YZ125 Jetting for Avgas

jridout12
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jridout12
So I’m new to the 2 stroke world and I have no experience with tuning the carb. I have a bone stock YZ125 and I have Avgas easily available to me and I plan on running it 50/50 with premium pump just as a cheap piece of mind to protect against detonation and pinging. I also run Klotz super techniplate 32:1. I’m just wondering what’s some good settings people have found on the jetting? I’ve rode the bike once with this setup and it seems to load up in the low RPMs but runs great in the high RPMs. Which tells me I could probably go a tad leaner. I know this is beating a dead horse but any help would be much appreciated
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CPR
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CPR 7/15/2021 2:31am
7/15/2021 2:31am Edited Date/Time 7/15/2021 2:34am
If you’re getting the avgas at a decent price, there’s no reason why you can’t run it straight, no need to add unleaded.
As for jetting, just start with what you had with it crisp on unleaded and fine tune if needed, shouldn’t be much different. Hard to give specific settings because it changes with where you are etc.
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alien
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alien 7/15/2021 4:45am
7/15/2021 4:45am
I run av gas mixed 50/50 with pump gas in my ktm 150sx and my yz125 before that. On the yz125 I leaned the pilot to a 37.5 and went up to a 500 on the main. You can run LL100 straight but I felt it made the bike run flat, it was much more responsive mixed 50/50. Another problem I had running AV gas straight was keeping the oil mixed with the fuel due to the specific gravity being much lighter.
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jridout12
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jridout12 7/15/2021 10:23am
7/15/2021 10:23am
alien wrote:
I run av gas mixed 50/50 with pump gas in my ktm 150sx and my yz125 before that. On the yz125 I leaned the pilot to...
I run av gas mixed 50/50 with pump gas in my ktm 150sx and my yz125 before that. On the yz125 I leaned the pilot to a 37.5 and went up to a 500 on the main. You can run LL100 straight but I felt it made the bike run flat, it was much more responsive mixed 50/50. Another problem I had running AV gas straight was keeping the oil mixed with the fuel due to the specific gravity being much lighter.
I think I’m going to start off just leaning the pilot out a bit, it does load up pretty bad in the low RPM’s, everything 1/4 throttle and above seems to be fine
sandhills
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sandhills 7/15/2021 10:25am
7/15/2021 10:25am Edited Date/Time 7/15/2021 10:26am
Controversial opinion: The jetting shouldn't be too far off pump gas. The octane is higher which means it resists detonation but the energy content is very similar. Not sure how oxygenation compares but that would be the difference, I think it's also similar.

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Ozy
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Ozy 7/17/2021 12:56am
7/17/2021 12:56am
jridout12 wrote:
So I’m new to the 2 stroke world and I have no experience with tuning the carb. I have a bone stock YZ125 and I have...
So I’m new to the 2 stroke world and I have no experience with tuning the carb. I have a bone stock YZ125 and I have Avgas easily available to me and I plan on running it 50/50 with premium pump just as a cheap piece of mind to protect against detonation and pinging. I also run Klotz super techniplate 32:1. I’m just wondering what’s some good settings people have found on the jetting? I’ve rode the bike once with this setup and it seems to load up in the low RPMs but runs great in the high RPMs. Which tells me I could probably go a tad leaner. I know this is beating a dead horse but any help would be much appreciated
you don't need to run 100% AV gas, 50/50 with high octane pump gas is fine. you will need to lean the main jet with higher octane
cwtoyota
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cwtoyota 7/17/2021 9:10am
7/17/2021 9:10am
jridout12 wrote:
So I’m new to the 2 stroke world and I have no experience with tuning the carb. I have a bone stock YZ125 and I have...
So I’m new to the 2 stroke world and I have no experience with tuning the carb. I have a bone stock YZ125 and I have Avgas easily available to me and I plan on running it 50/50 with premium pump just as a cheap piece of mind to protect against detonation and pinging. I also run Klotz super techniplate 32:1. I’m just wondering what’s some good settings people have found on the jetting? I’ve rode the bike once with this setup and it seems to load up in the low RPMs but runs great in the high RPMs. Which tells me I could probably go a tad leaner. I know this is beating a dead horse but any help would be much appreciated
AVgas runs a lot better in that bike than pump premium once you dial it in.

A good starting point is the 2008 stock jetting for a USA model YZ125.
I'm pretty sure they changed the needle number in 2011 and later models and I preferred the '08 needle out here on the west coast.

2008 Jetting (USA model)
Air Screw: 2-1/4 Turns (out from closed position)
Slow Jet: #40
Needle: 6BFY42-74 (3rd clip position, from top)
Main Jet: #430


My notes say the 2011+ needle is the 6BFY43-74.

*A note from the 2008 manual:
changing from 6BFY42-74 to 6BFY43-74 has the same effect as a rising of 0.5-clip position


I am currently running Sunoco 110 leaded mixed 50/50 with ethanol free 92 pump at 32:1 with Yamalube 2R in all my two-strokes. My YZ125 has a milled head, a VF-3 Reed and a pipe and the following jetting works well on the west coast:

Air Screw: 2-1/4 Turns (starting point)
Slow Jet: #37.4
Needle: 6BFY42-74 (3rd clip position, from top)
Main Jet: #410
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