only problem is i have only a .40 size o.s. bb i think its bb. i hate to spend over 100 bux on a new engine cant seem to get a used one. the old russian mds carb. is a piece of sh*t! other than that its a powerfull little engine. if i could get some kind of .48 carb. i could just slap it on and itll run better. Would a .40 size be too underpowered?