98 or 91 octane?

[SCTSCT]

I have a 17 Yamaha 90hp 4 stroke. Dealer recommends 91 octane. However I feel that the motor is not idling as smooth as I expected. It has gone for the 20h and dealer says all is fine. I wonder if this can be improved with 98? Or would 98 be bad for the motor?


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[scottar]

Check your manual. If Yamaha "say" it's ok, run a tank or two through it and see what happens.

[catshark]

stick with 91 as its recomended and would be regually topped up at the bowser meaning the octane is fresher, your just doing your dough otherwise, if you think it running not as smooth than you may need a second opinion and dont take the dealers word for it, find a old hand to have a listen to it but i would say petrol wouldnt be the cause, it reads like its only has 20 hrs on the engine, perhaps the engine is still 'tight' and needs more time to loosen up.

[up the creek]

definately get a second opinion, a good mechanic should be able to diagnose what it is, if its bad on 91 it might not matter what fuel, seems that way for mowers cars etc as they can be run on either and dont usually idle/play up on 91, id get someone to look at it. ascatshark says.

[SCTSCT]

Ok maybe I shd clarify. Idling is ok. But when I'm trolling at 700 rpm it does not feel as smooth.

Who can I bring it to for second opinion? In the shop when they run in tank it's always ok. I dunno if it's when out on ppb and there is some wave/swell, it affects the motor.




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[up the creek]

just a thought i dont know if its possible, but if you get a local mobile mechanic he may be able to meet you at the water and go out with you, i mean you pay by the hour surely a quick run is within reason to properly let him see what its doing, as u ask it is hard we all have gone from place to place (sometimes) and told the same thing which cost more and more and problem not fixed, so yeah see if you can get him in the boat.. hope that helps..

[stevej]

idling in a bucket v idling in gear with wave action and boat hull vibration two completely different things
they all seem rougher when trolling

unless your engine can detect octane rating and adjust timing to suit which next to non of them do your just wasting money
just buy the lowest rated fuel you can find with no ethanol

[Dignity]

idling in a bucket v idling in gear with wave action and boat hull vibration two completely different things
they all seem rougher when trolling

unless your engine can detect octane rating and adjust timing to suit which next to non of them do your just wasting money
just buy the lowest rated fuel you can find with no ethanol

Herein lies the problem, mate in the fuel industry says when they run low on 91 they often supply 95, very dependant on incoming fuel from overseas (a bit of a worry when most of our fuel comes from os). About 2 years ago most people were running 95 for about 4 weeks at one stage and didnt know it (also begs the question why such a price difference). If your motor was tuned with a tank full of 95 then it will run like crap once back on 91 All being equal most motors tend to be a bit rough at idle speed with a load on them.

[Jason Green]

Pretty sure it has a knock sensor in it and the ecu will adjust the timing dependant on what fuel octane is being used



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[SCTSCT]

Herein lies the problem, mate in the fuel industry says when they run low on 91 they often supply 95, very dependant on incoming fuel from overseas (a bit of a worry when most of our fuel comes from os). About 2 years ago most people were running 95 for about 4 weeks at one stage and didnt know it (also begs the question why such a price difference). If your motor was tuned with a tank full of 95 then it will run like crap once back on 91 All being equal most motors tend to be a bit rough at idle speed with a load on them.

So it might run smoother on 95?
As Long as it won't cause any harm I wouldn't mind trying it.


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[ranmar850]

As someone else said, they always run different at sea. if you are trolling in some wind/wave action,however small, you are asking the motor to maintain exactly constant rpm against a constantly variable load, at a very low power output. So there will be tiny variations in the engine speed, which you may be hearing as "not as smooth". Just a thought.

[the gecko]

Just had my Yammy 40 4st serviced by a yammy dealer, and he said to run it on 91.

Id been swapping between 91 and 98 over the years, and I always felt it revved a little too high on 98.

[TheRealAndy]

Herein lies the problem, mate in the fuel industry says when they run low on 91 they often supply 95, very dependant on incoming fuel from overseas (a bit of a worry when most of our fuel comes from os). About 2 years ago most people were running 95 for about 4 weeks at one stage and didnt know it (also begs the question why such a price difference). If your motor was tuned with a tank full of 95 then it will run like crap once back on 91 All being equal most motors tend to be a bit rough at idle speed with a load on them.

This. I used to do a lot of work for the refineries in QLD, its been that way for many years. Octane rating is a simply a measure of the fuels resistance to detonation, nothing more, nothing less.

If you are buying 98 for an engine that does not require it, your are just throwing away your hard earned.