how do i jet a 98' yz400f for 110?

3 replies to this topic
  • yz400fsmasher

Posted February 17, 2009 - 02:34 PM


im asking because i heard 110 octane fuel gives alot more bottom to mid range power. any suggestions?

  • grayracer513

Posted February 17, 2009 - 03:05 PM


You might find a 110 octane fuel that does that, but it won't be because of the octane number. Note that U4e2 and simi9lar fuels actually formulated for racing four strokes are only around 92-94 octane.

Octane number in and of itself has not one thing to do with power, or jetting, or anything whatsoever other than the resistance of the fuel to detonation under extreme pressure. In that regard, if your bike does not ping on 91, you will gain nothing from using a higher octane fuel than that.

It may well be, and in fact is, that there are racing fuels which can offer some gains in power, and many of these fuel are also higher than pump octane, but the octane is NOT where the power comes from.

Whether you need to change jets for a given fuel or not depends on the particular chemistry of the fuel itself. Basically, absent any recommendation from the fuel blender, you'll just have to try it and see.

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  • ace402

Posted February 18, 2009 - 08:22 AM


Typically you jet richer. and i'm 95% sure that you advance the timing because race gas tends to burn slower. Gray, correct me if i'm wrong but I believe thats the case.

  • grayracer513

Posted February 18, 2009 - 09:14 AM


Typically you jet richer. and i'm 95% sure that you advance the timing because race gas tends to burn slower. Gray, correct me if i'm wrong but I believe thats the case.

You are wrong. First, no one has yet said "race gas" was at issue, only high octane gas. Second, Race gas generally burns at least as fast as pump gas, if not faster. And third, octane number and burning rate are NOT related to each other in any way.

Altering the timing on a YZF can only be done by physically modifying parts of the ignition system. As built, it's fixed, and controlled by the CDI.

Some race gases are oxygenated. That is, they contain components that release additional oxygen into the combustion reaction as it takes place. These do often require richer jetting. Others jet just like pump gas. Some pump gases just yield less energy per pound, and need to be bumped richer because of that, but there is no rule of thumb as to one category of gas needing to be jetted richer based on what kind it is.

The only thing that will determine how a gas should be jetted is to run it, try it it richer and leaner, and see what it likes.

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