YZ125 2016 UK Jetting - Help!

PC recommended jetting is fine to use in the UK, it’s a safe base to start from.
Is your bike completely stock?

Low end throttle bogging is your pilot jet, if it’s too lean it’ll cause poor throttle response and bogging when snapping the throttle.

Wind the air screw all the way in, count how many turns it takes to bottom the screw out...wind the air screw out 2 turns, start the bike on the stand and test the new jetting.

If still no change then turn the air screw in 1/2 turn at a time and test after each half turn.

If winding the air screw in improves the issue slightly but not completely, then try going up a couple of sizes on your pilot jet...let me know how you get on

45 pilot will be rich for sure. Poke at it and try altering each circuit individually.

Start with the pilot circuit. Keep going down on the pilot keeping the air screw at 2 turns out until you get a "buwaaaaaap" when you stab the throttle". Then bump back up one pilot jet size. The "buwaaaap" means your too lean on the pilot. Don't worry, piston speeds aren't fast enough to cause a seizure while jetting the low end range. Hell, even wide open throttle on the stand with a super lean main jet won't cause a seizure. There is not enough load to get the exhaust gas temps up to seize it. Ride the bike with the pilot circuit "in the ball park" and evaluate throttle response.

Next, move onto the needle clip position. This is the most difficult. Chances are, changing the pilot circuit (leaning it out) will help, but the jetting will still need some improvement. With the bike on the stand, warm it up and slowly rev it taking note to how smooth the acceleration is. Fluffy and chunky sounding means too rich. Thin and dull sounding means lean. Too lean will also result in surging (engine speed increases and falls while holding steady throttle). Drop the needle on clip position (raise needle clip) to second position. Warm bike up and listen. Your goal is to get the engine RPM to match throttle I put in a smooth, seamless manner. I'd bet the second clip on the needle will be better. Experiment around with the option needle Yamaha gives you. I believe it's a half clip richer.

Main jet is pretty easy. Hold it wide open and get a good "clean sounding" rev until it hits max rpm. Too rich and it will never reach full rpm and cough/stumble. Too lean and it will basically shut off and cut out. A 420 main is probably a good starting point. It would take and act of God to seize a bike on the stand. Experiment on the stand to see what carb circuit does what and experience what "lean" sounds like and what rich sounds like. Spend an afternoon playing with it and riding it (short sessions) and evaluate. You can learn a ton. A properly jetted bike makes the throttle feel electric. The engine rpm matches throttle grip input exactly. Good luck! It sounds like you're 2 pilot sizes too rich and about a full clip too rich on the needle.

Direct YZ125 info:

Get the optional needle 6BFY42-74, start at clip 3.

Put in the 42.5 pilot and a 410 main.

You will be much closer to where you should be. You may need to go back to 420 or 430 for winter.