Math problem

johnnyvirgil

Cadet
Joined
Jul 10, 2023
Messages
10
Did something dumb. Had a 6 gallon tank with approximately 1.x gallons left in it. On autopilot, I dumped in 16oz of 2-cycle oil, and filled it up. How much fuel do I need to drain out of the tank and replace with straight gas to get the mixture back to 50:1? I feel like this is an SAT question that I would get wrong.
 

johnnyvirgil

Cadet
Joined
Jul 10, 2023
Messages
10
Did something dumb. Had a 6 gallon tank with approximately 1.x gallons left in it. On autopilot, I dumped in 16oz of 2-cycle oil, and filled it up. How much fuel do I need to drain out of the tank and replace with straight gas to get the mixture back to 50:1? I feel like this is an SAT question that I would fail.
 

Drivewayboater2

Petty Officer 1st Class
Joined
Aug 21, 2019
Messages
334
Well I’m no math wiz but here’s my thinking
50:1 ratio in a one gallon tank would mean 2.6 oz of 2 cycle oil is needed. In a 6 gallon tank you would need 15.6 ( roughly 16) oz of 2 cycle oil. Since you had only 1 gallon of fuel in the tank and you put in 16 oz. You would need to add roughly 5 gallons of fuel. Give or take. That make sense?
 

alldodge

Moderator
Staff member
Joined
Mar 8, 2009
Messages
42,054
I wouldn't do anything, Your around 43:1 mix right now so it may smoke a tad bit more but probably not that much. Could bur a 1/2 Gal of it and replace just the 1/2. For me I would just run it
 

johnnyvirgil

Cadet
Joined
Jul 10, 2023
Messages
10
Here’s the full story - maybe you have some suggestions. Last fall I blasted the engine (1977 evinrude 70hp) with fogging oil. Took the head cover off, and all the carbs and shot the crankcase with the fogging oil because I was afraid I had leaking exhaust plates and was getting some water in the lower cylinder. (Turns out I wasn’t) so I replaced all the exhaust plate gaskets, rebuilt the carbs, new thermostat and valves, new water pump, new plugs. New fuel pump. Put it all back together and it starts right up and runs, but smokes like crazy and when I run it in a barrel the water gets oily fast. If I run it for a minute or two, there are little black globs of something (soot? Oil?) floating around. I was hoping it was the fuel mix since I can’t think of anything else it could be, other than one mod I’ll get to ina moment. All OEM replacement parts. Nothing leaking from the carbs. Seems to idle ok and go into gear ok without stalling after I adjusted the idle. The only other thing I did was the recommended service to that engine to address a hesitation I was experiencing. The solution was to ever so slightly enlarge the holes in the carb high speed nozzles and replace the .030 idle orifice with an .032 orifice. That’s really the only thing different, other than the fuel mix being off. I was thinking since I dumped 16oz of oil into essentially 4.5 gallons of gas, it would be rich, but it’s enough smoke and oily crud that I wouldn’t feel right running it in a lake.
 

johnnyvirgil

Cadet
Joined
Jul 10, 2023
Messages
10
Well I’m no math wiz but here’s my thinking
50:1 ratio in a one gallon tank would mean 2.6 oz of 2 cycle oil is needed. In a 6 gallon tank you would need 15.6 ( roughly 16) oz of 2 cycle oil. Since you had only 1 gallon of fuel in the tank and you put in 16 oz. You would need to add roughly 5 gallons of fuel. Give or take. That make sense?
I think that doesn’t account for the mixed fuel that was already in the tank. I siphoned it out and did a proper mix with synthetic blend and the smoke is less but still what I feel is excessive. Might not be a mix problem. Can’t you edit ana delete posts here? I was going to delete and repost under engine repair but didn’t see a delete option.
 

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
18,039
My math says you have 3.09 oz of oil per gallon (about 40:1) . . . so you need to dump about 1 gallon and fill it up with straight gas to get back to 2.56 oz / gallon ( approx 50:1)
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,739
Remember, you fogged the crud out of it, Literally.

All the fogging oil has to go some where.
Out with the exhaust and into the motor leg.
Then it comes out the bottom end.

I would run it. Then add 1 - 1.5 gallons of straight gas and run it some more.
 

Lpgc

Petty Officer 1st Class
Joined
Jun 17, 2023
Messages
258
Been said you'd need 15.36oz in a 6 gallon tank, 2.56oz in a 1 gallon tank.

So if you already had a gallon of pre-mixed in the tank it will have contained 2.56oz oil. You added 16oz of oil, so now your 1 gallon contains close to 18.6oz. If you add an extra 5 gallons of fuel you'll have 18.6oz in 6 gallons (768oz) which is a ratio of 41.3:1.

If we're being fussy your 6 gallon tank should only take 6 gallons including the oil, so instead of there being 768oz of gas in the tank there'll only be 749.4oz plus 18.6oz of oil which is a ratio of 40.3:1. Still not far from 50:1.
 

racerone

Supreme Mariner
Joined
Dec 28, 2013
Messages
37,816
Motor cowling makes it a 76 model I think.---Motor will run / idle all day with 40:1 mix and do no harm.----Just another example of how the concept of 2 strokes and oil in gasoline is mis-understood.
 
Last edited:

johnnyvirgil

Cadet
Joined
Jul 10, 2023
Messages
10
Motor cowling makes it a 76 model I think.---Motor will run / idle all day with 40:1 mix and do no harm.----Just another example of how the concept of 2 strokes and oil in gasoline is mis-understood.
Yep it’s a 70673D. 1976. I know it’ll run and not hurt anything but I’d rather not smoke up the entire marina if I can help it. That’s really my only reason. I pulled the plugs last night ana they seem a little wet with oil. Maybe I’m not getting great combustion. Checked the compression and all cylinders are over 125, with the top one being around 130.
 

cyclops222

Lieutenant
Joined
Mar 21, 2024
Messages
1,289
There should not be a lot of smoke from the slight extra amount of oil in the tank. If there is. You are not measuring the amount of oil correctly. Or you are not mixing the oil and gasoline correctly / well enough.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
49,536
FWIW, I have an outboard at 24:1, one at 50:1. Both smoke about the same at idle

chain saws at 32:1, weed whackers at 32:1 and a moped at 40:1. They also do not produce much smoke

I love the smell of 2-strokes

That little extra oil in the fuel won't hurt
 

redneck joe

Supreme Mariner
Joined
Mar 18, 2009
Messages
10,903
Run it close to wot until you use a gallon, refill that gallon.

I think I remember that it needs 50 to 1 at wot but only 120 to 1. So at idle you are burning about 3x what it needs if current is 40 to1. At wot you would be pretty close to 50 to 1.


Burn it baby burn it...
 
Top