How come when I start my 91 Honda prelude si it shuts off?

How come when I start my 91 Honda prelude si it shuts off?

Have you checked for any code?
http://check-engine-light-codes.blogspot.com/2006/04/honda-1990-1995-obd1-code-retrieval.html

It could be spark/ ignition relate: Look through all five pages and do some testings:
http://easyautodiagnostics.com/honda/2.2L-2.3L/honda-ignition-system-tests-1

The two most notorious causes for Honda in the same era were the Ignition switch and the Main relay. You'll have to do some reading/testing and see which one would be the cause:
http://www.tegger.com/hondafaq/mainrelayoperation/index.html
http://techauto.awardspace.com/ignitionswitch.html

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