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Old 29 August 2006, 17:18   #1
Country: Austria
Length: under 3m
Join Date: Aug 2006
Posts: 5
Tohatsu M8A 8hp Fuel mix

G'day, i have just been given an unused 8hp tohatsu M8A. i has been under a mates house since new and has only been used for about 3 hours, i think it is a 1980's model. i was wondering if anyone out there knows what the post break-in fuel oil mix is for this particular motor. i have a feeling it is richer than 50-1. also does anyone know where i can find out what year it is. Thanks.

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Old 29 August 2006, 18:12   #2
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,618
According to the Tohatsu USA site, it's 50:1. Might be for a slightly later model, though; they had specs for the M8 (no "A".)

The only owners manual they have on the site is for the M8B.


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Old 29 August 2006, 20:30   #3
Country: UK - Wales
Town: swansea
Boat name: Too Blue
Length: 8m +
Engine: Suzuki DT225
Join Date: Mar 2004
Posts: 12,791
The more oil the longer it will last but the more likely you will be to foul the plug etc. Use a good quality oil and 50.1 should be ok for normal use - if you are going to use it hard it won't hurt to run at 30 or 25:1
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Old 29 August 2006, 23:49   #4
Country: Austria
Length: under 3m
Join Date: Aug 2006
Posts: 5
Thanks for the replies. i think ill run it on 50:1, i would think it has roller bearings so that will be fine.
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Old 30 August 2006, 07:01   #5
Country: UK - N Ireland
Town: Comber
Boat name: Gemini
Make: Tornado
Length: 6m +
Engine: Mariner 150 Optimax
Join Date: Aug 2004
Posts: 22
if you are going to use it hard it won't hurt to run at 30 or 25:1

While you are right about the plug fouling part, what you are suggesting about the fuel : oil ratio, in theory, isn't correct.

The problem is that running an excess amount of oil premixed in your fuel can actually make the engine run lean – the total opposite to what most people think. It’s a common misconception that more oil will make the jetting richer.

The oil lubricates the engine, but doesn’t fully burn in the combustion cycle so adding more oil does not make the air : fuel ratio (or jetting) richer in fact it does the opposite – it ‘takes up space’ in the fuel part of the air : fuel ratio thus reducing the amount of fuel in this ratio and leaning out the jetting. This is due to the fact the there is only a certain volume of liquid that can flow through the orifice of a jet and if there is a higher proportion of oil then there is a lower proportion of fuel per unit volume. What’s more the fact that you are also making the liquid more viscous, by adding more oil, further exacerbates the problem as even less liquid will flow through the jet while all the time the volume of air drawn through the carb is staying the same.

However, as you say, in practice it probably won’t hurt it - due to the fact it’s a low technology 8hp engine and the manufacturers tech to jet on the safe side anyway…
'You must always strive to be the best but you must never believe that you are.' Juan-Manuel Fangio
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