Main Jets Q....

40Pickup

40Pickup

Senior Member
How many numbers between to rich and to lean, are there some rule of thumb? I understand, many factors can affect this.
59A-B std. One 94 carb.

Thanks, Jorgen
 
Well, one possible point to examine would be altitude. The 59A carb had .051 jts standard, and the bulletins suggest .049 for 5,000 to 10,000 feet altitude and .047 for 10,000 to 15,000. I remember seeing a "Nairobi carburetor" in the Canadian parts catalog, and Nairobi is less than 2,000 feet above sea level...
float level can significantly influence jet behavior in a given carburetor, and plugged air bleeds in the nozzle bars certainly would. The same circuit delivers additional gas through restrictions in the power valve circuits which act as additional jets under high load.
 
There is actually a reasonably wide range of jet numbers which deliver similar "seat-of-the-pants" performance. To some extent the variance is controlled by the type of carb. In other words, a Stromberg might demonstrate too rich or too lean with fewer jet number changes than a Holley. The Holley does a better job of atomizing and therefore will run, without misfire, through a wider range of jet numbers. Probably 3 to 5 numbers would not be noticeable to the average driver but could be seen with a proper wide band A/F meter or on a set of clocks such as found at the drag strip. However, the best drag strip jetting might not provide the best overall street performance.

Tuning for best overall performance, when only external settings can be changed, is largely a trial and error situation. Although we do know the general parameters for timing and jetting, we must keep in mind each engine and car can require slightly different adjustments.
 
John, thanks for your reply!
"Probably 3 to 5 numbers would not be noticeable to the average driver", sounds reasonable to me!

Jorgen
 
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