Look at the RB Designs head mod in the technical writings here, to see what is involved in what PEB is speaking of.
It and jetting are the only upgrades my engine needed. It ran fine, but the difference in fuel mileage and extra low end torque make this $85 head mod the best money you can spend on the engine.
I was pissed at myself for waiting over a year to do it to my 2006 DE300. Due to more efficient fuel burn and less throttle used, my fuel use was cut in half.
That's why I did it to my 2011 EC250 as soon as I had put three hours on the bike (to make sure it didn't have any issues I might need to fix while I waited for RB to do the head mod and of course the time it took for shipping both ways.) It only took about a week, which I used to put the guards on the bike and have the suspension customized; which was another thing I kicked myself for delaying on the '06.
You might want to check your float level. Too low, and your carb can spit fuel out the overflow on hard bounces. There are tech posts that are GasGas specific on this procedure, both here and on youtube.
Is your bike running well, except for poor mileage?
When you learn your jetting, you can compare it to what others are using.
I run a N3CJ needle with the clip in the second groove from the top (2009 Yamaha YZ250 jet needle) There are other's that will work just fine. It depends on what power delivery you prefer.
175 Main
42 pilot (slow) jet
#7 slide
I do most of my riding under 1k', seldom get over 3k' in elevation.
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