Jump to content


Weber idle jet size help needed...

Recommended Posts

I have a set of triple 40's on my 240z that I'm trying to dial in.  I had a low RPM stumble that I cured by moving up in jet size from a 50F9 to a 55F9.  In the process of doing swapping idle jets, my idle mix screw is now only about 3/8 of a turn out and I'm having some rough idle issues and richness.  I'd like to try an idle jet that is right in between the 50F9 and the 55F9 but I'm not sure which one that would be - maybe a 55F8 or 55F2?  I've seen the chart showing the F# order from lean to rich (attached), but I don't know how a 50 series F chart compares to a 55 series F chart.  Is there any overlap or is it sequential?  BTW, the car runs great off idle.






idle jet sizing.jpg

Share this post

Link to post
Share on other sites
1 hour ago, 240260280 said:

What 40DCOE version do you have?  The later 150/151's have finer thread and the turn out at idle is usually ~ 2 turns.


Mine are OLD - type 2's

Edited by swa240z

Share this post

Link to post
Share on other sites

Fuel height is an important parameter affecting the slow the off-idle transition to the mains as well as the fuel richness when cruising on the highway.


30mm down to 27mm down are optimal for Webers.  It is a critical tuning parameter.  It is important to balance the fuel height with a friendly sized idle jet to get the best performance for the off-idle transition and the air/fuel ratio at cruise.  The number of turns on the idle enrichment screw are not important and come after finding the best idle jet and fuel level.


here is a nice blurb on fuel height: http://www.classiczcars.com/topic/44512-setting-weber-dcoe-fuel-level-with-caliper/



Edited by 240260280

Share this post

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Create New...

Important Information

By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.