Tempreture and jetting

MikeT

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#1
My bike, 1996 YZ125, has always run well, but lately it is blubbering a little and the plug is black. The recent rise in tempreture I know will make my bike run rich, but how many degrees is enough to make a difference? I know my bike is running rich because of the blubbering and black plug color. I did a full throttle hold and quick shut down to check the plug color and it came up black. My bike has one step leaner pilot jet and I raised the clip one notch. The rest is stock. Should I buy a new mainjet one size leaner and leave the clip where it is, or get a mainjet two sizes leaner and put the clip back in the middle?

1. Does one clip position equal one mainjet size?

2. How much does the tempreture have to go up to cause a need for a mainjet change?

My manual says that my carb has a 350 main and for higher temps use a 340. Am I "simulating a 340 by moving the clip one position??? Also do you guys change your main nozzle or just the main jet? I'm wondering if I should do both at the same time?


Thanks for your help MikeT
 
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#2
No, the main jet can't be "simulated" by any other circuits. Your best best is to buy the next smallest main jet and then do another full throttle run. Then once that part is right, adjust the needle and pilot. To adjust those circuits, you must do it by feel, not by plug color. For the pilot circuit, you might be able to clear it up by backing out the airscrew a little bit.
 

MikeT

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#3
What are your thoughts on the main nozzle?
 

RM_guy

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#4
It’s not just temperature

Humidity makes a difference too. My boy’s 125 runs great when it’s around 80 degrees at 50 to 70% humidity. We rode the other day when it was close to 100 degrees and 100% humidity. His bike had no low end and burbled on the top. By all rights I should have decreased the main and raised the clip but I knew (hoped) the temp would drop so I left it alone. I didn’t feel like messing with the jetting, it was too damn hot. I would have to change it back when it cooled down anyway and I’d rather it be too rich than too lean.

Buzz is right, you can’t simulate a main jet change by changing the clip. You need to lean out the entire system (air screw, clip and main jet) to compensate for the reduced oxygen content in the air due to high temperature and humidity. As Buzz said you can probably get away with just changing the air screw and not the pilot jet.

In any case, remember that you changed the jetting when it gets cooler and dryer. You run the risk of running too lean and seizing it you don’t rejet. I keep the bikes jetted for the “normal” temperature and just suffer through when it gets too hot. I hate to jet a carb so that may have something to do with it too. If you want to keep track of the different carb settings for various weather conditions you could just change the jets and settings before you ride.

Good luck
 

MikeT

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#5
I've been able to run in the winter with the pilot jet one leaner and it's fine. Now I'm looking at the main jet. You are right, come to think of it, the humidity has been 90-100% on the days I have ridden. I might just go one leaner on the main and watch it when it gets cold. The manual says to change the mainjet and nozzle when the temp goes above 25* Celcius (77* Far). Everyone knows we haven't seen 77 for about a month plus.