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Jetting a little closer look.


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Lately it seems there's been some renewed interest in jetting, this may be due in large part to RC's thread on the 300 kit and the use of a different slide. With the slide in mind I thought it might be helpful to provide a visual idea of how the individual jetting components effect the fuel mixture. The graphs we typically see for these components in jetting guides are informative but I thought a graph from JD's jetting guide might provide us with a clearer picture. The JD graph shows the difference in % of fuel and where in the throttle range that difference occurs. The box inside the graph provides the details for each jetting entry (max. of 5 entry's). The first entry shows up on the graph as a heavy black horizontal line at zero. The subsequent entry's then are portrayed on the graph as they compare to the first. If leaner they appear below zero, if richer above. I used the jetting from the stock 06' YZ 250 as the first entry (50 pilot jet, needle N3EJ in the 2nd clip position, 178 main jet, and the number 7 slide). The subsequent entry's are all leaner as that seems to be the direction most go from stock.

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This graph displays the jetting predictions for the following entry's.

2nd entry= blue line, represents a change in pilot size, from a #50 to a leaner #48.

3rd entry= green dashed line, represents a change of 1 clip position leaner, from clip 2 to clip 1.

4th entry= red dashed line, represents a 1 step leaner change in needle diameter, from N3EJ to N3EK.

5th entry= purple dashed line, represents a change in slide cut-away, from a #7 to a leaner #8.

And penciled in the entry at the bottom of the page= red line, represents a change in main jet size, from a #178 to a leaner #175.

Sorry about the size, was hoping for something larger, still I think you get a good idea of what affect a change has. It's clear that a clip position makes for a much greater change when compared to the other components, must be why they offer needles in 1/2 clip versions. On the other hand a needle diameter change of 1 size seems almost pointless, yet in 07' when Yamaha decreased it 1 size to the N3EW the mags raved about it. Just goes to show you never know until you try. ­čśĆ

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Very good information. Thank you for taking the time to do something like this. This type of visual chart is great for people like me. I'm not one of those guys that can read something and understand it without a visual.

You mention that Yamaha went to a leaner diameter needle in 07. I thought the N3EW was richer the the N3EJ? Unless I'm confused (which is very possible) and the decreased diameter is leaner.

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Very good information. Thank you for taking the time to do something like this. This type of visual chart is great for people like me. I'm not one of those guys that can read something and understand it without a visual.

Your welcome and I hear you on the visuals, I suffer from a lack of reading comprehension myself.

You mention that Yamaha went to a leaner diameter needle in 07. I thought the N3EW was richer then the N3EJ? Unless I'm confused (which is very possible) and the decreased diameter is leaner.

Yes the N3EW is richer and it does have smaller starting diameter.

Fuel flowing through the main jet is further metered by the needle due to the space left between the needle and the brass tube it resides in. The brass tube is often referred to as the "needle jet" or "nozzle". A smaller diameter needle will increases this space, which allows more fuel to pass by, thereby making the air fuel mix richer.

Here's a view from inside the carb. I took the measurements the best I could with the tools I have so while not perfect it's fairly accurate. I set the slide and needle along side so that I could see just where the needle and it's relative parts were at different throttle openings. The main jet is located at the bottom of the tube, The nozzle is the reduction at the upper end of the tube.

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In the pic above the upper blue line on the needle is the point where the needles straight diameter meets the needles first taper. The lower blue line is the point where the first taper meets the second taper. This is closed throttle, notice that the nozzle contains a portion of the straight diameter and a portion of the first taper.

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Now at 1/8 throttle we can see that the straight diameter has been removed from the nozzle. From this point on the starting diameter has no affect. This is basically what the diagram showed in entry #4.

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