I have a 1961 Evinrude 5.5 HP that does not idle well or start well after several starts and stops. I took out the spark plugs and found them oil fouled. I have been using a 24:1 mix ratio that someone told me is the correct one for that motor, but I think it is too much oil. What is the recommended ratio, 24:1, 50:1, or something in between?