Hi, I have a 2009 Husqvarna TE 250, spec states the engine as 12.9:1 compression ratio. The question I have, is what octane fuel rating is recommended for this bike? Thanks, Rob

Sounds like pre ignition Knock....Try going to a Higher Octane Fuel........ Any Octane under a 100, does not work well over 8.5 compression Ratio.......You could retard the ignition timing, a few degrees if you cannot get a higher grade of fuel.............

no
it is not mandatory to use any octane rated fuel
however if the manufacturer recommends 91 octane, then that will be the fuel most suitable to run the engine economicaly
If you use 98 octane there may be some noticeable improvements in performance but that comes at a much higher cost for the fuel
The general rule is
lower rated octane for lower compression engines , lawn mowers and the like
medium octane for the average range of vehicles
and high octane range for high compression high performance engines

the higher the compression ratio the higher the octane rating required
normal compression ratio 10 or 11:1 will run on 91 or 95 octane rating
higher will require 100-110 octane rating or aircraft engine gas
superchargers only increase the amount of air available for combustion
it is the compression ratio that determines the octane rating required

Hey LeDonta,
The octane rating of the gasoline has nothing to do with the "quality" of the gasoline.. quality is purity, quantity of bad "fillers", quantity of good "fillers", and cleanliness of the transportation tanker and the stations tanks in the ground.
the octane rating is a rating of how fast the gas burns in the combustion cycle... the higher the octane rating the slower the gas burns.. and the lower the octane, the faster it burns.. The compression ratio and ignition timing are the main factors in determining what octane an engine requires. The higher the compression ratio and thus, the more advanced the ignition timing, the higher the octane required. Racing engines (as a rule) require a higher octane...
USE WHAT YOUR OWNERS MANUAL SAYS TO USE!!!.. if it says 87,, use it, .. using 93 is a waste of money!!
Want to learn what is actually in gasoline.. read this:
https://en.wikipedia.org/wiki/List_of_gasoline_additives

welcome to usa forum.
no year, no country stated, (hopeless for sure)
we dont have KB's here, sorry!
so why not be the first and post country.?
as engine spec. vary by that.
Africa, not same as Australia. (etc)
no year, stated.
nor if engine is turbo or not.!
engine spec.s vary by lots of things, including compression ratio.
etc. and evolve in time.

Check with the dealer for the correct octane rating to use in you machine as it depends on the compression ratio and tuning specs

No, Your vehicle needs premium fuel.

you should have a factory affixed sticker inside gas compartment cover listing premium fuel only or also listed in your owners manual. Otherwise if you notice that you're vehicle is making a pinging sound while accelerating or climbing a hill for instance then you may want to step up to 89 or 93 octane and try to avoid purchasing fuel with ethanol added it's **** and degrades your fuel quality/octane ratio

You are experiencing detonation. Main causes are:

• Low octane fuel - Use higher octane rating fuel.
  
• Incorrect ignition timing - Readjust timing
• Too high compression ratio(due to engine mods such as skimmed head or high compression pistons) - replace modified parts
  
• Air/fuel mix too lean - Replace fuel filter and readjust.