I have a 33hp 1965 evinrude o/b that is in excellant shape in appearance and performance but last weekend the low speed mixture adjustment lever has all of a sudden got so loose that even just running up the lake it vibrates right over to the rich side and the motor stalls as soon as I slow down, I tried tightening the nut behind the screw but it did not work. Could someone please tell me how to adjust the lever so it will not move once I set it up? Is there a spring on it like some small engines have on there needles, I have not taken it apart yet. Thank you 