In my professional opinion its a choke issue.... second problem can be an over kill carb for a SBC. Unless the motor has been bored out to the max and its loaded with racing parts to have a motor that puts out 600+hp and made to run over 5000rpm..... the 750 is an overkill ...it will baxically flood its self out on cold starts and give you crappy gas mileage...... the most you need for street power is no more than a 600cfm.

My 2 cents