Jump to content

YZ125 Jetting Question.


Recommended Posts

Anybody running a really large mainjet in their YZ125? I've been running a 490 for a while but when I replaced my top end this weekend the entire underside of the piston was a black hot spot, not ashy but you can tell it's been burning premix. Also I can never get a good chocolate color on the plug, even after 20 hrs on the plug. It always comes out a really light tan color closer to the white side of the spectrum.

I mainly ride motocross at a fast intermediate pace but occasionally do an 1.5 hr cross country race through the woods. I've never had any spooge and just re packed the silencer and it was dry, rusty red color.

I thought I might have an air leak so I did a leak down test and over 30 min I only lost .25 pounds so the motor is all tight and sealed up.

I just never read or heard of anyone running more than a 450 main and I'm wondering why my bike would be so far off from others.

The bike is a 2014 with port work, milled head, squish set at .035. Running VP 110 with Yamalube 2-R@32-1.

Anyone out there running 500+ mainjets?

Link to comment
Share on other sites

Is it possible that a previous owner installed a JD kit?  I've seen that his custom needles are larger in diameter than stock, and he typically specifies a much larger main than stock, at least from the couple of kits I've used.

 

I have also had strange jetting issues with my 06, but not as weird as needing such a large main.  It ran fine all summer long with stock jetting (woods riding and racing almost exclusively--very little track), except that I was running the accessory 43 needle in a position that would be 1/2 leaner than stock.  Then the weather gets cooler, and the bike starts running rich and fouling plugs????!!!!?  I checked all the usual non-jetting things, and finally settled on a 400 main, stock needle 1 clip leaner, and 35 pilot.

Edited by rpt50
Link to comment
Share on other sites

@ Shawn250. I never thought about checking the float height because the bike runs so well. But I will check it tonight and adjust it to the specs in the manual.

@ RPT50. I'm using either the stock needle or the 1/2 clip leaner optional needle, can't remember off hand but I have it wrote down in my manual. Also a 37.5 pilot. 2 1/4 out on the air screw.

Link to comment
Share on other sites

Checked the float height and it was spot on at .400" above float bowl.

Here are a couple of pics, as you can see the bottom of the piston is very dark and not a lot of wash on top of the piston. Plug is also very light colored although it appears darker in these pics.

I think everything looks ok except for the bottom of the piston, maybe I should stop worrying about it, I've put 33 hrs on it without any problems. Just don't want to leave any horsepower on the table. image.jpgimage.jpgimage.jpgimage.jpgimage.jpgimage.jpg

Link to comment
Share on other sites

Well guys I put 2.3 hrs on the bike this weekend and made some adjustments to the jetting and timing. I went out for the 1st Moto and thought that the bike ran good but decided to try a 460 main the next time out. I reset the air screw and it ended up at 2 turns out so that was good and I headed out. The bike was a little more responsive and ran great. Then I advanced the timing a little and went back out, it ran even better. I made these adjustments after talking to Derrick with HP Bikes and he was super helpful. Thanks man!

Here's the crazy part... I just pulled the plug to see how it looked and it was a perfect dark tan color! The best color I've gotten on a plug since I bought the bike. The only thing I can figure out is that I had to have an air leak with my V Force reeds, I've read that this is a very common problem. I swapped them out with the stock reed block on this rebuild and then did the pressure test, so who knows?

Link to comment
Share on other sites

Create an account or sign in to comment

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

Reply with:

×
×
  • Create New...