Please read my yz103 piston

Hello I have a 2002 yz85 with eric gorr's 103cc 52.5 mm piston big bore. I have 17 hours on the big bore and replaced the piston. How does my piston look? I'm not a piston reading expert like some of you guys here. I do see a hot spot on the underside of the piston. The top does have some wash but is white and ashy

The engine is set up for race fuel and low/mid porting. Bike has 170 psi compression after 10 kicks. I run 50/50 112 race gas and 93 pump fuel. For oil i have been using bel ray H1R at 32:1. I am at sea level and ride between 45-90 degrees. I am currently running a 138 main, 42 pilot and a NAPE needle from a kx85/100 on the 2nd clip. The needle is leaner then the stock yz85 NBKF and helps low/mid. I have ran this jetting for 5 hours. The plug is a nice tan color and dry. I would post a pic but I can't get my camera to focus on such a small item.

ImageUploadedByThumper Talk1382836729.474056.jpg

ImageUploadedByThumper Talk1382836755.690033.jpg

ImageUploadedByThumper Talk1382836776.670472.jpg

ImageUploadedByThumper Talk1382836812.487164.jpg

ImageUploadedByThumper Talk1382836833.267957.jpg

ImageUploadedByThumper Talk1382836854.515711.jpg

ImageUploadedByThumper Talk1382836868.858439.jpg

ImageUploadedByThumper Talk1382836910.505384.jpg

For the first 10 hours after I installed the big bore I ran 3rd clip 45 pilot and 142 main. At 12 hours I went down to a 140, 42 and 2nd clip.

This bike is a adult play bike. I'm 175 lbs and ride woods riding. This bike is always wide open as you have to ride a 85 that way. I sprung and revalved the bike for my weight and its a blast to ride! I ride this bike more then my 295 and 478. I want your recommendations on my current jetting and any suggestions on where to go

Thanks for your guidance.


Looks hot in the center of it and the under side is getting the dark hot spot.

That's  an unusual piston crown burn. I can barely see a definitive piston wash, at 17 hrs you should see more wash.. I'm also concerned why the center of the piston is completely clean of carbon. Doesn't look like detonation, but that doesn't mean it's not happening. You can be getting detonation and not hear it. What are you running for fuel ? What did Gorr recommend for octain ? I would email your pics to Gorr and ask him for feedback. Me personally would err on the side of safe and bump the main up a couple sizes and see how the engine runs. Where your a 175 lb adult flogging and 85 that engine is getting heavily loaded which festers detonation, and you don't want a burndown.

Edited by Polar_Bus

If you do nothing about this...the next photo will be a hole in the piston, lol. The temp in the middle of the piston is so high its just burning away the soot plus, the underside where the brown spot is...the fuel mixture is burning underneath...way too hot. You say you ride wide open???, hope your not using the stock plug, go out and buy 2 plugs that are colder in the heat range and get the NGK iridium, these plugs will burn off the center electrode before a meltdown happens. Also check for air leaks, carb boot, reed cage, all gaskets and seals. Lets us know, thanks!.

Thanks for the recommendations polarbus. For his race gas motors he recommends 100 octane, I'm at 102.5 with the 50/50 mix but I can bump that up to 75/25. This thing has alot of compression 170 psi at 10 kicks. At 15 kicks it goes to 180. When it was a fresh 85 it was 135 psi.

I can bump up the main a few sizes and lower the ignition timing which Should help. The plug is a nice tan but the ground electrode is the same color as the porcelain. Thats kind of weird. I will play with my camera and get a pic of it

I forgot to mention I did a bottom end at the same time as the 103. Also put new vforce3 reeds in as well. It holds 6 psi consistently so there are no air leaks. My air screw is at 2 turns with the 50 degree weather we have now. In the summer it's 2.5 to 3 turns out. If I go lower on the pilot it would be a 40 which seems really small. This bike stock is 48 pilot, 135 main and second clip NBKF. I'm using a NAPE off a kx85/100 at 2nd clip. I can run 3rd clip for a safety margin

I think you need to be running full on race gas not mixing it with pump just like my 06 144 gorr did for race gas, I have to run Trick 114 which burns in the bike at like around 110. So far I would say your lucky you didn't lock it up.

I can try full race has and see if that helps. On the phone eric says he builds the race gas motors off of 100 octane so people could run Aviation gas 100. I can try straight 112. I got the remnants of a 55 gallon drum left. Probably 20 gallons that I can't use in my car anymore. I didn't hear any pinging but the bike does have a shorty and is very loud. even louder then my 295. I will lower timing from 1.2mm to .9mm. Bump up the main to a 142 and run 3rd clip. Maybe switch back to the stock richer needle.

One suggestion make single incramental changes and test. Dont go changing multiple things at once. This way with a single change you observe and document the results and will know if your moving in the right direction or not.

Where is your timing set in thousands BTDC?

Currently I am at 1.2 mm which is .047", this is his recommended setting for the low/mid porting. Stock is .035" or .9mm. I am going to set it back to the stock .035" and run it for more control over detonation.

If this engine is like his 295's then he builds them with alot of compression. I have 170 psi on ten kicks with this 17 hour old piston. Hopefully with the retarded timing and more octane I can stop the detonation and then richen the jetting one change at a time.

