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10/11/2025
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I’m running ethanol free 93 Octane mixed 32:1 with HP2. I recently changed the main jet from a 178 to a 175 and the pilot from a 50 to a 48 in my Yz250. The bike runs crisper and more snappy in the low end. The bike with the stock jetting would wheelie from throttle in 3rd gear, but since changing the jetting it will no longer. Feels like there’s less power in the midrange now. Is this a clip position issue or should I go back to the 178 main jet? The reason I decided to rejet was heard that they run rich stock. Should I richen the needle clip position or just revert back to the 178 to see if that improves it? I have about 1.5 hours on the new jetting.
Just heard this the other day to change the needle and slide
I went ahead and swapped back to the 178 if that doesn't improve it I'll try 48/175/3rd Clip Position combo.
You're probably on the right track with the main jet. It's easy to change out and you can always go back quickly if that's not the right fix.
It rode a lot better, but when I installed the 178 I misaligned the carb into the intake boot. I fixed it before I rode it with the 178. I'm wondering if I possibly had an air leak from having the carb misaligned so the 175 main jet didn't get a fair chance.
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^Check the spark plug to see.
You can avoid the carb boot hell by removing the float bowl drain plug and accessing the main jet with a 6mm socket driver. This is what I do for main jet-only changes. It's far faster and easier; just loosen the boot clamps and swivel the whole carb assembly. Remove the three screws on top so that the slide cover comes off, but don't remove the slide. Now you can rotate the carb the other way so you can access the drain plug. It helps to remove the countershaft sprocket guard, and don't forget to ride out most or all of the fuel with the gas off before this procedure. (Or have a handy receptacle to capture the gasoline in the carb.)
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