Thanks to all the responses to the "strange boiling sounds" from my 1994 XR600!
Here is the question that was posed:

Pop Quiz: My XR600 had developed an odd problem that no one has been able to identify. If you have a guess, Email Me! T he symptom is this: After the bike has been worked a bit and the engine's hot, turning the bike off in a quiet environment results in a "boiling over" sound from the downtube/frame neck directly behind the steering head. I've found that opening the oil filler cap sometimes stops the sound. I initially thought that maybe I'd gotten water in the oil, but an oil change didn't make a difference (nor could I see any "white" oil, a sure sign of water). It's not boiling fluid trapped in the spark plug detent of the engine. Any ideas?

Here are a sampling of the many replies I got:

There is a gusset welded to the frame at the steering head to reinforce
the area. It (the gusset) creates a void between the frame backbone and
the gusset that is welded all around. A pin hole in the weld will allow
moisture to collect in this void. After running the bike hard this moisture
expands/boils and causes the noise you hear, to prove this drill a small
hole in the gusset without going into the backbone if that was the problem
it (boiling noise) will stop.

I welded a tower to this area (Scotts steering stabilizer) with a tig welder
and pressure built up in this void and kept blowing out the weld till I
relieved the pressure with a small hole.

Let me know if this was the problem.

Ron Peck

** Probably the most likely answer, considering the sound first showed up the
day after riding 25+ river crossings. However, I seem to remember the sound
would stop when I opened the oil reservoir cap. Hmmmm... -Jeremy


From Fri Nov 29 03:18:00 1996

I think that it might be "breathing" air into the oil inlet
somewhere....that's a sign which is seen whenever there is a leak or an
illegitimate "intake"...try replacing the cap or the washers associated to
the frame oil container.


From Sat Nov 30 01:50:21 1996

The boiling sound that your're referring to is probably caused by the 
oil gurgling back past the checkvalve. Next time this happens, check 
your oil level and I'd bet it's low.o

** I seem to remember it not being low. In fact, the first time it happened, it was high! -Jeremy


From Sat Nov 30 19:36:36 1996

first check,

oil level
oil pump,
go to hisperia and have 8 beeers. weeel see you there


My XR650L does the same thing after heavy flogging through deep sand washes.
I think that it must be overheating - it can burn up to a half a quart after
a five mile sand wash!  Sometimes when that heavy thing is in the sand, I'd
be near redline in second gear (digging a trench while trying to get the
thing to float on the sand) while only doing a registered 25 miles per hour.
It definitely is getting very hot. I've thought of installing an oil cooler,
much like the one that is on the XR400.

My guess: your oil is indeed boiling

After the Moonrock X National Hare & Hound on October 6, my bike was a full
quart and a half low on oil! It went somewhere burning to the inside of my
engine or bioling away.

Sean Hollingsworth

**Yikes! I sure hope it's not boiling.  I would suspect boiling oil would
lead to an engine siezure. Also, I've never found my bike to be low on oil.
I have a feeling Sean's bike is burning oil - bummer. -Jeremy

From Tue Dec  3 19:05:10 1996

I believe it's your gas boiling in the gas tank.
If so, nothing is wrong. It's just hot.
Pat Phillips 

** ACK! Now my gas is boiling!? I don't think this could be it either. I think
if my gas was hot enough to boil, it would be a very explosive situation. ;)
I also had several comments that it's just the breather tube, gas cap, etc.
I have a one-way valve on my gas vent line and suspect that it would prevent
that from happening. Also, the sound's coming from the frame neck immediately
behind the steering head, not the top of the gas tank.... -Jeremy


From Wed Dec  4 00:42:17 1996

How about expansion noises from all the heat of your hot bike?  Good luck,
happy hunting, and have a blast on the trails.  Just don't stop to listen!!

** Hah! That's what I've been doing - ignoring it. Last ride out I didn't
notice it at all. Did the bike "dry out" if it was indeed water in the neck?
Or was there just too much noise from other bikes to hear it this past time?
Or was I just too busy riding to notice it? 
Maybe I should ride the bike, wash the bike, and ride it again?  
I didn't change oil between the last time it "boiled" and this time.