83 YZ125 K Restoration

7/23/2025 5:16pm
YZ125KAUS wrote:
Well the rider is finished, motor ran in and I've been taking it for a few blasts.Its absolutely ripping and going well but I'd like to...

Well the rider is finished, motor ran in and I've been taking it for a few blasts.

Its absolutely ripping and going well but I'd like to know from jetting experts their thoughts on this plug after about 30 mins of riding. To me it looks rich. Currently at 1.5 turns out on the pilot screw, 290 main, 65 pilot and needle circlip in the centre position. Fuel is 32:1, using 98 RON fuel and motul 800 oil. Any advice greatly appreciated.

100002431910000243161000024320

That's a real nice resto job you did. Congrats on how it came out! The plug looks a bit rich. In order to get it dialed in you'll need to know which jetting circuit it was on when you shut it down. Doing plug chops is the best way to figure it out. Do them at 1/8 throttle, 1/2 and full throttle. Do them in that order, one at a time, and pull and read plug each time. 1/8 throttle will be the pilot jet, 1/2 throttle will be the needle jet/jet needle. and full throttle will be the main jet. You should do them in the specified order due to the pilot jet affecting everything else. That said, you'll want to see a light tan color on the center electrode insulator. A whitish tan is too lean, where a brown to black color is too rich.

1
YZ125KAUS
Posts
111
Joined
2/26/2023
Location
perth, WA AU
7/24/2025 5:58am Edited Date/Time 7/24/2025 5:58am
YZ125KAUS wrote:
Well the rider is finished, motor ran in and I've been taking it for a few blasts.Its absolutely ripping and going well but I'd like to...

Well the rider is finished, motor ran in and I've been taking it for a few blasts.

Its absolutely ripping and going well but I'd like to know from jetting experts their thoughts on this plug after about 30 mins of riding. To me it looks rich. Currently at 1.5 turns out on the pilot screw, 290 main, 65 pilot and needle circlip in the centre position. Fuel is 32:1, using 98 RON fuel and motul 800 oil. Any advice greatly appreciated.

100002431910000243161000024320
That's a real nice resto job you did. Congrats on how it came out! The plug looks a bit rich. In order to get it dialed...

That's a real nice resto job you did. Congrats on how it came out! The plug looks a bit rich. In order to get it dialed in you'll need to know which jetting circuit it was on when you shut it down. Doing plug chops is the best way to figure it out. Do them at 1/8 throttle, 1/2 and full throttle. Do them in that order, one at a time, and pull and read plug each time. 1/8 throttle will be the pilot jet, 1/2 throttle will be the needle jet/jet needle. and full throttle will be the main jet. You should do them in the specified order due to the pilot jet affecting everything else. That said, you'll want to see a light tan color on the center electrode insulator. A whitish tan is too lean, where a brown to black color is too rich.

Thanks for the advice. Just so it's clear in my mind, I'll put a new plug in the bike, start it up and let it warm up idling and then ride it with only 1/8th throttle input. Then switch the bike off and then read the plug and do a plug chop. Then repeat this process for 1/2 throttle using a new plug again etc.

7/24/2025 6:30am
YZ125KAUS wrote:
Thanks for the advice. Just so it's clear in my mind, I'll put a new plug in the bike, start it up and let it warm...

Thanks for the advice. Just so it's clear in my mind, I'll put a new plug in the bike, start it up and let it warm up idling and then ride it with only 1/8th throttle input. Then switch the bike off and then read the plug and do a plug chop. Then repeat this process for 1/2 throttle using a new plug again etc.

correct

 

1
YZ125KAUS
Posts
111
Joined
2/26/2023
Location
perth, WA AU
7/25/2025 10:43pm

This is were im at after a few hours of tuning today. Definitely heading in the right direction by the looks of things.1000024646.jpg?VersionId=

2

The Shop

7/26/2025 5:31am

Definitely looking better. I wouldn't go much leaner. I prefer to do my needle tuning based on throttle response and feel though, rather than plug chops. I do my idle jet and main jet with plug chops

1
YZ125KAUS
Posts
111
Joined
2/26/2023
Location
perth, WA AU
7/26/2025 8:52pm
Definitely looking better. I wouldn't go much leaner. I prefer to do my needle tuning based on throttle response and feel though, rather than plug chops...

Definitely looking better. I wouldn't go much leaner. I prefer to do my needle tuning based on throttle response and feel though, rather than plug chops. I do my idle jet and main jet with plug chops

Not planning on going any leaner. Will leave it there for a few rides and have another look. Needle is still in centre position at the moment.

2
YZ125KAUS
Posts
111
Joined
2/26/2023
Location
perth, WA AU
7/30/2025 8:01pm

Been out for a few more laps on the old girl and she seems to be running sweet still. Pulled the plug again after giving it a gut full and it's still looking a nice tan / brown colour so probably leave it there.

 

1000025062
3
8/2/2025 6:47am Edited Date/Time 8/2/2025 6:47am
YZ125KAUS wrote:
Been out for a few more laps on the old girl and she seems to be running sweet still. Pulled the plug again after giving it...

Been out for a few more laps on the old girl and she seems to be running sweet still. Pulled the plug again after giving it a gut full and it's still looking a nice tan / brown colour so probably leave it there.

 

1000025062

Looking good. Now if you need to adjust the needle, that'll be by feel. If it's blubbering between 1/4 and 3/4 throttle, lower the needle clip 1 position and retest. Repeat as needed. If it feels flat in the same range, like it's not pulling hard, then lower the clip 1 position, retest and repeat as needed 

1
YZ125KAUS
Posts
111
Joined
2/26/2023
Location
perth, WA AU
8/5/2025 3:00am

Who said 83 yz125k tanks are impossible to find ?  😉 

1000025481.jpg?VersionId=QBG VoPaU26SKiybgWPgcMiBJK81000025482
2
YZ125KAUS
Posts
111
Joined
2/26/2023
Location
perth, WA AU
8/7/2025 12:42am

And another tank i just got... taking my total to 6 now. I need help 😄 

1000025544
2

Post a reply to: 83 YZ125 K Restoration

The Latest