help! new 20hp kohler wont stay running

hi new to the site but hope 2 help and get help! i just put in a new full/compleet 20hp kohler motor in a walker mower and it will not stay running! it runs 4 around 4 seconds then shuts down. i have done the folowing.

`made shur spark stays till it cuts out
`has fule pumping in to carb till it cuts out
`allso fule is in the carb bowel and flote moves freely

Please any help will be grate! thanks