pre detenation problem 09 yz450f

3 replies to this topic
  • Darren450

Posted April 16, 2012 - 05:32 PM


I ride in the woods. And I find that when I come to a climb and don't have the room to hit it at a good speed. My bike pre detenates. Making it feel like i'm jerking up the side of the mountain. Front sprocket 13t and rear 49t. Buying a 51t and hoping it'll help. My manual says I should have 95 or higher octane. Our pumps only go to 91.
And my bike is stock all round
Some of my questions are:

What else can I do to elimenate the problem?
Would changing the stock exhaust to a full fmf exhaust help?
My local mx shop has 87 octane race gas high oygenated. Should I mix the 2 like half and half?
Do I need to do a jetting?

Any help would be appreciated.

And my yz has only 15hrs on it. Bought it brand new bike

  • grayracer513

Posted April 16, 2012 - 06:35 PM


The manual says you need 95 Research Octane. That roughly equates to 86 Motor Octane. Gas pumps in the US are marked with the AKI (anti-knock index) number, which is an average of the two (R+M/2), or about 90. Your pump 91 is all that you need.

Pre-ignition is caused by hot spots in the combustion chamber, not by low octane (unless it's really way too low). Detonation is caused by the heat and pressure in the combustion chamber rising to a level that exceeds the ability of the fuel to resist being ignited by it (octane rating). This normally occurs after normal ignition begins. There is no such thing as pre-detonation.

YZ450's running on any decent grade of pump premium rarely display any tendency to detonate or ping. What does happen at low RPM is that the engine's very low rotating mass and the fact that it fires every second revolution sets up a pulsing load in the chain that causes it to slap against the swing arm very audibly. That's much more likely what you are hearing than a pinging condition.

To eliminate it, use a lower gear, slip the clutch a bit, add a flywheel weight, or go faster.

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  • Darren450

Posted April 20, 2012 - 01:07 PM


cool deal man. Very informational. Definitely the best info I have gotten. Still pretty new at this. Learning as I go.

  • Yamaryder29

Posted April 21, 2012 - 04:39 AM


You may wanna try a 50 rear before you go to 51. I have a 50 on the rear and am very happy with it.

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