Posts
110
Joined
11/20/2021
Location
Logansport, IN, USA
Race bike is causing issues and not sure where to start.
2020 yz250 pro circuit cylinder and head has 26 hours on the build.
Son was doing some hot laps yesterday and stopped for a second to take his gear off and the bike died… it would not restart, not even an attempt at a fire.
checked plug and changed it, has spark and wet.
tried starting fluid direct into cylinder and fuel. nothing!
did notice right away the compression felt down by just the feel from the kick starter? Even with this it should still try to fire?
pulled the head cap off and the inner oring was pretty torched but still in tact. Piston and cylinder still look good… not sure where to go from here besides ordering a piston and rings.
Would be worth checking your reeds if you are doing a top end. It shouldn’t make it not start, but does make starting much more difficult.
Also not likely but just checking the boxes, make sure your gas cap isn’t blocked up. I had a yz250 stop running but still had spark. Lots of trouble shooting to have a guy say “try my fuel cap, yours could be clogged”. Swapped em and fired right up.
hope you get it fired up asap
My buddy who was fast (raced pro sx) would break pistons in his YZ if he went more than 20 hours. There is a chance it cracked or skirt broke, might be worth taking the cylinder off as well and looking at the entire top end/rod situation. Good luck, hope it isn't major.
Take the cylinder off and look inside. Check reeds too. Maybe even check carb. Who knows maybe a jet worked its way loose and too much/little fuel actually makes it into the engine.
Got the cylinder off, piece of the bottom of the skirt broke off and I think moved the ring and shifted it to where it wasn’t lined up with the little pin or whatever it’s called to close the ring gap, half the ring was stuck flush and took heavy force to get the ring off the piston. Not sure how this happened ???
it appears the cylinder is okay. Thanks god because it’s a high dollar pro circuit deal 😵💫😵💫
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Where's the piece that broke off? Either in the bottom end or in the pipe most likely.
We are gonna go with out the pipe 😵💫😵💫😵💫
Why does it look like the piston crown was kissing the head squish region? It looks like a witness mark all around the crown of the piston. Also, there is another mark on the exhaust side of the piston crown where the piston smashed the debris up into the head.
Odd that it broke an exhaust skirt. Usually the intake side breaks from too much piston to cylinder clearance. Something doesn’t add up here.
Yeah I really don’t know? This engine was done by pro circuit and its at 26 hours. Only thing we did was go from 40:1 to 32:1 and maybe it leaned it out to much?
Btw no up and down play on the rod only side to side
Looks like it ate something.
You said the ring end gap had moved, is the ring end gap locating pin still intact? I've seen them pop out, doing similar damage.
Any chips in the cylinder plating?
I recommend when you go back together to go with the Racer Elite piston and not the Racers choice. It's a much better piston.
I've been through a bunch of YZ pistons. If you are hard on the bike I would change the piston every 20 hrs. I would also look at the Wossner piston over the Wiseco. I agree with DynoDan on the exhaust side. I have only broke the intake side of the skirts.
The pin was still there! No damage to the plating so maybe we got ultra lucky. The only thing I can think of is we added a sand tire to the bike and he was doing motos in deep wet sand without a jet change maybe it just got to hot?
What is the difference between the two pistons ??
Yeah, one size large main and a "9" series plug would be the way to go on a full mod in deep sand with a scoop. Those things will put some load on an engine.
The Racer Elite series basically have all the trick they have learned from the four stroke pistons. They are amazing. I'm surprised PC didn't recommend it.
THE 2-Stroke Piston Kit: Wiseco Racer Elite - YouTube
Little update, cleaned everything. Installed new Weisco piston. Moved needle to clip 3 in the carb. It has a 180 main 50 pilot.
hopefully moving to clip 3 from 2 will richen it up enough to be a bit safer “if that’s what’s did it”
fired up first kick!!!!
let’s hope it’s good to go.
There you have the reason why piston brands advice 15 hours max...
If you going the same way, we will see you back in a 25 hours. That clip lower is not gonna change it.
Well with post like this it's giving me a squint look at the plug and piston
Why cheap out on a piston with to many hours in a race engine (cilinder/head) what you had to pay for at PC?
Taking a risk with your son, hope That engine not gonna quit mid air.
Always good info here to broaden my horizons. Thanks for sharing!
Pit Row
We are going to be changing every 10 hours from here on out. Lesson learned.
If you do the Racer Elite piston you won't have to change it every 10 hours. Just the ring, maybe.
How long is that piston good for ?
I ran my first one almost 100 hours in about a year and a half. I can't seem to keep and hour meter working, but I've been running the one in my bike now about three years. I ride almost every weekend once the weather gets decent. I think I've done 6 ring changes on it, and it still looks and specs good. The armor fit coating is still intact at the last ring change around thanksgiving. The squish is set pretty tight at .043", but the compression is lower than a stock YZ. Maybe it's the oil, or air filters, but I've never seen a piston that wears so well.
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