Short version first, trying to rejet 2018 YZ125. I’m using the instructions from “motocrosshideout” and everything is great till the end. The plug reading after going through the gears and hitting the kill switch. I pull the plug and there is no color change at all down in/on the porcelain. Its just as white as a brand new one. There is some very slight carbon on the side electrode. I have went from a 420 main to a 480 main and don’t see any difference. I even put everything back to where it was originally with another new plug and no change. Are you able to see some color change after such a short run? I don’t want to keep running it for the fear of it running lean.
Ok here is the in-depth. My buddy bought this bike, its in great shape and the previous owner bought it for his wife. They didn’t ride the bike much because it didn’t run good and was hard to start and the carb leaked as soon as you turned the gas on. The bike looks like it was maintained by someone that raced at least a little, suspension has been done front and rear with some inked in notes on top of the forks. Im pretty sure it had some sort of aftermarket pipe on it because the stock pipe that’s on it now doesn’t have a scratch\dent on it which is why I think it didn’t run good from the start (jetted for the aftermarket pipe). To start with I put a new jet block gasket and new float needle and seat in and set float height using the sight glass method. The would run and no leaks but was sluggish in the middle. So we decided to rejet. Bike came with 37.5 pilot 420 main the stock 43 needle and clip at the top position and the BR9EIX plug that was in it was covered black and wet. I looked up what was stock and put that in 40 pilot 430 main and same needle with the clip in 3rd position. Using the “motocrosshideout” method followed to the T. everything was going great bike running much better in the middle. Did the plug check and no color on the spark plug. Went to 480 main (the biggest I have) new plug and no change. I put all the original jets in and the clip back to where it was tweaked the air screw to get it to ideal right. Put another new plug (BR9EIX) in did another run and still no change.
Are you cutting the threads away to expose the porcelain?
no i haven't cut any plugs. i was looking at the pics that are on the jetting guide and the "desired tan color" shown has the tan color all the way to the tip. question i have is the plug that has the tan color all the way to the tip a plug that has been run a while? i just did a google search and found that if you cut the plug that color line should be around 2mm but that was from someone that was running race gas. I'm running pump 93 if i cut the plug should i be looking for the same 2mm line?
Same 2mm line tends to be optimal and the only correct way to read if a main jet is rich or lean is with a new plug and a plug chop. The tan color tends to appear on plugs that are run for a period of time.
Plug color reading and plug chops have minimal benefit for MX jetting. A wide open plug chop is only useful for setting the main jet. And the truth is we spend a small percentage of the time truly using the main jet on an MX track. Yes, I know some of you are going to say you are a true bad boy or you ride sand tracks and you are wide open 90% of the time, but if you had data collection you'd likely be shocked to see you're not. Also, things like fuel type, race gas, etc affect the color of the plug a lot and can lead you astray trying to chase plug color. With some fuels you are never going to get a "mocha brown" plug and will confuse yourself trying.
For the recent YZ125's, the stock jetting is very close and only requires small tweaks for fuel and conditions. I see you are in SC and I assume it's muggy hot summer there now. I suspect you'd be best off slightly leaner than stock settings. I would start by putting in the 42-74 needle on clip 2 or 3 and you'll probably end up back near those 37.5 and 420 jets. I assume you are running 32:1 and pump gas?
The Shop
Luxon 4-Post Bar Mounts
$189.95 - $239.95
Free shipping: VITALMX
DeCal Works Huge Plastic Inventory of UFO and Polisport kits.
That is the plug from running the 470 main jet. We are no where near pinning the throttle just don't want to burn it up.
If you're running these huge jets and still think the bike is lean, I'd start looking for air leaks.
The jetting that is listed above, should be damn close to what you need. If your plug looks that lean after using a 470 main, I'm going to say that you have a mechanical issue (air leak) somewhere in the engine, and not a jetting issues.
Post a reply to: jetting/ plug reading help