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Location
Timbuktu, WA, AU
So I’ve joined the pre-mix club with a ‘23 YZ250!
But the more I google, the more lost I get…
I’ve owned 4-strokes for years, done most of my own maintenance and I’m a Diesel Mechanic by trade so I have decent mechanical skills. But I’m an over thinker…
So my plan is to run Motul 800 @ 40:1 with pump 98 and switch from the stock pipe and muffler to pro circuit once it’s run in.
Do I need to re-jet it and if so, where do I start?
There is no easy answer to this since it will depend on how hard you ride and in what conditions. There are two things to consider, getting it to run well and then safe. Generally i think you could let it be if you run hardpack but should go up on jet size if you pin it in heavy sand.
This guy on youtube is quite handy with the YZ 2-strokes. Watch his videos if you have the time.
https://www.youtube.com/watch?v=fSDCIwX93Bg
Thanks mate, I will give some of his stuff a watch! I ride mostly hard pack, I definitely won’t be sitting on the pin and not looking to make it rip out of my hands. Just want it to be reliable and sound awesome!
Stock jetting on the YZ's is good, so start there.
With mods these bikes like more fuel, so look to increase the main up 1-2 sizes when you add the pipe.
Keep an eye on the plug as a guide, but I've found from testing with a mate on his YZ250 that these bikes aren't too finicky and work well across a variety of conditions with 1 setting.
Stock YZ250 jetting is pretty close and easy to fine tune. Running 40:1 (vs 30:1) is going to richen your AFR slightly so you'd need to lean the jetting a hair. 98 octane is also going to richen it slightly. Since you'll be on hardpack and don't want the bars ripped out of your hands, running it slightly rich will soften the snap.
If I was you, I'd start off stock and see where it's at and if you feel you need to tune it. I think you'll need to lean it out a little if anything. I'd have the N3EJ and N3CJ needles and 2 sizes up and down of the pilot and main jets on hand.
The Shop
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
Luxon 4-Post Bar Mounts
$189.95 - $239.95
It's true that they run well across a lot of conditions. I "jet it and forget it" on my '06.
Thanks for the info! The last thing I want to do is to end up damaging anything. Got a good starting point I think.
40:1 vs 30:1 is less effect of AFR than a change in jet size!
I generally have had great luck using the JD jet kits. They have everything you need and a detailed sheet of what to install for your elevation and riding conditions. Makes it super easy and never steered me wrong.
Hence my use of the word "slightly"
The OP is not familiar with jetting, so throwing in oil ratios having as effect, even small, on AFR is not helpful.
Stock jetting is close, but the OP is in Australia, our fuels are different, most bikes need to be leaned out.
Thank you.
Jetting is never perfect on a two stroke at all rpm and throttle settings, if the rpm is in the middle of the powerband it will run lean, at low and high rpm it will run rich...
Many four stroke riders lug their bikes out of corners, two strokes don't like being lugged, ride it in the powerband, your clutch is your friend!
Fuel octane is marketed differently in Australia and the US, in Australia Research Octane Number (RON) is used in the US Motor Octane Number (MON) is used, 98 octane (RON) fuel in Australia is equivalent to 90 octane (MON) in the US. The stock bike will run fine with 98 octane (RON) fuel.
The YZ250's jetting is close, slightly rich but rideable.
Welcome to the two stoke brotherhood!
Im looking forward to getting out there for a rip! I believe rich (as in, more fuel) is better than lean as it will run slightly cooler than running it lean?
Correct.
It will also run cooler if it's not ridden in the power band!
There is nothing like the sound and feel of a two stroke on the pipe.
I have a 2023 YZ250 that I am trying to figure all of this out as well. Stock jetting is 178/50 (I think), and after installing an FMF system I had my mechanic adjust mine to FMF's recommended spec:175/48. I ride small tracks in the Florida heat at sea-level. It doesn't bog, fires right up and it is a fire breathing dragon at any RPM.
My only complaint is the oil it spits. The FMF pipe/silencer do not fit snug together like the factory exhaust does, so there is a slight gap, ever so small but enough for unburnt oil to leak out down onto the swing arm. Even with the rubber exhaust-ring on it, it still drips. I have not fouled a single plug.
I currently run 927 @ 32:1, and the bike has about 6 hours on it total. I understand they dump a ton of oil in the mix on that first tank of gas for break-in, and maybe I lugged it a little too much when I first got it. I know 2-strokes by nature do not generally like to lug, they need to be rung-out. Still trying to educate myself on how to cut down on the spooge from tail pipe and pipe/silencer connection.
My brain says "go to 40:1, that will lean it up some because I am technically dumping LESS oil in the gas at 40:1", but I just learned that doing that makes it technically run more rich, whereas adding more oil like at 28:1 or 30:1 actually makes everything run LEANER because it's getting less fuel and more oil. I understand the piston likes more lubrication and you don't wanna lean it out, but I remember my KX250 in the 90's was ran at 40:1 and blew up twice.
Comments/Advice welcome here lol
To seal the pipe/silencer joint, push an 18x1mm O-ring...
https://www.ebay.com/itm/293855867794
into the pipe's join...
If you lug the bike and use lots of throttle while lugging the rear fender will get covered in oil, ride in the power band!
Thank you! You are the man! I was searching Home depot for one of these buy they don't have this exact size, I tried high-temp silicone but it was uneven and made a mess.
My mechanic keeps telling me "Get on the pipe and go man!"
Get some lower flashpoint oil, that 927 is gonna have dribble if you are not hammering the piss out of the bike and jetting not perfect for the occasion. Regarding the exhaust junction, use a KTM part or tighter fitting high temp hose with zip ties or clamps.
Pit Row
I'm not 100% sure of the size for your FMF pipe, you might want to buy a couple of sizes, smaller diameter o-rings will stretch.
The dealership gave me a bunch of Yamalube with my bike, I can switch to that for starters.
I used to have a chart of all the oils/flashpoint. Now that you say that I remember the 927 being high and is prob causing this due to my riding style.
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