Rebuilt Engine Quits

[bmw633]

Hi,

I took the head to a machine shop to do a valve job and they also honed the cylinder walls. I put new valves in, and reassembled the engine, which ran great for about 400 miles. My daughter was riding it to work when it died on the side of the road. It didn't seem to have any compression and it was backfiring through the carb. Since it was the first time I had gone through an engine, I was depressed that I had screwed up something.

Today I started the tear down and discovered that one of the cotter valves had fallen off one of the intake valves, leaving the valve partially open, hence no compression and backfiring through the carb. The valve did not appear to be damaged, nor did the piston. I have a valve spring compressor, so I found some other cotter valves on a junk motor and swapped them out. Tomorrow I am going to

Why would this happen? It looked like it fit fine when the head came back from the machine shop. Any ideas?

Seth Anderson