fuel ratio for older outboards

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Aug 8, 2015
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CONDENSED SERVICE DATA shows 16:1 fuel ratio for motors before 1959 and 24:1 ratio for motors after 1958.
Does this fuel ratio still apply when using today's fuel and tcw-3 oil?
 

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ondarvr

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In the older motors you still need to use the original ratio.
 

F_R

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Throw that chart away. In most cases listed there, there is no difference in the motors between 1958-59. Use what your specific motor calls for by the mfr as built. Or make your own decision as to what is "right". Everybody else does.
 

racerone

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Fact------What happens to the oil once the fuel goes through the carburetor is not well understood !!
 

Scott Danforth

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Fact------What happens to the oil once the fuel goes through the carburetor is not well understood !!

it gets sucked thru the crank case, oiling the bearings and thrust surfaces and bottom of the pistons. then as the piston passes the ports, it gets sucked into the cylinder where it is compressed slightly prior to the introduction of a source of ignition (spark). at which point the fuel/oil/air is ignited and the resultant explosion pushes down on the piston, which in turn pushes on the oil film in the bearings which pushes on the bearing rollers which pushes on the connecting rod........
 

Vic.S

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Fact------What happens to the oil once the fuel goes through the carburetor is not well understood !!


it gets sucked thru the crank case, oiling the bearings and thrust surfaces and bottom of the pistons. then as the piston passes the ports, it gets sucked into the cylinder where it is compressed slightly prior to the introduction of a source of ignition (spark). at which point the fuel/oil/air is ignited and the resultant explosion pushes down on the piston, which in turn pushes on the oil film in the bearings which pushes on the bearing rollers which pushes on the connecting rod........

Eventually though it all must leave the engine by one route or another ... it does not, cannot, build up in the bearings and crankcase,

Some will be burnt and leave as CO[SUB]2[/SUB] and water just like the hydrocarbons in the gasoline. Some will not be burnt and will leave as oil discharged with the cooling water to pollute the environment until degraded by the natural chemical and biological processes.
 

Vic.S

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Throw that chart away. In most cases listed there, there is no difference in the motors between 1958-59. Use what your specific motor calls for by the mfr as built. Or make your own decision as to what is "right". Everybody else does.
But does that chart not merely reflect what the original owners instruction manuals would have said.

however there is an item on the Engine FAQs board by Chinewalker offering advice on fuel/oil mixtures
http://forums.iboats.com/forum/engi.../162218-fuel-oil-mix-most-motors-covered-here
 

racerone

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Scott, that is an interesting explanation you gave.-----But I suggest you do some more study on the 2 stroke motor operation.----Fuel and air charge are NOT " sucked " into the cylinder.---Fuel and air charge are compressed at up to 160 PSI in the cylinder , which i think is more than " slightly " as per your post.
 

Scott Danforth

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compared to a boosted diesel, 160psi is slightly compressed. however most 2-stroke outboards are only 120-130psi

didnt think we wanted to get into reed valves or pressure waves of exhaust (port vs loop scavenging, etc) as a response to your post (#4) so I went with "sucked" as a simple explanation, along with a highly generalized comment on fuel/oil lubricating the bearings for a reason.

We could also go into lubrication film and the requirement of surface energy to keep the lubrication film intact. or how important the lubrication film is to prevent metal to metal contact. however suffice it to say, if the film is compromised (especially at high RPM), motor goes KAAAATHWAK! and stops running. which leads back to the OP's post on fuel/oil ratio.

if the motor is designed to run on 24:1 or 16:1, run it there. do not run an earlier motor on late motor fuel ratios of 50:1.

Side-note: Dad spent a few years in the Tecumseh 2-stroke lab running motors from 16:1 to as low as 200:1. the higher the oil content, the higher the emission output and the higher the load capacity. the lower the ratio, great for emissions at steady state load, however the minute change in temperature or load and the motors ate themselves in a few revolutions. lots of pictures of carnage
 
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Thanks for all the input, i just got a 1956 johnson 10hp QD-17, and was trying to find the right ratio for it. The condensed service data is the only mention of 16-1 ratio i have found so far.
 

