Why change Carb Jets?

[silvadw]

Hi all Carb experts
I am unsure as to what changing jet sizes does to a car, or what symptoms dictate when to do it. Do you have any general advice?
When is it required to change the primary and secondary jets for Idle/Main/Air correction and Emulsion tubes? The only thing I can find is a description on the movement of the mixture screw too far in/out being the idle is Rich/Lean. But is this the primary Idle or both Primary and Secondary idle?. What to do with the other jets e.g. does changing the secondary main, increase power at the top end on the revs?

I just acquired a 34 ADF and single plane manifold for my 79 spider 2000. It has the following primary/secondary jets
Idle 50/90
Main 120/135
Air correction 145/180
I didn't check the emulsion tubes.

Any assistance will be gratefully received. I visit Chicago, Austin Tx, and Northern Cali often enough to buy any respondent a beer or three (also goes for Curly in Oz).

Copied to Engine and Tuning

[baltobernie]

Entire books have been written on carburetor jetting, so it is impossible to answer your question in a single post.

But ... generally speaking, when you are installing a different carburetor on your engine, first you make sure that everything else is working properly; ignition timing and advance, cam timing (do the marks line up?), fuel system clean, etc.

Next, you jet the carb to specs closest to an engine similar to yours. Somebody here can probably tell you what the jetting for an ADF-equipped Spider is (was). Then, you begin with the idle circuit, making sure that secondary (if equipped) is completely closed and there are no vacuum leaks anywhere. You go up or down in size if you cannot achieve the correct A/F mix with the adjustment screw. The idle circuit on a Weber carb is something of a misnomer; it provides much of the fuel over the entire RPM range, so you've got to get it right before moving on to the main jet or emulsion tubes.

John Passini's "Weber Carburetters, tuning tips & techniques", ISBN 9781855207592 is a great resource.

[silvadw]

Sage advice, Thanks!!

[toplessexpat]

Here's an extract from the standard thoughts on Weber setup...

If the mixture screw is more than 2 1/2 turns out turns then the Idle jet is too lean (too Small). When the mixture screw is less than 11/2 then the Idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is not opened more than 11/2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it is really a lean condition. See pictures and notes in the tech 2 article supplied in the kit instructions, view and please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size. JET KITS are available if needed.

EXAMPLE With the speed screw set at no more than (1 1/2) turns in after contact with the stop lever; and the best idle occurring with the mixture screw set at 3 turns from bottom, indicates the need for a larger Idle jet. Achieving the best idle at under 2 turns indicates the need for a smaller idle jet.

The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation. This is the reason that the Weber should give a fuel economy improvement over most factory carbs along with significant performance gains. In the worst case you should not see a significant fuel economy loss over stock, while improving HP & Drivability.

It's never failed me running a variety of single carb set ups (34ADF,38ADL etc).

http://www.redlineweber.com/html/Tech/c ... _best_.htm for more.

A

[silvadw]

thanks Topless, i get to Austin & Dallas occasionally. Where are you expat from? I presume UK given the car history. for the link, i feel a little sip may be in order for when i next get that way

[toplessexpat]

Always up for a sip (or three).

Yep - originally from the UK, now domiciled in Houston and currently sat at Austin Bergstrom airport!