why can i only use my pull start after my o/board is warm?

Hi,i recently bought a s/h boat and motor,the motor is a 1986 40 hp Mariner,my problem is this,when i try to pull start the Mariner it just wont let me pull the cord out to start it,its almost like the motor is jammed or something,but if i use the electric start it turns over easily and fires away and runs without hesitation and after about 10 minutes i turn the motor of and then only after the motor is warm can i pull start it.Also if i leave it for a week after using the boat i have to use the starter coz the pull start seems jammed again, any suggestions ? Thanks.