Why the changes in fuel/oil mixes?

[00scoots]

I'm kind of new to outboards.<br /><br />I inherited my dad's 14' runabout. It came with a 1978 35hp Evinrude and the manual says a 50:1 fuel/oil mix.<br /><br />I bought a 1960 3hp Evinrude and it has a 24:1 fuel/oil mix.<br /><br />I also just bought 2 well used Matin 60 outboards and thay have a 16:1 fuel/oil mix.<br /><br />All of the newer outboards usually have a 50:1 mix and older outboards (like pre 1964) have richer mixes. Is this because of lower compression ratios (?) in older outboards or does this have to do with older metals in the outboards or because newer 2 cycle oils are better lubricating?<br /><br />Just curious. I use Mercury or OMC 2 cycle oil.

 

[Paul Moir]

Re: Why the changes in fuel/oil mixes?

Scoots, I can't speak for your Martin but I can for that 3hp Johnson. <br />If you take that engine apart until you expose the crankshaft, you'll see a few things that are different from a modern outboard. The bearings for the crank and the ends of the connecting rods are simple plain babbitt bearings. Babbitt type bearings rely on riding on a film of oil when operating normally, or else they heat and wear. Automotive engines use this type of bearing on their crankshafts since there is lots of oil available, and they are inexpensive.<br />A modern outboard uses nice roller or sometimes ball bearings in these places.<br /><br />Secondly, there's the matter of crankcase sealing. In order to pressurize the charge properly, the crankshaft must be sealed from the outside of the block. The shaft oil seals on the 3hp are pretty basic, and depend on being covered in a lot of oil.<br /><br />I hope this answers your questions.

 

[Clams Canino]

Re: Why the changes in fuel/oil mixes?

Let me add a follow up. In the case of Mercury, most all the post WW2 motors had no friction bearings and touted "full jeweled movement", which meant all the moving parts had ball or roller bearings.<br /><br />So speaking *strictly* to Mercury, you can run most anything from the 50's and 60's on 50/1 TCW-3 regardless of the instructions. This is because the OIL got better, not the motor.<br /><br />Most any outboard with full ball and roller bearings can run on 50/1. Just be *sure* it has no bushing or friction bearings first! <br /><br />-W

 

[00scoots]

Re: Why the changes in fuel/oil mixes?

Thanks for the replies.<br />If I understand correctly, "friction bearings" are what some would call a "bushing" and are usually made of brass. I can understand the need for a richer oil mix in these outboards.