Gas to Oil Ratio

[jathomps04]

I have a 1981 40HP 2 cylinder Mercury outboard engine. I have no idea what the oil to gas ratio is. Does anyone know? If not, what would be a good ratio to use if I don't know? 50:1?

 

[imported_Curmudgeon]

Re: Gas to Oil Ratio

50:1 will work just fine ...

 

[jnewtonsem]

Re: Gas to Oil Ratio

The larger mercs from the 70's and early 80's need to burn premium gas because of today's crummy gas. High probability of burning a piston if you use regular gas. I've heard the smaller mercs have the same tendency. Also use a good quality oil (synthetic) for added protection.

 

[jathomps04]

Re: Gas to Oil Ratio

Thanks for the input guys. I'll be sure and use premium gas with a 50:1 ratio.

 

[Texasmark]

Re: Gas to Oil Ratio

My '02 service manual, which covers '94 and newer, says min 87 octane (r+m)/2 with TCW-II oil at a 50:1 ratio. I realize we are into TCW-III oil today and that will do just fine as petroleum upgrades are designed to be backfittable.<br /><br />One of the personal problems I have with premium fuel is age. I have asked fuel retailers about how much mid and premium fuel they sell and it's nil. Gas prices are so high that most folks that used to use it quit. I get good fresh 87 so I decided to use that, as recommended, rather than buy the higher octane gas. I do use the syn or syn-blend oil however, the best I can buy (Merc or Pennzoil) and add Sea Foam at every fill-up.<br /><br />Mark

 

[dajohnson53]

Re: Gas to Oil Ratio

Originally posted by jnewton:<br /> The larger mercs from the 70's and early 80's need to burn premium gas because of today's crummy gas. High probability of burning a piston if you use regular gas. I've heard the smaller mercs have the same tendency. Also use a good quality oil (synthetic) for added protection.

These questions are made because of curiosity, not argument. Are you saying that regular octane gas from years ago was better than same octane gas today and therefore one should use high octane today when regular would have been fine in an earlier time? I certainly don't disagree that you should burn the octane that your engine's specs call for -if it calls for premium, burn premium or you might have detonation issues. However, if an older engine calls for regular, are you saying that because modern gas isn't as good, you should burn premium?<br /><br />What is meant by "crummy gas"? I've heard it said before, but I really don't understand why the quality would actually be lower today. I just hear it said, but not why it is. I'd say that I'm pretty biased against this general kind of statement because it's said about a lot of stuff that is actually better today than years ago. Cars for instance- they are incredibly better than they were 30 years ago in my personal experience.<br /><br />I've never had a fuel related issue in 30+ years of operating cars, boats, etc. I'm in the high-30s cylinder count including 4 vehicles, two outboards and various small engines. Maybe modern lawnmower, car and outboard engines have been designed to accomodate new-ish "lower quality" fuels, but in my rigs I have a mix of very old (20+ years) and very new no fuel issues with any. Maybe I'm just not perceptive to subtle performance issues, but I certainly don't see shortened lifespan at all. <br /><br />RE: synthetic 2 stroke oil. Is it really better at protecting? I understand that another benefit might be smoother operation and less smoke and smell, but actual measurably better protection than properly mixed TCWIII? And how would this relate to the crummy gas situation? <br /><br />My two outboards are 20+ and 30+ years old and as far as I know have only burned regular gas and non-synthetic TCWIII oil (I take that back - I burned a gallon of full synthetic through them once but didnt' notice any observable difference so I didn't bother buying any more). Both run pretty much perfectly and have good compression.

 

[jnewtonsem]

Re: Gas to Oil Ratio

The old Mercs were made when gas was leaded or the old "white" gas which was unleaded but a premium blend. Also before ethonol, MTBE, and other oxiginates. During the 60's through the 80's merc dominated the racing circuits and they were constantly pushing the envelope for performance. When unleaded gas was mandated, the big mercs started burning pistons especially the #1 and #3. Over the course of several years merc reduced compression, retarded timing, lowered top end rpm, etc, to try to correct the problem. But they finally stopped making all the old in-lines big and small in the late 80's. <br /><br />Using premium gas prevents predetonation which is what causes the cylinder to burn. The synthetic oil is added insurance that the engine is getting proper lubrication. There is still a hard core group of "old racers" and in-line devotees and they swear the key to longevity with these engines is premium gas and synthetic oil mixed no less than 50:1. <br /> <br />Don't get me wrong the in-lines are great engines and if treated right will give you long performance. I have a '85 115 and it runs like a champ. But the previous owner, the mechanic who works on it and has worked on mercs for 30 years all say the same thing. The key to long term performance is premium gas and good oil. The mechanic is the the one who told me the smaller mercs of the 70's and 80's have the same problem.

 

[dajohnson53]

Re: Gas to Oil Ratio

Thanks for the explanation jnewton. That makes sense to me, a layman. We don't have any oxiginates around here (no mtbe, no ethonol) so that's not an issue. My large engine ('83) is not a Merc, but is also post-unleaded and of the low compression design common to some of that era OMC V6's, so I interpret this to mean regular fuel should never be an issue for it. My little 9.8 Merc is '74, but even so, frankly I'm not going to mix premium fuel for it. I just run it off my main tanks, 99.9% trolling and only about 50 hours per year.

 

[Texasmark]

Re: Gas to Oil Ratio

Hey j, Back when I was growing up, white gas was 75 octane when regular was 92 (motor method). It was in a pump with a glass top and you cranked the top to what gallonage you wanted and then stuck the nozzle in your fuel inlet and let her gravity feed. If you selected too much, tough luck as there was no such thing as the auto shut off nozzle.....it just poured out on the ground.<br /><br />Then along camd Amoco in the '60's and put out 95 octane White premium.<br /><br />So what white gas are you talking about.<br /><br />Mark

 

[jnewtonsem]

Re: Gas to Oil Ratio

D.-I think you are right about the OMC's even in the 70's they were lower compression than the Mercs, so they weren't as susceptible. I think in the 70's the Merc's had compression of about 150, my '85 has about 130, I think the stock OMC's were 110 or 115. Also, with your 9.9 probably won't have the problem because you're not pushing it. Texas- I'm talking about the Amoco 95 white gas. The 92 regular would be equal to today's premium.<br /><br />Disclaimer- I'm not an expert here, mostly repeating what I've heard