Yamaha 4hp 2 stroke Outboard Model 4AC Petrol Oil Mix ?

[Jim@sea]

I have a Yamaha 4hp 2 Stroke Outboard Model 4AC.
At sometime in the past it was sold by an outboard dealer as a "Refurbished" Outboard and they put new Stickers on it.
One sticker says 100:1 But when I read a manual it says 50:1. OK I know I can out more oil in the petrol but I would like to know what the mixture should be . Please ?

 

[Refueler]

50-1 .....

Many outboards changed from 50-1 to 100-1 in answer to Green lobbying .... but it was a move that not long after manufacturers reverted to 50-1.

Honest ..... 50-1 - you will not regret it.

 

[jwilson]

100:1 was a marketing choice, but it was not actually that good for the engine.

 

[Refueler]

100:1 was a marketing choice, but it was not actually that good for the engine.

As I said - it was a measure to placate the 'greenies' and to try save the 2str ...... in the light of 4str coming along.

 

[VicS]

50:1 is the safe mix for all premix 2 stroke outboards since the mid 1960s. Before then they usually used an oiler mix

Edit
I should have said most not all . Seagulls being the notable exception.

 

[Refueler]

50:1 is the safe mix for all premix 2 stroke outboards since the mid 1960s. Before then they usually used an oiler mix

All ?? There were still O/bds such as Seagulls being produced into the late 60's + with 25-1 ..... but I agree that generally 30-1 and up to 50-1 was common.

 

[jwilson]

All ?? There were still O/bds such as Seagulls being produced into the late 60's + with 25-1 ..... but I agree that generally 30-1 and up to 50-1 was common.

In about 1967 or 1968 I bought a new Seagull 40+: the manual specifically said to use lowest grade petrol with a 10:1 mix of straight 30 motor oil, which is what I ran it on for over 10 years, first on a dinghy, then on a small cruiser, sometimes hour after hour at full throttle, pushing a ton of boat. The most reliable and easiest starting engine I've ever owned. But noisy, smoky and oily......

 

[Refueler]

In about 1967 or 1968 I bought a new Seagull 40+: the manual specifically said to use lowest grade petrol with a 10:1 mix of straight 30 motor oil, which is what I ran it on for over 10 years, first on a dinghy, then on a small cruiser, sometimes hour after hour at full throttle, pushing a ton of boat. The most reliable and easiest starting engine I've ever owned. But noisy, smoky and oily......

Interesting as I had 3 Seagulls and I was told to run on 20 .. 25-1 ...

The only two downers ..... the flying rope that could inflict serious on others in dinghy ... so I used to tell them to look away and head DOWN !! Second was the fuel valve !!

 

[VicS]

All ?? There were still O/bds such as Seagulls being produced into the late 60's + with 25-1 ..... but I agree that generally 30-1 and up to 50-1 was common.

Interesting as I had 3 Seagulls and I was told to run on 20 .. 25-1 ...
The only two downers ..... the flying rope that could inflict serious on others in dinghy ... so I used to tell them to look away and head DOWN !! Second was the fuel valve !!

I guess I should have said most, not all

Seagulls from IIRC 1968 , can be coverted to run a 25 :1 with a change of metering needle or jets depending on the carb. Before 1968 they had different ( smaller ) main bearings which makes then unsuitable .
Towards the end of their reign some models were supposed to heve been suitable for 50:1 .... I don,t think any survived that .

I have converted both of mine ( '73 and '74 models to run on 25:1 ... Makes them a lot less messy

All the history etc is on the SOS website

 

[ChromeDome]

FWIW this Service Manual states 100:1 on pg. 2-2, oil type TC-W3 but as others said, that ratio might not be the best.

The manual does however cover a lot of other details so worth downloading if do not have it already.

 

[DownWest]

I have a 3.5AC Yam and have always run it on 50:1

 

[pyrojames]

I have one that I bought in 1996. I have always run it on 100:1, and it still runs fine, although it has had a couple of swampings, so it is a bit rattlely , but I cant blame that on the oil mix!