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

[raul10141964]

3 possibilities
1- the machine chop install it wrong
2- the spring is to weak
3- over rev the engine

[bmw633]

I noticed the cotter valve did not have a groove on the inside. It was smooth, so maybe that is all it needs. I don't have the tool that measure spring tension pressure, but it took a bit of squeezing the valve spring compressor tool to replace the cotter valves. I am almost positive she didn't overrev the engine. I put a front sprocket with an extra tooth on it to lower the revs.

Thanks for the reply