Rob's Bike

After a fun and successful 50CC with Steve, I was looking forward to a more
sedate return to Tucson via stays in New Orleans and Steve's home in Clovis.
The French Quarter was cool (my first visit), but after leaving N.O. the
next morning, we gassed up at Natchitoches where Steve asked that we trade
bikes so he could get a quick feel of the Bridgestone BT020s I had recently
mounted.  Well, quick it was as Steve pulled over to the side of the
Interstate just 11 miles out.  The bike had suddenly started to run on one
cylinder and Steve had wisely pulled the clutch and hit the engine cutoff
switch as he coasted to the shoulder.

Roadside troubleshooting verified good spark and fuel delivery, but no
compression on the right cylinder.  I cranked the engine with the right
plug removed and Steve noted an object fly out the hole.  Later, I would
learn that was probably a piece of the piston crown.

Long story short, "The Woman" pulled Steve's MC trailer all the way from
Clovis to bail me out.  I continued on home with the tow car and trailer and
tore the bike down two days later.  Here's the damage assessment:


1.  Large 1 3/8 by 5/16" piece of the piston crown just below
the lower exhaust valve was gone.  Evidence of high piston
temperatures.  Top compression ring is hammered tight by the
piston crown.  The second compression ring is partially stuck, too.

 2.  Cylinder head is cracked between the two intake valves.  
Evidence of severe collisions within the chamber. Three out 
of the four valves clearly bent and the fourth likely bent.

3.  Cylinder is OK!  No scoring and the original crosshatching is evident
after 111K miles.

4.  New parts cost to repair (excluding labor): ~$1600.  I bought a used 12K
mile '94 RS motor from Iron Horse for $900 and will swap out the two top
ends myself.

Why did this happen?  I'm not quite sure.  Even though the bike had 111K on
it at the time, this is not big miles for a well-maintained BMW.  Something
had to have made the bike, at least the right cylinder, run very hot.

In my experimentation with external fuel filters, I originally tried a
smaller Deutsch and Fram filter than the one currently recommended in my
article at http://www.ibmwr.org/tech/r11tech/extern_fuel_filter.html.  On a
Three Flags run (with this smaller filter), the filter plugged and caused
loss of fuel pressure and power.  That had to have been an extremely lean
condition for the bike, heating things up, I suspect.  Then later on, with a
fresh smaller filter, I was returning home from Ron Ayres home at Plano, TX
when I did some fast speeds west of Big Spring, TX and the bike started
running on one cylinder.  On this occurrence, one of the plug side
electrodes (I don't recall which cylinder) was melted onto the center
electrode, again a high heat situation.

This indicated to me that the small filters were not the way to go and
tended to inhibit fuel flow and pressure.  What I now also think is that I
heat-damaged my engine with the small filters and that this latent damage
became catastrophic coming home from the 50CC.

Lesson learned: Do not go to a smaller filter than OEM.  I'm going to
research an even larger filter than the current Deutsch FF401/424 to assure
constant fuel pressure under worse case conditions.  More to come, and I'll
modify the external filter article with new filter recommendations.

Rob