R/C Tech Forums - View Single Post - Mid range needle?
View Single Post
Old 06-29-2005, 07:31 AM
  #11  
duneland
Tech Elite
 
duneland's Avatar
 
Join Date: Sep 2004
Location: NW Indiana, USA
Posts: 2,403
Default

AFM
A very well written explanation of carb function. I do disagree on a couple of small points. I don't think that the engine does 90% of it's work in the infield! In off-road, yes. On-road is another story. Especially as it relates to the low-speed adjustment. I have found that tuning for on-road requires that the balance between low-end mixture and high-speed mixture, must be very close. That is, operating temp is mostly governed by the high-speed adjustment, and low-speed needle must be adjusted for smooth transition from low to high. Where as off-road allows for a richer low-end adjustment to keep operating temps down. The additional RPM experienced in on-road racing, has a profound affect on my tuning approach. Driving style has a lot to do with it... If you pin the throttle comming out of corners, your low-speed adjustment has little affect of anything (unless it is way-off one way or the other) as the low-end needle exits the seat at approx 1/2 throttle. If you use a lot of partial throttle movement in the infield, then low-end setting would have a larger affect on operating temps.
Now, on to mid/spray-bar/seat postioning in the venturi. First, I don't think you can move it far enough (and remain within practical limits of the o-rings to seal, and other physical limitations) to provide any change in fuel atomization. The opening of the spray-bar would have to be very near the wall of the venturi, to have an adverse affect on atomization. Relative pressure in the venturi does not change drastically with lateral movent. It does change drastically with vertical movement.
Any attempt to change the point at which the low-speed needle exits the spray-bar, relative to throttle position, is conteracted by, the need to re-adjust the low-end needle. In other words; if you move the spray-bar to the left .5mm, you will have to move the low-end needle .5mm to the left (to keep the same mixture at idle). The needle will still exit the spray bar at the same throttle position as before.
duneland is offline