Picked up an unused 10 year old 5hp johnson. I say unused in that it was bought as an auxilary motor but was never needed. Anyway, problem with it is that it won't stay idling for long. Even with the throttle stop adjusted for a fast tick over, it will die after a minute. When this happens it will only start if you pull out the choke and push it back in immediately. Doesn't seem to matter if the motor is hot or cold. At full throttle it runs fine, the plug is the correct one, is a nice tan colour and not fouled. I stripped the carb and blew out all the jets, but it was spotless anyway. Any ideas?