knocking to get louder.
Try again in fifth gear(might need to raise the RPM to keep it from stallin'.
If she knocks, doesn't matter if it's piston slap or a rod bearing..., you've got a job ahead of you.
*Make sure the carbs are properly synchronized first. An out of synch carb can cause that cylinder to knock, too*
Did this noise just suddenly appear?
Check everything before doing anything drastic.
One person's definition of a knock is another persons definition of valve noise or a loose timing chain.
Make sure everything is hooked up correctly and it's not the vacuum advance chattering back and forth.