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  #1  
Old 04-20-2011, 09:04 PM
FNG FNG is offline
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Default 01' YZ125 jetting

Stock:

460
25
6BGK9-75 3RD

Pro Circuit

410
30
STD 2ND


02' YZ125:


420
30
6CHY5-79
3RD

Pro Circuit

390
27.5
6BGK9-76
3RD


I plan to run 420 main with the 30 pilot. Stock needle in #2. 1000ft elevation 60-85 temps. 40 year old weekend warrior on a track with some trail.


Any experience or knowledge on the subject would be greatly appreciated.

Thanks
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Old 04-20-2011, 09:22 PM
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If you feel like one of the listed jetting is accurate at x elevation I have posted conversion charts for elevation and temp.

http://www.allthingsmoto.com/forums/...jetting-51176/

If you look at the bottom of that you will see a chart that you can figure out the conversion factor for any elevation and temp. The problem is you need to know what the IDEAL jetting is at some elevation.

If that's the case, then your good with this info, if not is there a reason your looking to jet? What does the plug look like? Do you get surging (fuel starvation) or fouling (IE a problem) that you are trying to correct?

Thanks
Scott
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Old 04-20-2011, 10:27 PM
FNG FNG is offline
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I like my machines to be dialed in stockers motor wise. Over the years I have done my share of porting, pipes, compression and the like but no more. I want it right, but not on the edge.


The 01' and 02' seem identical in regards to the motor/carb/pipe config. The part #'s are the same with the exception of the first digits which indicate the year of the bike, I think. The timing is the same as well so why is the jetting SOOOO different? The 01' and 02' seem like a 2 year run then Yamaha made changes again in 03'.

01' is the first year of the 38MM carb so did Yamaha miss the jetting that much? Seems so by PC jetting specs because going DOWN 5 sizes on the main is a huge drop UNLESS it was pig rich to begin with. Secondly the 02' is down 4 sizes on the main from 01' and PC went down 3 MORE sizes from there.

The only conclusion I can come up with is the 01' is way rich. And we all know PC is not going to send their pipes out with "on the edge" jetting so, again, there is more room in the jetting from there as well IMO. Yes, the bike runs good, a little spoogie, but I can't sleep at night knowing I could be 4 or more sizes fat on the main! LOL!


I was hoping some other riders on here would have experimented with this model and would have some experience to share.

Last edited by FNG; 04-21-2011 at 12:08 PM.
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Old 04-20-2011, 10:50 PM
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Scott IE Mxtras is the one who will know that info... I am sure he will chime in as he is the model specific Yamaha guy. He seems to know all the details of the blue beasts...

Thanks
Scott
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Old 04-21-2011, 12:12 PM
FNG FNG is offline
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Thanks for the info Scott. I will be waiting patiently for him to chime in.

Chris
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Old 04-21-2011, 02:07 PM
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Unfortunately, I went to 250's in 01, so I don't have records for the 125s.

Sorry!

I never take any jetting advice from anyone - I always develop my own data from plug reads and sound.

Scott
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Old 04-21-2011, 04:38 PM
FNG FNG is offline
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I agree completely with doing your own jetting and I plan to do just that but want a better baseline than stock. I figured if nothing else, I would find someone like yourself that can at least validate some of what I am finding and offer some wisdom or experience.

My past experience and the information I have found suggest I am on the right track. Being relatively new to MX bikes, but having 30+ years of snowmobiling. Modifying, drag racing and mountain riding with the usual porting, pipes, heads and big bores that go along with it I have a good base to start from.


If the info I provided was for a 250 instead of a 125, would you be in agreement with me, or would you go a different direction?

Thanks
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Old 04-25-2011, 08:43 AM
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The general rule of thumb is one jet size change for each 1000Ft of elevation, so if you are higher, you need smaller jets, so I think you are on the right trail so to speak.

Keep in mind your compression pressure can have a substantial impact on how the bike reacts to jetting. If compression is low, you are going to get more spooge no matter what you do. Some spooge is a good thing - more oil is a good thing, too.

Scott
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Old 05-10-2011, 10:34 PM
FNG FNG is offline
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After a day of tuning I ended up with the stock 25 pilot, stock needle in #2 with a 450 main. When it gets above 80 degrees I will be able to go down one more on the main.

Not gonna think about it anymore, when it stops raining I am gonna ride!
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