oktane???

[pellesmil]

I have a yamaha 75 hp OB. 92mod this is a enduro, a lowcost mod. (probably sold in Sweden) without el. starter.
My question for the crowd is: have you eny experience with low vs. high octane on this engine.
My choice is to go for 98 oct but since i can´t find eny info in the manual i wonder if others have had problems regarding this issue.

This is what i have dicovered during the last 3 years (octane related)

with 95 oct:
-kicback in start line at warm engine(arm= )
-at high water temp and after a while with high power, the engine misfires
and it seams like the ignition is to far advanced , no power at all over
1500rpm
-lower power in general
-more expencive to feed

with 98oct
-starts easier
-runs smoother
-more power (rpm+ 300 5knots )
-runs cheaper
-more expensive fuel (+5-7%)

In Norway we can still get 98 oct, but they are talking about skiping the 98,
and have the 95 as the highest

What i have heard is that they have stopped selling 98 in Denmark, how is it in
UK (and the rest of the world)????

Let me know please!

[Petrov]

Tjenare pellesmil;

Sweden still have 98

[jwalker]

I'd be surprised if you require more that 92octane (RON) for such an engine. It sounds as though you possibly have a fault or a mis-setting. From what you say, it may well be over advanced.

[bogib]

We have choice of the following
92
95
99+

Bogi

[TIM]

It sounds like a misfire with the engine to me.... and the high grade fuel is just 'covering up ' the problem....

From memory , it thing that engine is CDI ignition. so , proably no fault there, Have you changed the plugs ???? Can you change the plug leads....

If the plug leads are expencive , you can wrap insulation tape around them to see if this solves the problem... if it does , then change them

[pellesmil]

When i bought the boat 3 years ago it was just serviced, and some of the problems (misfire) accured 1/2 year afted that when the water temp was as high as 22 d. C (not often in Norway), the other "problems" (low power) are present at all temperatures.
I´m gonna do a full tuning soon, i guess any wrong timing will show up then.
I have a problem accepting that there are other serious problem with the motor, for eg. last winter i had my boat under a plastic wrapping and it was left outside with only new gear oil and some alcohol on the tank, then in april revealed after loads of snow and temperatures below -25 d. C.
At the third pull it started and ran like a dream.

Thank you for all answers

Pellesmil

[codprawn]

In the UK the most common fuel is unleaded 95 octane. 97 octane lead replacement is still around and super unleaded is 98 octane but stupidly expensive - then again ALL our petrol is stupidly expensive - 1.21 EU per litre for normal unleaded - 98 octane is about 15 eu more!!!

Although not many people realise it 97 octane proper leaded petrol is not banned but it is a very restricted supply.

The higher the octane rating the better usually but an engine has to be set up for it.

During the war Spitfires used 150 octane!!!

[pellesmil]

Back again after a tune up on my sweet 75 hp, ignition is suposed to be 2 deg. ATDC on idle and 22 deg. BTDC on full advanced.
The engine was quite ok at idle, around TDC adjusted it back to 2 deg. ATDC, at full advanced it was correct.
This could be the reason for the cickback, but maby i have to live with some of that, i guess it is not normal to have a 75hp as manual start.
Soon i´m gonna replace the sparkplug leads, just in case, and thru the fall i will be doing some tests on eqonomy vs. efficiency in the octane field.
Be prepared for some stunning resultes from Pellesmil´s nautcal test laoratories.
Have fun and ride the wave.

Pellesmil