[Old_Dude]

When I switch to the largest cog in the freewheel the chain slips automatically from the middle to the the small chainring.

As far as I know the front derailleur is adjusted properly. When I look down the chainline with the chain on the large cog going to the middle chainring, the angle looks way too sharp (for lack of a better word) compared to a "normal" bike setup.

I'm wondering, could the rear of the frame be bent somehow, or the bottom bracket too far out, or something? BTW, if it matters, the bike is a Gary Fisher Marlin - all drivetrain components are stock.

 

[alex spencer]

looks like the chainline is a bit out, try fitting a shorter axled BB.

 

[Old_Dude]

Fixed the problem!

With the bike on the stand, I looked down at the chainrings from directly above. I put the chain on the middle chainring and slowly turned the crank. I could see a "bobble" or bump in the middle chainring (this was more obvious when the chain was on the ring for some reason).

Holding the crank in position so the bent portion of the crank was accessible, I took my 10 pound hammer and whammed the bump - a little at first and then gave it just the right amount of taps to get it all lined up (the chain was still on when I did this).

Works like a new one again.

Go figure.

Please keep in mind, this crank set is very cheap (chainrings are all pressed together, making disassembly impossible) and I would never do something like this or recommend anyone else doing this to a decent set of rings.

Happy riding!