i hope i am putting this in the right section. i have a early 31.8 with over the hub exhaust i am figuring mid to late 70s. this motor would not run at all unless you sprayed gas into the carb. i wanted to upgrade the carb to a 30hp anyway so i changed the carb to a 30/35 now the motor starts up but now it will back fire and quit the only way to keep it running is to partially choke it. when it back fires it does so through the exhaust it has good spark i swapped the cdi box still the same i even switch back to a good 25 carb with it doing the same thing i checked the key on the flywheel and it looks fine i am confused any help would be greatly appreciated . by the way compression is 120 in both