F_R

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I believe the '56 10hp came with a 6 gallon tank. Instructions on the side said mix 1/2 pint of oil per gallon of gas (16:1), or 1 quart per full tank (24:1). Two reasons for the difference. 1). Outboard oil in those days came in 1/2 pint cans and/or 1 quart cans. So, convenience and ease of measurement. 2). They didn't trust people to accurately measure the 1/3 pint of oil for a gallon, so better too much than too little. Thus the 1/2 pint which was a safety margin. 5.5hp motors came with a 4 gallon tank, marked 1/2 pint per gallon, or one quart per full tank (16:1).
 

F_R

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I could point out a few errors in the guy's video, but I won't. But I will tell you that the 1956 5.5 and 7.5hp motors came with a 4 gallon tank, part number 376590. The 10, 15, and 30hp motors came with a 6 gallon tank, part number 375777. Look it up in the parts books if you don't believe me. And read what I wrote above, concerning what it says on the side. OK, back to the video. I will say that his 10hp motor may have come with a 4 gallon tank when new. But if it did, it is because the DEALER swapped a 4 gallon instead of a 6 gallon when he sold it. But the tank that came with the motor in the box from the factory was a 6 gallon (10hp and up).
 

HighTrim

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I have been reading F_Rs posts for about a decade now, at a couple different forums. He is always correct. He knows this vintage motor like you know the back of your hand!
 

TN-25

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All interesting and valid points brought up by everyone. One thing I can add is that the oil requirements also should be based on the motor construction. The QD (10-horse) motors have roller bearings and can be run on modern oils at 24:1, while the 5? & 7? motors are bronze bushed bearing motors that need the film thickness of the oil. Run a bronze bushed motor at the originally intended gas / oil ratios even if you are running modern TCW-3 oil. 24:1 could do but 16:1 is better for the bronze bushed motors.
 

oldboat1

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That summary up in #1 looks like something from an old Intertech summary (looks kind of familiar). But then Intertech also distinguishes between horsepowers/years with what would be contradictory info. According to a 1983 Intertec manual, a J/E 10 between '55 and '63, for example, would use a 24:1 mix, same for the 5 1/2. But the mix for the 7 1/2 is 16:1.

Evinrude specified 1/3 pint per gallon for the 10hp between 1956 and 1961 (instr. rev. 12/60). That would be 24:1 for the OP's '56. If it were mine, I would run it on anything between 16:1 and 24:1, looking for best operation of the particular motor.

Think a lot of folks believe that a richer oil mix produces more smoke, more nastiness, but that isn't correct. A well-tuned '56 or '57 runs as clean under the old specs as '70s - '80s 2-cycles with their leaner mix.
 

tjandrews

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I've been using what I thought was a 24:1 mix in my 1954 QD for the last 20 years or so without any apparent ill effects. But my brother has been the one filling and refilling the tanks on our annual fishing trip. It was only this year that I noticed that, according to the station pump, I was NOT getting six gallons of gas into that tank. And it makes some sense, when you think about it.

The empty tank shell may indeed have a capacity of six gallons. But, add the pickup tube and gauge float/linkage, and the capacity drops to more like 5.75 gallons. Add a quart of oil, and the gas you can put in drops to 5.5 gallons. Leave a little head space for thermal expansion, and you probably have more like 5 gallons of gas in there. One quart of oil to 5 gallons is 20:1. And apparently, THAT'S the mix I've been using all these years.
 

interalian

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I've been using what I thought was a 24:1 mix in my 1954 QD for the last 20 years or so without any apparent ill effects. But my brother has been the one filling and refilling the tanks on our annual fishing trip. It was only this year that I noticed that, according to the station pump, I was NOT getting six gallons of gas into that tank. And it makes some sense, when you think about it.

The empty tank shell may indeed have a capacity of six gallons. But, add the pickup tube and gauge float/linkage, and the capacity drops to more like 5.75 gallons. Add a quart of oil, and the gas you can put in drops to 5.5 gallons. Leave a little head space for thermal expansion, and you probably have more like 5 gallons of gas in there. One quart of oil to 5 gallons is 20:1. And apparently, THAT'S the mix I've been using all these years.

That's why I always do my mix in Jerry cans, then pour into the boat can. Boat can is too small to fit 25L of fuel, and we buy oil in 500mL, 1L or 4L bottles.
 
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