Weber DCOE Jetting Criteria
Posted: Wed Jun 29, 2005 7:55 pm
Due to two-phase flow behavior the amount of air allowed to mix in the emulsion is by far the dominant factor controlling the curve of the mixture strength versus rpms graph. The tendency is to install a much larger air corrector jet then is actually needed. This approach is reinforced by the jet sizing relationship formula stated in the Weber Tuning Manual. Some simple testing of jetting combinations will soon prove that that formula to be dead wrong. What you want to see in a graph of your engine is one with the rpms on the vertical column and mixture strength along the horizontal and your's charting a nearly straight vertical line over the desired mixture strength. This is quite possible and easily achieved it turns out with some swapping and sucking.
The approach to take is to step down the air corrector jet in size until the measured mixture stabilizes at some mixture strength at all rpms and loads. Deviations of one part air to fuel is about as good as the carburetor can be expected to deliver though. When the mixture does not change much regardless of the load and the rpms you've found the nominal air jet size to use. Now adjust the mixture strength by sizing the mainjet to obtain the desired value.
The same procedure applies for the idle jets. Start with a large diameter F-numbered jet and step down in size with just the F-numbers until the mixture stays steady from just off idle until about 2500 rpms. Once you find the right amount of air flow then stay with that F-number and change the jet fuel size to get the desired mixture strength. You can do this measurement by removing the emulsion tubes and driving the car.
I'll show how this is all done in the proper sequence in the video I plan on doing. I know, I know, it can't be this simple, right? Oh yes, yes it is. Easy peasy stuff!
The approach to take is to step down the air corrector jet in size until the measured mixture stabilizes at some mixture strength at all rpms and loads. Deviations of one part air to fuel is about as good as the carburetor can be expected to deliver though. When the mixture does not change much regardless of the load and the rpms you've found the nominal air jet size to use. Now adjust the mixture strength by sizing the mainjet to obtain the desired value.
The same procedure applies for the idle jets. Start with a large diameter F-numbered jet and step down in size with just the F-numbers until the mixture stays steady from just off idle until about 2500 rpms. Once you find the right amount of air flow then stay with that F-number and change the jet fuel size to get the desired mixture strength. You can do this measurement by removing the emulsion tubes and driving the car.
I'll show how this is all done in the proper sequence in the video I plan on doing. I know, I know, it can't be this simple, right? Oh yes, yes it is. Easy peasy stuff!