Help Yamaha Zeal - hesitation below 6000rpm

I have a feeling there is a whole thread dedicated to this, and several others... couldn't find it

Higher octane may provide a little extra power and/or fuel efficiency or they may not. It doesn't matter much which you use if you don't mind paying for it and (whether real or placebo) there is some perceived benefit. "recommended 91" does not mean "only" it is a minimum standard not a maximum one. Higher octane levels should be fine.

Prolonged ethanol mixed fuel would probably not be recommended for carburetors with rubber seals etc. I've run it on occasions when I felt like being cheap with no noticeable affect, but I haven't done it on a pre-1990 bike/car.

 

I find it curious that the change on Octane rating cured this misbehaviour - it sounds somewhat counter-intuitive...not saying it didn't happen and that the cause and effect wasn't correct, trying to get at the rationale for the conclusion...

Interesting document on the different RON (Octane) fuels - there is a small difference in Stoichiometric ratios between the 91 & 93 -> 95 % 98 premium

http://www.bp.com/content/dam/bp-country/en_au/media/fuel-news/fuel-octane-power.pdf

Speculation - Now this would only have noticeable effect if the carbs/tuning was on the edge of the acceptable fuel ratio envelope with lower octane fuels and then falling outside this envelope with the change in the stoichiometric (air/fuel) ratio with higher octane fuels.

From the above document
91 & 93 yields max power @ 12.5:1 Air/Fuel ratio, 95 & 98 yields max power @ 12:1 Air/Fuel ratio, so higher octane fuels would run very slightly leaner in a carbed system running unchanged between both fuels.

Given that emulsion tube wear tends to make the FZR/Zeal series run rich at lower revs, lets ignore that possibility as it's one more complication/variable.

Hypothetically raising the needles one clip position should cure the midrange stumbles with premium fuel if it was running just a bit too lean and struggling.

It would be an interesting test at the very least.

Everything else RON related in engines needs ECU's which can alter fuelling/injector duration and spark timing changes, none of which happens on Fizzers and Zeals.

I do remember the lowering of Australian RON fuel requirements and several decades ago there was the discussion which surrounded the whole normal versus premium fuels when they started appearing. At that time there was only one car which was capable of taking advantage of higher octane fuels - A Turbo Saab, which had an octane sensor and the capability to increase engine performance parameters to take advantage of increased ping/knock resistance...

 

Yeh tend to agree with that statement.

The theory is the theory but in practise there's a lot of variables that come into play as to whether or not you actually see the prescribed benefits. On the FZR250 I got 20km more out of a tank of 95 than 91 and all seemed to run OK. But no noticeable difference on the (fuel injected) 650.