>From what I've seen, 4-speed clusters were pretty much standard on post-war Italian racing bikes.
I had similar shifting problems with the Ancora. The chainring isn't perfectly round, and the freewheel has a high spot in it's rotation. During pedaling, the high spot in the chainring and the high spot in the freewheel meet every few pedal strokes, causing the chain to become very tight and creating a popping sound. Turning the cranks by hand, the chain would jam as you described. Combined with a bad chain line, this could be what's causing your chain jam and crashing noise.
Turn the cranks by hand and look for tight spots. Both the chainring and the crank "spider" can be out of round. If so, you can try unbolting the chainring and rotating it forward one bolt hole at a time.
The problem can be exacerbated by using a new chain on an old chainring - how well does the chain fit the chainring? Does the chain want to float above the teeth?
After shifting, you need to make sure you provide a bit of chain slack as you close the quick release - I do this by pulling the shift lever out with my fingers as I push the qr lever closed with my palm. I don't need to do this on the Bianchi, but on the Ancora it's a must!
Check that the chain line is centered - a straight line back from the centerline of the chainring should pass through somewhere between the 2nd and 3rd cog of the cluster. Too far to one side or the other, and the chain can bind on the chainring teeth.
Steven Maasland mentioned my movies of shifting the Cambio Corsa... if you've not seen them before, you can watch them here:
Downshifthing the cambio Corsa:
http://www.wooljersey.com/
Upshifting the cambio Corsa:
http://www.wooljersey.com/
And don't miss the improvised camera rig I installed to make the movies:
http://www.wooljersey.com/
Aldo Ross
Middletown, OH, USA
>
\r?\n> Aldo, thanks for the reply!
\r?\n>
\r?\n> yes, its on the Learco. I have already moved the shifter up a bit, so now
\r?\n> i got a bit more than 1cm (1/2 an inch) clearance above the rear
\r?\n> chainstay. i cant vary that much in order to keep all gears rideable.
\r?\n> (Side question: was a 4-speed cluster customary in 1940? Sheldon says they
\r?\n> started to be around not before the 50ies?)
\r?\n>
\r?\n> it behaves like this: the chain returning from the rear sprocket hits the
\r?\n> front chainwheel a bit on the side. that causes either a very loud
\r?\n> crashing noise or a minor "jam" about every 10th crank rotation (which can
\r?\n> be solved by backpedalling). i had similar effect with a bike converted to
\r?\n> sinlge speed, when the chain line wasnt aligned perfectly. Might also be
\r?\n> an issue of chain tension, is it maybe too high?
\r?\n>
\r?\n> Martin Appel
\r?\n> Munich, Germany
\r?\n>
\r?\n> ---
\r?\n> *looking for "Automoto" fork crown medals! please send mail :)
\r?\n>
\r?\n> *visit the search engine for classic racing bike related stuff:
\r?\n> http://www.google.com/
\r?\n>
\r?\n> *view my classic vintage lightweight bicycles at:
\r?\n> http://www.wooljersey.com/
\r?\n>
\r?\n> Aldo Ross schrieb:
\r?\n>> What exactly does the 1/8" chain do, or not do, when you try to shift?
\r?\n>>
\r?\n>> Is this on your Learco Guerra?
\r?\n>
\r?\n>
\r?\n>
\r?\n> I noticed in the pictures on
\r?\n>> wooljersey.com that the shifter is mounted too low on the seatstay.
\r?\n>> Compare this photo:
\r?\n>>
\r?\n>> http://www.wooljersey.com/
\r?\n>> ...with these two:
\r?\n>>
\r?\n>> http://www.wooljersey.com/
\r?\n>> http://www.wooljersey.com/
\r?\n>> Aldo Ross
\r?\n>> Middletown, OH, USA
\r?\n>>
\r?\n>> ----- Original Message ----- From: "Martin Appel" <martin@team-ware.de>
\r?\n>> To: "CR RENDEZVOUS" <classicrendezvous@bikelist.org>
\r?\n>> Sent: Saturday, April 07, 2007 10:51 AM
\r?\n>> Subject: Spam:***, [CR]right chain size for cambio corsa?
\r?\n>>
\r?\n>>
\r?\n>>> X-SpamDetect-Info: ------------- Start ASpam results ---------------
\r?\n>>> X-SpamDetect-Info: This message may be spam. This message BODY has been
\r?\n>>> altered to show you the spam information X-SpamDetect: ***: 3.000000
\r?\n>>> Aspam=3.0
\r?\n>>> X-SpamDetect-Info: ------------- End ASpam results -----------------
\r?\n>>>
\r?\n>>> i recently aquired a cambio corsa bike. now i wonder what might be the
\r?\n>>> best chain to use with it? I tried a standard inexpensive 1-1/8" chain
\r?\n>>> since i figured that was the best match to the standard chain in 1940
\r?\n>>> (right?). On a testride i found the performance left to be desired, it
\r?\n>>> basically teft me with only one gear of 4 to use.
\r?\n>>> Would a 3/32" be a better fit?
\r?\n>>>
\r?\n>>> Martin Appel
\r?\n>>> Munich, Germany
\r?\n>>>
\r?\n>>> ---
\r?\n>>> *looking for "Automoto" fork crown medals! please send mail :)
\r?\n>>>
\r?\n>>> *visit the search engine for classic racing bike related stuff:
\r?\n>>> http://www.google.com/
\r?\n>>>
\r?\n>>> *view my classic vintage lightweight bicycles at:
\r?\n>>> http://www.wooljersey.com/