Tohatsu 50hp two stroke

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Post Options Post Options   Likes (0) Likes(0)   Quote Kandrew Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 1:21pm
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2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke.

As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves.

The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats.
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Post Options Post Options   Likes (0) Likes(0)   Quote Kandrew Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 1:42pm
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Originally posted by Steps Steps wrote:

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.
Not on a 2 stroke compression in a negative it robs HP. to improve a 2 stroke motors volumetric efficiency or its ability to use higher octane fuels is done by altering the exhaust port shape length and height in relationship to the combustion chamber and by improving the exhaust system. Big job to do on an outbourd motor.

Ignition timing is important on all engines but WOT timing is more important on 2 strokes.
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Post Options Post Options   Likes (0) Likes(0)   Quote Pcj Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 2:11pm
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Originally posted by Kandrew Kandrew wrote:

2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke.

As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves.

The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats.
Older 2s was told to run 95 or betterless smoke and more economical.running 95 I am saving roughly 2 ltd against 91. Advised by mechanic and South Auckland marine
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Post Options Post Options   Likes (0) Likes(0)   Quote Pcj Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 2:12pm
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And no change to timing.
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Post Options Post Options   Likes (0) Likes(0)   Quote smudge Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 2:20pm
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The ports take care of the intake and exhaust timing on a 2 stroke, effectively the same function as a cam in a four stroke engine. Two strokes do indeed have lower compression ratios as Kandrew says and that is why they are happy using lower octane fuels. The ignition timing is very critical and a pre-ignition scenario (detonation) is not what you want in a 2 stroke engine but you wont need to change it if you put in a higher octane petrol. Outboards are relatively low state of tune  beasts. A 90hp Etec is 1300 cc approx 60hp/l and CBT's little Kawasaki is 45hp from 350cc -approx 130hp /l. Bear in mind that bike is from 1972 or thereabouts and two stroke technology improved in leaps and bounds in the 80's with electronic ignitions, variable exhaust port timing, reed valves and carburettors that went directly into the crankcases. Things improved massively again with direct fuel injection and the corresponding refinement of all the other stuff. A modernish 250cc motocross bike engine puts out more hp than CBT's 350 from the 70's, close to 50hp. Of course you don't want to be rebuilding your outboard every 100 hours though. Be nice if the motor had lots of low end power too, which is easy to get when the hp per litre is lower.

But yeah to answer Pierro's question, Gull fuel potentially takes its toll on some rubber components due to the ethanol and crank seals and possibly injector parts are the things that will contain those. Plus we should ask a mechanic what the real truth is  Big smile
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Modern engines won't have CDI, that was a thing way back.

 My typo Should be the "power pack"
 Timing, change octane, the time it takes for the fuel to ignite across to  combustion chamber changes with octane, then time to max explosion changes which means a timing change.
Run an engine , espec modern engines, not timed correctly to octane, they die.
 Kandrew covers it well in his post above.

 So boils down to ...just run the right octane
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Post Options Post Options   Likes (0) Likes(0)   Quote Kandrew Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 3:05pm
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Originally posted by Pcj Pcj wrote:

Originally posted by Kandrew Kandrew wrote:

2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke.

As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves.

The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats.
Older 2s was told to run 95 or betterless smoke and more economical.running 95 I am saving roughly 2 ltd against 91. Advised by mechanic and South Auckland marine
ok fair call but 91 to 98 I wouldn’t except to see a saving with the price of 98. Remember not all 91s are the same and the ratings in the US which is what most of the manufacturers recommendations are based can be way over stated. No need to alter timing as you say.

But at the end of the day my 115 opti runs fine on 91 and it’s heaps cheaper because petrol prices are just ridiculous at the moment.
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Post Options Post Options   Likes (0) Likes(0)   Quote Pcj Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 3:35pm
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Originally posted by Kandrew Kandrew wrote:

Originally posted by Pcj Pcj wrote:

Originally posted by Kandrew Kandrew wrote:

2 stroke motor have a lot less compression than 4 strokes, for this reason they don’t make use of the extra benefits of higher octane fuel. Increasing compression in a 2 stroke robs the engine of hp because there is only 2 cycles and the extra compression competes with the engine ability to reach TDC on the compression/ignition stroke. So there is no benefit in using 98 in a 2 stroke.

As for timing, ignition timing can be changed on a 2 stroke, but dynamic or engine timing is set by height of the ports in relationship to the combustion chamber. No cam or valves.

The problem with ethanol is all the plastic and rubber parts in the fuel system that ethanol eats.
Older 2s was told to run 95 or betterless smoke and more economical.running 95 I am saving roughly 2 ltd against 91. Advised by mechanic and South Auckland marine
ok fair call but 91 to 98 I wouldn’t except to see a saving with the price of 98. Remember not all 91s are the same and the ratings in the US which is what most of the manufacturers recommendations are based can be way over stated. No need to alter timing as you say.

But at the end of the day my 115 opti runs fine on 91 and it’s heaps cheaper because petrol prices are just ridiculous at the moment.
using z 95 in car/outboard less smoke,starts easier,oil using the mercury brand silver something.  bio fuel contains who knows what,sugar cane,cardboardetc, ran bio in the ute(designed to run on it)13lt to 100km open rd,95  8.5 lt  to 100km open rd.
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Post Options Post Options   Likes (1) Likes(1)   Quote Pierre'o Quote  Post ReplyReply Direct Link To This Post Posted: 02 Dec 2021 at 4:10pm
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Cheers everyone. I'm staying with Z91. Same price as the (marina discounted) Gull 98.
I've drained most of the Gull 98 out of the tank and topped up her car. There's a wee bit of 98 in the bottom of the tank and in the fuel line (and carb.) Will fill this 45l tank with 91. That should dilute the last of the 98 - happy days.
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Post Options Post Options   Likes (0) Likes(0)   Quote Bounty Hunter Quote  Post ReplyReply Direct Link To This Post Posted: 03 Dec 2021 at 9:54pm
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i thought the number one reason to stay away from Gull 98 was the ethanol content?

my understanding is that petrol/gas is hydrophobic (repelling water) and ethanol/biofuel is hygroscopic (attracting water)

fuel mixes of up to b10 appear to be okay in cars that get used regularly and crew through it before it goes nasty - but in a boat that sits unused, the ethanol will continue to absorb moisture to the point where a phase separation of the fuel/water occurs

the puddle of water in the bottom of the tank will then start causing corrosion problems and possibly being picked up by the fuel pump and pulled through the fuel injection system
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Post Options Post Options   Likes (0) Likes(0)   Quote Pcj Quote  Post ReplyReply Direct Link To This Post Posted: 04 Dec 2021 at 3:09am
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Carburetors: O-rings and rubber carburetor parts on older engines tend to get hard and brittle when exposed to ethanol and then break off in bits and pieces causing clogs, misfires and shutdowns. ... Ethanol, however, has more oxygen and affects the air/fuel ratio, causing engines to run leaner and hotter.12/04/2012
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