All good advice here so far.


  • Looks hot on crown and under crown (lack of carbon and burning charge in crankcase)
  • No wash near the port entries (lean)
  • Plug as described seems hot (either lean, advanced, or too hot of a heat range).


As Polar says, 1 change at a time. I'd start with going up 1 size on the main, maybe 2, unless that looses a notiable amount of power. That would be the safest change, and in my opinion the most likely to get some of the heat out of this thing.  Does richening the needle a clip hurt midrange that badly? If not, I'd error on the safer side there as well.


It sounds like you have adequate octane. Unfortunately splitting the difference between the two fuels being mixed doesn't really work, but we don't have a better way to figure it. You're 102.5 estimate might come out 100, might come out 105. No way of knowing because of the chemical makeup of the fuels.


I would then also experiment with a colder plug. The way you describe the ground strap sounds like it's either too hot, or too advanced.   With a 96mm rod and 47.8mm stroke 0.9mm advance should be 14.1 degrees and 1.2mm advance should be 16.3 degrees. Not that big of a departure, but if it's on the ragged edge....

Anyway, see what a cooler plug does to the ground strap and overall plug color. If it doesn't seem to help, then back off timing closer to stock. A big bore should want a touch more advance though, so I don't think it's that big of a deal.

Thanks for the advise Adam. Richening the needle one clip does not make a drastic change in drive ability so I will start with that and one richer main jet. With the ignition retard and one cooler range plug that should be a good start. I'd rather have it safe from detonation then squeeze every hp out of it. I'd also rather have it too rich and work down leaner then run lean like it was.

Tonight im putting the new piston and doing a post rebuild leak down test before putting the carb and pipe back on. I Always like to know its sealed before running the bike

What would you guys advise for a service interval for pistons on this bike? I changed this one at 17 hours. I have a plan to prevent detonation. Should I do 15. 20 maybe? What do you guys think.

Looking at that piston you have a lot of things going in wrong direction. It shows a lot of hot blow by bellow rings, dark outside edge on piston and burnt center.

I think your jetting is off all way around. Pilot to rich needle to lean and main to lean.

If you have the right needle it should gain torque and increase mid by adding fuel.

But if your muddy on bottom it will blubber and feel lazy.

I don't setup any of my 85 through 112cc motors to run over 1 to 2 turns out on air screw.

If you change pilot to run at 1-1/4 to 1-1/2 turns out on air screw then find needle with the right fuel curve, much richer then you are then up your main it should clean up your piston and take heat out of the motor.

Second, your compression in psi isn't a lot of help to designate what fuel you should be running.

Your corrected and uncorrected compression ratio is what is really important.

Depending on your blow down and total exhaust duration could be 12-16to1.

Very few 112cc and smaller motors can benefit from straight race gas as the flame front speed needed at high rpm is hard to obtain with c12. Vp did not design there c12 for engines turning over 10,000rpm.

My theory is your trying to cover a rich pilot and slide by running lean needle and main and your missing out on a lot of potential bottom and mid because of it. You may have great over rev to but your not making any power.

Just use this theory, engine is nothing more than an air pump, the more air and fuel we can pump and trap the more power you can make.

Guys I put another 15 hours on this bike and put a new piston in. This time looked a lot better with the changes I made, no white ashy deposits on the top of the piston.

What do you guys think of this piston?

ImageUploadedByThumper Talk1424615328.131934.jpgImageUploadedByThumper Talk1424615364.948932.jpg ImageUploadedByThumper Talk1424615452.896438.jpgImageUploadedByThumper Talk1424615473.821679.jpg

ImageUploadedByThumper Talk1424615513.490767.jpgImageUploadedByThumper Talk1424615535.901878.jpgImageUploadedByThumper Talk1424615574.902319.jpgImageUploadedByThumper Talk1424615590.343662.jpgImageUploadedByThumper Talk1424615613.925438.jpg

I went from the previous 142/45 NAPE kx85 needle on the 2nd clip to the stock yz85 NBKF needle on the 2jd clip, I went to 145/45 air screw 1.5 turns out. Piston still has a burn mark on the bottom of the piston, so it looks like I'm still lean, but going in the right direction. The burn is the entire bottom of the piston crown.

The plug looks great though, and doesn't look lean so I'm confused about the burn on the crown.

I also retarded the ignition timing from 1.2mm to the stock .9mm, this eliminated the ashy deposits on the crown. I'm running BR10eg plugs

I'm Running 75 percent 112 octane Sunoco/ 25 percent pump 93 octane. Previously I was running 50/50 which came out to a 102.5 octane

I had eric gorr machine me a extra pump gas head which I put on now instead of my race gas head.

Squish with the race gas head measured at .065". I didn't measure the new pump gas head squish yet.

I'm running Klotz Super Techniplate at 32.1

I'd appreciate if you guys could read my piston, I always thought you want a dime sized burn on the bottom of the piston, Mines the whole length, I will post a pic of the plug too.

Can I get your guys opinions on this Piston. I'm heading in the right direction but I still got a large burn spot on the bottom of the piston.

Thank you


Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now