Posts
29
Joined
6/3/2018
Location
Batavia, NY
US
So I’ve recently been considering correcting the squish on my yz125 (with the goal of more consistent jetting and fuel economy). I tested the current squish with the stock head. I did 3 tests and they all came back at .044 inches. The measurement was taken at the outer edge of the combustion chamber. The bike currently runs well on 90 ethanol free fuel with no signs of detonation after 210 hours on the head. Is it possible to have the squish tightened to .040 and still be able to run pump fuel? This is my first go at correcting the squish on one of my bikes, so any input/advice is appreciated. The bike is used as a moto/ harescramble bike.
Bone stock I recommend atleast a mix of c12/pump. They detonate with decent riders on pump. Just saying
Like @williamsmotowerx said, the stock YZ's can use a little better fuel with a fast rider and great jetting.
I cut my squish clearance on the YZ125 down somewhere between 0.027in - 0.030in and run a 50/50 mix of leaded race fuel and ethanol free pump fuel. That combo has run reliably in my YZ125s and bikes I've prepped for friends for many years.
We have 92 octane premium here and lots of local stations offer an ethanol free option.
At 0.040in to 0.044in squish clearance, you could mix about one gallon of 110 leaded into four gallons of ethanol free pump premium to protect your engine from any chance of detonation.
I don’t mind running 1 gallon of race fuel to every 4 gallons of pump fuel. I just don’t wanna have to run straight race fuel in it. I plan on tuning it after the shop gets the head back to me. I’ll play with the timing and jetting to fine tune it.
On a slightly different note, I didn’t think you could go as tight as .027 to .030 on the squish. Did you open the dome back up to drop the compression or just shave the head without opening the dome back up?
Chamber volume is not too small in my experience. I just cut them into that range, install a reed system, Bill's pipe & silencer or FMF pipe & PC silencer and jet them for 50/50 race gas / pump gas. The YZ125 runs really strong with that setup and they're super durable engines.
I built a 2021 YZ125 for my buddy's kid in August with 0.030in clearance, FMF, VForce... Easy to ride and lots of power from mid through top.
On the YZ250, I'm pretty careful with the chamber volume and I cut them to 0.045in clearance.
19.0cc on those is pretty aggressive while 20.5cc runs strong, but remains really easy to ride.
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Your location says NY. I seem to recall a few threads on here of guys saying race gas is hard to come by in NY.
both squish and compression affects how fast combustion completes which affects the ignition timing need. Increasing the squish means you also need to retard the timing. That increases power and avoids detonation.
NY is a big state but no problems getting race fuel from many locations…Upstate 1hr north of Albany, I can get Vp in 5 gal cans at various shops, 93 non ethanol at the pump, and Sunoco 110 leaded and 94 ultra unleaded at pump…we also have a ton of Mx tracks to ride….if it was not for 8 months of winter, I would never leave😂
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