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Old 03-17-2016, 08:44 PM
Koa Koa is offline
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Default 02 cr125 jetting for higher altitude

     

Hey there,

Right now I currently riding an 02 cr125 that I have jetted just right for where I ride which is around 1000ft. Breathing through an FMF gold series fatty and shorty pipe on the stock mikuni carb. Running 32:1 amsoil dominator and premium fuel. Br8 plug.

Question is, At the end of April I'm headed to Virginia city Nevada for a 4 hr offroad race that I have not done before. Im getting there late Friday and the race is early Saturday so I wont have time to fine tune the jetting. As far as I could figure out, the altitude there is between 5000-7000ft. That is a pretty big margin for this finicky mikuni. Any suggestions on jetting the bike for that race? At least to get be within range to where Im not fouling plugs all day or running it lean. Don't know much about the course but I hear there are some technical areas and also some open road for some distances where I'll be wide open. Im hoping I geared the bike right and now trying to figure out where to put the jetting.


Also, a spark arrestor is required so I'll be swapping my shorty pipe out for the turbine core 2 which may effect the jetting a bit.

Pretty risky jumping up to that altitude going in with blind jetting and not being able to run the bike up there before the race... hoping maybe someone has the same bike and rides that altitude..? Or anyone with suggestions.
Thanks
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Old 03-19-2016, 12:50 PM
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JMV JMV is offline
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You need to get a manual. This is the jetting chart from the manual. You can use it as a reference as to how much you need to change your main, pilot and needle relative to where you are at now.
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Old 03-19-2016, 11:07 PM
Koa Koa is offline
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Thanks

I do have a jetting chart. Problem is, I've used that chart when I first got the bike to jet for where I ride now in accordance to altitude and temps. First time out I fouled 4 plugs on a 3 hour ride and was so rich my swingarm was covered in oil. I ended up leaning out my main and pilot circuit with the air fuel 1-3/4 turns out to get it dialed in pulling through all rpms smoothly and no more fouled plugs. Basically it is jetted for 2500-4999ft according to the chart. But I ride from 900-1200. And the bike runs great with that jetting. Top end is pretty fresh and no other issues with the bike as far as I know. So for whatever reason, the pipe or whatever it is, that chart for me is just a basic reference that gets you close and takes the guess away. Maybe this mikuni carb is tired. pwk a/s is in the near future.

Ill get her figured out. Probably shoot as close as I can and take a few jets with me. I can change them out in about 3 minutes trail side.
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Old 03-20-2016, 09:12 AM
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JMV JMV is offline
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Like I said, use the chart as a reference. If the jetting chart tells you to go 3 mains leaner from the stock setting to 8000ft, then take your jetting and move it 3 jets leaner. Same with the pilot, needle and AS. The jetting chart is based on changes in air density with elevation and temperature change so even though their baseline setting might be a little off from what you have found to work, the impact of the density change will be in the ballpark.
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Old 06-13-2016, 11:17 PM
Koa Koa is offline
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Haven't posted in a while
Race in Virginia city NV didn't go as good as planned.
Lost compression about 25 minutes into a 4hr race. Thought it was a fouled plug, swapped it out and discovered I had no compression. Finally got the time this weekend to tear the top end apart only to find that a piece of metal got into the cylinder and trashed my piston and head, pinching the ring closed loosing compression. Cylinder seems to have made it out unscathed somehow. At least as a visual. Looks like maybe some bearing material or something from the bottom end. Some play in the connecting rod so it looks like I'm due for a full tear down.

On a good note, the bike was running good for the race so the jetting worked out. the plug had good color and felt great. Heat was not an issue.

I don't think I want to tackle the bottom end myself and planned to go with an EG big bore kit when this one got tired so now I'm wondering if I should just send him the whole thing for a full rebuild, new crank and a 144 kit. We don't have many reputable shops around here to do that work at a reasonable price.

Probably around 50hrs of hard trail riding on this piston.

According to the piston wash, it has been on the rich side. Which it has had some time running rich but recently have (according to ride and plug color) been right where It should be







Last edited by Koa; 06-13-2016 at 11:22 PM.
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Old 06-22-2016, 02:51 AM
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JMV JMV is offline
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Looks like your big end bearing failed, sending needles or parts of the thrust washers through the top end. The biggest you can big bore that year is to 134cc.
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