Wondering about the need for "variable" oil mix.

[Dave1027]

I have an old 2 stroke 1988 Mercury 100 hp outboard. It has the auto-blend oil feature with an oil tank under the cowl and a small driveshaft driven oil pump that mixes oil with the gas before it's delivered to the carburetors. The oil pump varies the oil ratio with a linkage connected to the throttle. It's said to vary the ratio from 50/1 to 100/1 depending on the rpm, lowering the ratio the 100/1 at idle. I'm thinking the engineering behind that is to help the engine the idle with less smoke?

But I'm questioning the logic of this system. Think of it this way. I'm idling along, oil is mixed at 100/1. Suddenly I nail it. Now I'm at full throttle but I'm still using 100/1 mix in the carb bowls! I continue cruising at full throttle and the mix in the carb bowls has now increased to 50/1. I reach my destination and come off plane. Now I'm idling but the engine is still using that 50/1 mix.

I like the auto-blend feature so I don't have to manually mix the oil in the fuel tank. I'm thinking about disconnecting the linkage between the oil pump and the throttle and locking it down at 50/1. Is that a bad idea?

 

[Chris1956]

Newer oil injection systems do not very the oil ratio. My '98 Johnny runs about 60::1 all the time. The advantage is that I get exactly 60::1 and freshly mixed fuel.
A lot of people will tell you to deep six the Autoblend. I have no experience with it, so I have no opinion.
Outboards can idle quite well with 50::1 oil mixture, provided the carbs are clean and of a decent design.

 

[racerone]

On an 88 model 4 cylinder 100 the oil pump is driven by a PLASTIC gear .---Scary stuff that is and I have seen that gear stripped..-----But you are very correct in your thinking on the mixtures changing.---Bearings need just as much ( if not more ) oil at lower speeds-----It is also proof that many folks are not well versed in lubrication in 2 strokes.-----That motor will idle all day long at 50:1 mix.

 

[Faztbullet]

All the oil pumps on Merc are variable..even your OMS pump is variable.

 

[flyingscott]

I have an old 2 stroke 1988 Mercury 100 hp outboard. It has the auto-blend oil feature with an oil tank under the cowl and a small driveshaft driven oil pump that mixes oil with the gas before it's delivered to the carburetors. The oil pump varies the oil ratio with a linkage connected to the throttle. It's said to vary the ratio from 50/1 to 100/1 depending on the rpm, lowering the ratio the 100/1 at idle. I'm thinking the engineering behind that is to help the engine the idle with less smoke?

But I'm questioning the logic of this system. Think of it this way. I'm idling along, oil is mixed at 100/1. Suddenly I nail it. Now I'm at full throttle but I'm still using 100/1 mix in the carb bowls! I continue cruising at full throttle and the mix in the carb bowls has now increased to 50/1. I reach my destination and come off plane. Now I'm idling but the engine is still using that 50/1 mix.

I like the auto-blend feature so I don't have to manually mix the oil in the fuel tank. I'm thinking about disconnecting the linkage between the oil pump and the throttle and locking it down at 50/1. Is that a bad idea?

Your engine has been working like that since 1988 and still running strong. Why the change now? What about the bottom 2 cylinders that get VERY LITTLE oil when idling? I think you are overthinking this.

 

[Dave1027]

Thanks for all the replies. Gives me a lot to go on. The main reason I brought it up is because the linkage between the oil pump and throttle on my engine is breaking and I was thinking I'd eliminate that part.