What's that noise?

I climbed onto my wife's '82 XJ650 for a quick ride into town. I pushed the choke on, turned the key, rolled the throttle a little, and hit the starter button...

Vrrroooommm! She started up after a few revolutions and began to warm up. I dropped her off the center stand and began to turn the bike around. By the time I got the bike pointed in the opposite direction, I could hear a noticeable thunk, thunk, thunk, thunk coming from the bike. I revved the engine and the thunking matched the engine speed. A horrible thought crossed my mind, "Is that a rod knocking?!?" I just had to drain the crankcase because of a stuck float and a leaking petcock that flooded the engine with gasoline. Did it wash out one of the bearings?

I figured out what the noise was. I'll give you a hint: the engine is fine. Can you guess what was causing the noise?

 

Sounds like a classic case of bad muffler bearings.

Did I accidentally tune in to the Puzzler on Cartalk?

U got me. If the bike were still on it's center stand I might say you had it in gear an the clunking was the drive shaft balking at no resistance... You have me curious. Let us know when you've stumped enough people.

 

How 'bout another hint? The noise was more pronounced when the fuel tank was getting to the point of needing to be switched over to reserve.

 

Vacume connected to tank petcock, the diaphram inside the petcock?

 

You forgot to bolt down the tank or you didn't put the rubber isolator between the tank and the frame...

or both.

 

82maxim400 got it correct! (Sorry for the long response delay... I went for a ride. 8) ) It seems the spring wasn't quite strong enough to keep adequate pressure on the valve which caused it to pulsate when the weight of the fuel got to around half a tank. So, every time the #3 piston went down, the valve got a round of vacuum which caused the valve to bang into the petcock body. Hence, the thunking noise. Strange!

 

I had that happen on my xj750. Sounded horrible with close to an empty tank.

 

I have never heard of that before.
Interesting.

Glad it wasn't anything serious.
Wow!