I recently read an archived post on jetting and how jetting was dependent upon relative air density. Some friends and I were going to go up to Prescott AZ to do some trail riding and I knew my bike would run rich there because the elevations are around 5500-7500 feet depending on where you are. So the need to re- jet was apparent. I was taking my 02 CR 250, so I immediately went out near my house and tried to jet the bike according to the information I received from this website and Eric's book. I did the circles for the slow jet and jet needle. Then, I did the wide open throttle check for the main. Once I got all that the way I thought it should be I notated the following:
(T)emp = 31C
(P)ressure = 706 mmHg
(H)umidity = .27 or 27%
So now I understood that I should compute the Relative Air Density. So I used the following:
Air Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * ((P-MN*H)/760)
I was still missing the (M)oisture (N)umber or Saturated Vapor Pressure. This number I found in a chart that shipped with my Davis Instruments Perception II, which was used to calculate the atmosperic conditions above. According to the chart at 31C the:
(MN) = 33.74
Now I used the variables and formula from above to calculate the RAD for my jetting area just outside Phoenix. That number came out to be:
RAD = 1.06
So if I have done this correctly, my bikes jetting is at 100% for me when the RAD = 1.06
So, Extending this to Prescott. I get there and compute the RAD using the same method as above. The RAD at that time in Prescott was 0.95. Now I need to compute the percentage of change in RAD and use that ratio to re-jet for Prescott. If 1.06 = 100% or 1.00 then 0.95 = 90% or .89 rounded to the nearest hundreth. So my change in RAD is 10% or .10
In Phoenix I ran:
30 Slow
380 Main
Stock Needle 3rd Position
So in Prescott I should run:
30 x .90 = 27 (nearest jet size is 27.5) so 27.5 Slow
380 x .90 = 342 (nearest jet size is 340) so 340 Main
Stock Needle 2 positions leaner (1 position for every 2 Main jet sizes)
I ran this jetting and the bike ran well in High altitude. So did I leave anything out or not account for anything that I should have. Or is it possible that jetting can be this easy?
(T)emp = 31C
(P)ressure = 706 mmHg
(H)umidity = .27 or 27%
So now I understood that I should compute the Relative Air Density. So I used the following:
Air Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * ((P-MN*H)/760)
I was still missing the (M)oisture (N)umber or Saturated Vapor Pressure. This number I found in a chart that shipped with my Davis Instruments Perception II, which was used to calculate the atmosperic conditions above. According to the chart at 31C the:
(MN) = 33.74
Now I used the variables and formula from above to calculate the RAD for my jetting area just outside Phoenix. That number came out to be:
RAD = 1.06
So if I have done this correctly, my bikes jetting is at 100% for me when the RAD = 1.06
So, Extending this to Prescott. I get there and compute the RAD using the same method as above. The RAD at that time in Prescott was 0.95. Now I need to compute the percentage of change in RAD and use that ratio to re-jet for Prescott. If 1.06 = 100% or 1.00 then 0.95 = 90% or .89 rounded to the nearest hundreth. So my change in RAD is 10% or .10
In Phoenix I ran:
30 Slow
380 Main
Stock Needle 3rd Position
So in Prescott I should run:
30 x .90 = 27 (nearest jet size is 27.5) so 27.5 Slow
380 x .90 = 342 (nearest jet size is 340) so 340 Main
Stock Needle 2 positions leaner (1 position for every 2 Main jet sizes)
I ran this jetting and the bike ran well in High altitude. So did I leave anything out or not account for anything that I should have. Or is it possible that jetting can be this easy?