Tohatsu 50hp two stroke

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Post Options Post Options   Likes (0) Likes(0)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Topic: Tohatsu 50hp two stroke
    Posted: 26 Nov 2021 at 8:07am
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Can I change from 91 to 98 petrol safely?
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Post Options Post Options   Likes (0) Likes(0)   Quote Keith C Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 8:12am
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Why would you?
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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:17am
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Pierre, if the motor is only rated for 91 octane there will be no performance benefit from using a higher octane. I'm not a mechanic but have been around 2 strokes most of my life and have seen this discussion many times. What is your reason for wanting to change?
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Post Options Post Options   Likes (0) Likes(0)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:43am
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Thanks Smudge. The reason I'm asking is because there is only 98 available here at HMB, no 91. And it's a lot easier than dragging the boat to the nearest gas station. So it's a price decision, not performance. :)

Will the motor need a re-tune for 98? 

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Post Options Post Options   Likes (0) Likes(0)   Quote Pcj Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:43am
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presume you have a 2020/2021 model seeing how new boat. If it says run 91 stay with it,98 becareful as most are blended bio fuel type,BP/Gull are not sure about mobil. 
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Post Options Post Options   Likes (0) Likes(0)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:44am
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Hi PJC,
it's GULL.
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Post Options Post Options   Likes (0) Likes(0)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:48am
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The book says "Unregulated Regular Gasoline : R+M/2: 87 or higher RON: 91 or higher".
I have no idea what this means hahahahaha
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Post Options Post Options   Likes (0) Likes(0)   Quote Pcj Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:49am
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Originally posted by Pierre'o Pierre'o wrote:

Hi PJC,
it's GULL.
DO NOT USE 98 GULL. Talk to your O/B mechanic first,mine said run away.
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Post Options Post Options   Likes (1) Likes(1)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 9:55am
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OK, I'm running away....
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Post Options Post Options   Likes (0) Likes(0)   Quote spin king Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 1:10pm
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please explain reasoning
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Post Options Post Options   Likes (0) Likes(0)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 26 Nov 2021 at 5:55pm
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me, or PCJ?
My reasoning is convenience, but I don't want to damage my motor
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Post Options Post Options   Likes (0) Likes(0)   Quote neil_cb125t Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 11:21am
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So Im not a OB mech, im a motorbike guy. but my explanation.

The design of two strokes mean the cranks are lubricated by the oil that is either pumped into the crank directly or mixed with the fuel (which is pumped around the crank) to lubricate it. Two strokes are basically a "dry sump" hence why there is no oil to change like your std car.

As the crank in most OBs are lubricated by oil being mixed into the fuel, then being mixed around the crank - this means the TYPE of fuel plays a part in the process. 

WHY

A Two stroke crank has rubber seals that segregate each cylinder "web" off and ensure additional air does not get sucked in. Extra air being sucked in = leaner mixtures which will hole a piston and blow a motor.

The Ethanol blended fuels has been known to be "not as good" for rubber parts. On your car there are very few rubber parts that the fuel is directly in contact with. In your Tohatsu, there is. And they are pivotal to your engines reliability. 

Accordingly my old 1973 3 cylinder Two stroke Kawasaki motorcycle (very similar to an OB motor) should not run on ethanol blended fuel - even thou the 98 (octane rating) is desired, that blend is not. 

Octane rating (the number 91,95,98) is loosely about how stable the fuel is when it gets or is about to be ignited - which is more pivotal when you have a higher compression motor or a boosted/turbo motor. I.e your subaru WRX with a big turbo wants 95 octane Min.

I have attached a picture of a motorcycle two stroke crank - you can see the black rubber seals sitting in-between the main bearings. Note the only way to change these if they become damaged is to strip the motor all the way down to the crank as pictured, and then pull each part of the crank apart...........consider this open heart surgery and replacing the seals/valves inside the insides of your heart.



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Post Options Post Options   Likes (0) Likes(0)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 11:26am
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Cool, thanks. I've drained (mostly) the 98 out of the tank and will fill it with 91. Will the small amount of 98 in the fuel line and carb etc. do any harm?
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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 11:34am
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A Mach 111 CBT? Petrol with ethanol is also more likely to absorb water. Not a good thing for a modern outboard motor.
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Post Options Post Options   Likes (0) Likes(0)   Quote neil_cb125t Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 12:02pm
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Close - its a little s2a 350cc :-)
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Post Options Post Options   Likes (0) Likes(0)   Quote neil_cb125t Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 12:02pm
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na should do any harm - throw it in your car :-)
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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 12:42pm
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Originally posted by neil_cb125t neil_cb125t wrote:

Close - its a little s2a 350cc :-)

Nice, drum front brake?
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Post Options Post Options   Likes (0) Likes(0)   Quote Durban Quote  Post ReplyReply Direct Link To This Post Posted: 01 Dec 2021 at 5:09pm
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Unleaded fuel burns cleaner, extends engine life and promotes good starting by reducing the build up of carbon deposits .Premium fuels should not contain more than 10% ethanol
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 10:04am
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Not to use methanol fuels above a certain %,  will also be in the outboards instruction manual...
As  per comments above dont use it.. slow death to the engine.

Will the motor need a re-tune for 98?


Yes. timing is very critical, the days of swing back and forth on a dizzy (4S)( same  principle applies to any petrol internal combustion engine) or by 'ear' long gone pre 1980s
Timing on modern outboards.. at least 90s up.. is very sensitive.. and not like your old magneto lawn mower or vintage motor bike.
Get it wrong, can start stuffing pistons, over heat CD! and power packs...
 Again slow death..
Even back in the day, tune by ear, and or just swing on a dizzy, so many engines went down and usually got blamed on assembly or some other BS.

Timing is a combination of cylinder pressure (not ratio) at point of ignition, and fuel octane, at given rpms and load....octane also determines the speed at which the flame goes across the chamber...higher octane slower speed.
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Post Options Post Options   Likes (1) Likes(1)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 10:54am
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No you won't need to retune an engine for a higher octane fuel. That's my opinion, maybe an actual mechaniccan verify. Modern engines won't have CDI, that was a thing way back.
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