Oil Mixing or Premix for 55 Gallon Fuel Tank

[AbeToor]

I have a Rigid Raider 19 with twin 2001 OMC 70-HP engine motors with automatic oil-mixing. I'm thinking about scrapping the oil mixing system and going to pre-mix but have some questions.

--With cost in mind, what are the most accurate fuel gauges for an integral 55 gallon tank?

--How do you maintain constant premix ratio in larger integral tanks?

--Other than reading smoke and flash point of a Molotov cocktail, can oil-fuel mixture ratio be accurately tested in the field?

Thanks,
Abe

[jimh]

I can't offer any real first-hand knowledge about which fuel tank level indicator system would be the most accurate. I suspect that sounding the tank via a ullage port would be the best method. We used that on our sailboat. We had a stinky, diesel-soaked wood dowel rod that was used to sound the fuel tank. This was a very low cost system; the dowel rod cost less than $2. Since most outboard boat fuel tanks usually don't have a ullage port, I don't think that method would work for you.

The only way to maintain a particular pre-mix fuel-oil ratio is by always adding oil in the same proportion to added gasoline. In that method, you rely on the accuracy of the flow volume indicator on the pump dispensing the fuel, and on your arithmetic in calculating how much oil is needed.

I have no knowledge of how one could measure the fuel to determine its oil and gasoline ratio once it has been mixed.

Note: the term "oil injection" is not appropriate for your outboard engine. Evinrude called their oil mixing system an oil mixing system (OMS). Mercury used to refer inappropriately to their system as oil injection, but there was no injection going on; it was the same system as OMC, a simple mixing of oil and gasoline upstream of the fuel induction device, usually a carburetor. More modern two-stroke-power-cycle engines like the E-TEC use oil-injection at numerous ports on the cylinder walls to provide proper oil distribution. Older two-stroke engines just mixed the oil with the gasoline and let it enter the cylinder that way.

[jimh]

I am guessing that your inquiry about conversion to pre-mixing gasoline and oil in the fuel tank is perhaps due to concerns about the OMS pump. The OMS pump does have many rubber or other flexible parts, and the one in your engine is probably 17-years-old. With today's gasoline-ethanol blended fuels, the OMS pump may be due for replacement. I looked up the BRP part number for a 2001 Johnson 70 engine: a replacement OMS pump is $590. I can see why perhaps you don't want to spend over $1,200 in replacing those fuel pumps. However, if the pumps on your engines need replacement, the non-OMS pump is $488--not much of a savings. For the extra $100 per pump, I'd stick with the oil mixing system.

[flymo]

Abe, why do you want to change over to run premix?

[AbeToor]

Thanks for the good input and "injection" clarification Jim, I'll give my options more thought.

Flymo, main reasoning behind premix is less mechanical chattel and hopes to reduce risk of offshore powerhead confetti. That said, aside from adding another algorithmic booby trap to my recreation, premixing a belly tank seems to leave room for error in the long run. Hard to imagine maintaining a stable 50:1 ratio after dozens of partial refills cap offs and seasonal down times with stagnant fuel in the tank. Thoughts?

Since the motors haven't been fired up in 15 years it's pretty good odds the pumps are toast, and as Jim stated, sticking with the OMS should be a modest added expense, albeit I'd replace everything including remote oil tank.

So far it appears kindred wisdom leans toward a new oil mixing system which I'm not opposed to, still just kicking the carcass for a pulse.

Any additional input for upgrading to OMS or premixing is appreciated.

Abe

[Vance's Revenge]

I chose to Premix my 1992 Johnson GT 200 since it was new. It is on my river boat and I run a jet pump on the motor most of time. I had found that motors with jet drives that were premixed far outlasted the motors VRO systems. The jets are so inefficient that they lose 30% of their power so the motors are ran at far higher rpm's on average. As an example; my boat is 20 feet long with a 6 foot wide bottom, when I had four customers plus myself and gear the boat would fall off plane when throttled down below 3500 rpm. That combined with the thirst of those old two strokes means a crazy amount of fuel being burned!

My motor is still running strong after 25 years which is amazing. I guided with this motor for over 12 years.

Premixing is simple and you don't have to be a math wiz. Just fill the tank in 6 gallon intervals in your mind. If you feel your getting close to the top start figuring in 3 gallons. 16 ounces of oil in 6 gallons is 50 to 1 which is the ratio you want for your motor. Of course that means 8 ounces in 3 gallons. My tank on that boat is a little over 40 gallons and I don't usually fill it completely. If you have multiple tanks always start with your main tank you are premixing so you don't get confused.

I would never use a gauge to determine the mixing ratio. None are accurate enough. With a 50 to 1 ratio there will be more oil at idle speeds than your VRO system so you will smoke a little more at idle but it is not a big deal.

Be sure and cap off the nipple on your VRO/fuel pump correctly and if I remember right you have to unplug the VRO warning wire connection.

Vance

[jimh]

There is no good reason to abandon the OMS system. It works better than pre-mixing. When you pre-mix you are prone to UNDETECTED errors. For example, you mix oil at the wrong ratio; or, you forget to mix oil completely. There is nothing in place to monitor for these errors. If you make them, they go undetected. The engine may be damaged by improper lubrication, or may run poorly due to too much oil in the fuel.

With OMS there are alarm systems monitoring for problems. If the oil reservoir runs low, you get an alarm. If the oil mixing is not working right, you get an alarm. If there is a fuel flow restriction, you get an alarm.

I had a 1992 V6 Evinrude with OMS. It ran beautifully. Around 2009 I sold it; it was then 17-years-old. I told the buyer he should plan on replacing the OMS pump in the next year or two. As it turned out, he did have a problem with the OMS pump at age 19-years-old, and replaced it. As far as I know, that engine is still running and running with the new OMS pump. It is now 25-years-old.

Pre-mixing is a pain with large tanks and large fuel purchases. My boat had a 77-gallon fuel tank. We often bought fuel in volumes of 50-gallons. You don't just dump in a gallon of oil and add some gasoline. You need to use a funnel at the fuel tank filler inlet, slowing adding the oil into the stream of the gasoline being added. It is a two-person operation. It makes adding fuel a troublesome operation.

If you have a boat with two 6-gallon plastic on-deck tanks, pre-mixing is not hard. You throw in a pint of oil and fill the tank with 6-gallons of gasoline.

[AbeToor]

Thanks Jim, still on the fence but the input is appreciated. Because I'm adding an additional battery, console space may be tight for the remote oil reservoir. I found a larger console from a Mako 19 to replace the micro unit Raiders were outfitted with which could resolve that concern though. Will have the new (to me) console after the new year.