Yamaha 3.5 air cooled 2 stroke - what oil/ratio?

Molteni

Molteni

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Just acquired an old (1970s?) Yamaha 3.5 air cooled 2 stroke outboard motor.
Runs well :)
The casing says 1:100 oil added to the fuel but the previous owner said 1:50
What's the recommended ratio and is there a recommended make/type of oil?
Also, are spares available for this age of O/B?
Thanks in advance.
 
Molteni said:
Just acquired an old (1970s?) Yamaha 3.5 air cooled 2 stroke outboard motor.
Runs well :)
The casing says 1:100 oil added to the fuel but the previous owner said 1:50
What's the recommended ratio and is there a recommended make/type of oil?
Also, are spares available for this age of O/B?
Thanks in advance.
Click to expand...

I dont think you will have much luck finding spares except from a secondhand dealer such a Bill Higham Marine. Yamaha don't appear to support anything any earlier than 1984.

I think you should use a TCW3 2 stroke outboard engine oil. Any idea what the PO used ? TCW3 or an air cooled 2 stroke engine oil ? Quicksilver is a readily available brand of TCW3. Premium grade is adequate.

If you are gentle with it and winterise it properly you will be able to run it on 100:1 but not a fraction less oil. If it's is been running on 50:1 satisfactorily keep using that... it's the safe mix for all premix outboards since the late 1960s. 50:1 will leave enough oil coating the internals for it to survive even if you forget to fog it at the end of the season. If there is any tendency to foul the plug try 75:1.

I'd guess a SAE 80 or 90 outboard gear oil for the gear box.
 
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If one is to compare equal volumes of 100:1 and 50:1 petroil mixes then the 50:1 will contain less fuel, consequently weaker mixture. Weak mixture = hotter running. What is the consensus?
 
Well done on finding one of these in good working order. I ran one for over 25 years up to 3 years ago. Luckily I had another which I robbed for spares to keep the first one going. A great outboard: 2 stroke, air cooled (no water pump to flush or go wrong), twist grip throttle so you can sit well forward in the dinghy and control speed with tiller extension, proper all round tubular carry handle, decent sized fuel tank, 3 shallow water drive angles at the push of a lever, forward & neutral ...... Wish they could make outboards like that now!
I always ran mine on 100:1
 
Dave100456 said:
I think the OP is wanting to know petrol/oil ratio. Are you referring to air mixture?
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If you reduce the petrol content for a given volume of mix then one will reduce the air mixture. The same volume of air will be drawn through the venturi sucking the same volume of mix through the jet albeit with less petrol leading to a weak mixture. See other thread concerning British Seagull, when converting from 10:1 to 25:1 then the needle is replaced for a different profile. The same applies in reverse.
 
third engines said:
If one is to compare equal volumes of 100:1 and 50:1 petroil mixes then the 50:1 will contain less fuel, consequently weaker mixture. Weak mixture = hotter running. What is the consensus?
Click to expand...

third engines said:
If you reduce the petrol content for a given volume of mix then one will reduce the air mixture. The same volume of air will be drawn through the venturi sucking the same volume of mix through the jet albeit with less petrol leading to a weak mixture. See other thread concerning British Seagull, when converting from 10:1 to 25:1 then the needle is replaced for a different profile. The same applies in reverse.
Click to expand...

Making big changes to very oily mixes will affect the running eg changing a Seagull from 10:1 to 25:1 requires modifications to Amal and Villers carbs and as I have found serious re tuning of the fuel air mixture however I don't think you will find any significant changes with less oily mixes . After all usually a more oily mix ( double the oil) is recommended for commercial operation and during the break-in period..
 
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I have one of these engines and use 50:1. Starts every time, even after a winter with old fuel. The engine number dates it to 1980. Only drawback is it is a bit noisy.
 
I had one, it was probably the most reliable outboard ever, until the day i had it serviced, dont know what he did to it but it was never the same after that. I stripped it & rebuilt it including new crankcase seals, there was little, wear everything was spot on & yet i never got it to run again as it once had. In the end it beat me. I took it to a boat jumble & sold it cheap to a giant tattooed bloke whose knuckles dragged on the ground. "Does it start" he asked, I was honest & said "It is a pig to start but it might if we are lucky". Of course the bloody thing started first pull & he bought it on the spot, he wandered of with it dangling from one paw. About that time i decided it might be an idea to give up boat jumbles!
 
I did a google on "Yamaha 3.5 air cooled outboard 2 stroke manual"and there is a lot of info there.

Maybe the Mariner 3.5 is a re-badged Yamaha according to one suggestion. Fuel mixtures are also mention there.

clive
 
third engines said:
If one is to compare equal volumes of 100:1 and 50:1 petroil mixes then the 50:1 will contain less fuel, consequently weaker mixture. Weak mixture = hotter running. What is the consensus?
Click to expand...

There will be such an effect, with marginally more likelihood of detonation from the weaker mixture. But the effect is slight: I'd take it seriously with a hard-run Yamaha road bike of the same vintage, much less so with a mildly-tuned outboard. As Vic notes, 50:1 is the design fuel/oil ratio in certain situations, anyway. If the OP has any concern it's a simple matter to do a plug chop (tip: take oars ;)). A photo will give a better idea of ideal plug colour than words can: Google will find plenty on-line.
 
coopec said:
I did a google on "Yamaha 3.5 air cooled outboard 2 stroke manual"and there is a lot of info there.
Maybe the Mariner 3.5 is a re-badged Yamaha according to one suggestion. Fuel mixtures are also mention there. clive
Click to expand...

It is, with very minor differences such as with or without a drain on the carb bowl.
On my last boat, it was the only o/b that would fit in the locker. I went through about seven of them (both Yamaha & Mariner) because I was robbing parts to maintain them. The cheapest I bought was £32 which was cannabalised to get two others back into working order. There was a good market for selling the working spares. Eventually I found one that had been bought by an inland fisherman for use on a reservoir before he found out that motorised boats were not allowed on the reservoir. I bought it at 30 years old and never having had fuel in its tank! It is now in my garage because just after I bought it I changed boats. The current boat came with a Malta clamped to the back, leaving the 3.5AC with just 10 hours use in 40+ years.
Great machines, but as someone said - a little noisy.
I do have some data on it somewhere in the archive if anyone has a specific problem other than the mix.
I managed to buy some spares from the very helpful Yamaha dealer in Lymington
Cheers
Bob
 
I have a similar ish small 2st. The manual says 100:1. I have alway used 70:1. Runs perfectly.

Maybe 12 years or more ago I had another the same which I ran on 50:1, which occasionally fouled plugs.
 
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