Mixture screw adjustment
 

The purpose for this thread is to help myself and other understand certain aspects of the 48 IDA.

During my search for information to get these carbs to work correctly on a 427 FE it has been mentioned that the mixture screws need to be 3/4 turn to 1 full turn out to achieve proper tuning.

I have also read that the mixture screws must be set at 1 1/2 to 3 turns out for a proper tune.


From what I can understand the mixture screws are merly a volume screw for the mixture that has already been set by the air holder and idle fuel jet.


Comments have been made that the mixture screw is only for idle. I would like to add that it is only for the idle circuit. Driving at 2000 rpm the mixture screws do have an impact on the operation of the carbs.


Again, I am not out to prove anyone wrong, just looking to start a friendly thread in the operation and understanding of the 48 IDA.

I have seen many time including myself of people asking for help in tuning their Webers and only to receive partical information with no end result if it worked for them or not.


Bottom line I agree with 1 comment that has been made in regards to the 48 IDA, "Every Engine is different even if it has the same exact componets"

If you have were able to apply a standard jetting to A 427 fe with successful results of a final tune, please post your experience.

I am pretty sure, that the Weber tuning guide suggests to have the idle screw open about 3/4 of a turn to 1 turn.

On my car, the best idle is achieved at about 3/4 turns open, but that setting leads to exhaust popping on deccel and ocasional light "sneezing" at cruise .

Opening them up another 1/4 turn gets rid of the popping and sneezing.

My take is that the idle screw does have a noticeable effect as long as the car is running on the idle circuit. (up to maybe 2600)


The five biggest improvements were:

- getting, reading & kind of understanding the Weber Tuning Guide

- reading Eljaros Posts

- documenting each change in an Excel table

- Drilling the 3rd progression hole

- Opening up the venturis to 40 mm

The idle mixture screw adjusts the volume of air/fuel emulsion discharged through the idle mixture port as set by the idle fuel jet/air bleed combo, and works in the same principle on any fixed choke carburettor, more critical on IR manifolds with one venturi per cylinder.

The mixture screw still contributes while going into progression and does have a bearing on progression mixture, but as the throttle opens further it's contribution is proportionally less, depends on number of progression holes, certainly critical for the "popping on decel" syndrome, so as others have said in other threads 3/4 turn out is the norm.

Any more than say 1 1/4 turns out says the fuel jet is lean.

This general rule applies to any performance Weber carb, IDA, IDF, DCOE etc.

By staying with the notion of 3/4 to 1 turn on the mixture screw, I would like to know how your car truely idles. For instance, many people on CC have been using the magic values to tune their FE. If I am not mistaken it was as follows:

120 holder
60 idle jet
210 air corrector
F 7 e_tube
160 main
37 choke
55 exbypass
45 pump jet

With this setting many people felt this was the proper jetting but only to find out years later thanks to Eljaro that there was a completely different setting for the proper mixture.Which every engine will more likely have a different value.

This magic setting for years must have fouled plugs, poor drivability, poor fuel economy along with incorrect fuel mixture.

I have noticed that the mixture screws do make a difference in drivablility not only idle.

So, if I am correct the challenge is to have a proper mixture at idle and off idle until about 3000 rpm when the main circuit is at 100 %.


It seems as if the idle circuit will carry the motor up to about 2000 rpm and then begins to taper off while the emulsion tube and main jet are progressivly turning on.

So, from about 2000 - 3000 rpm the following jets that need to be correctly tuned are as follows:

holder, idle jet, bypass (only used when pressing of pedal) pump jet, emulsion tube, main jet
The mixture screws will control the "mixed" volume throughout the idle circuit.

Is this correct?

Check out eljaro's last post on his settings for his 482 ci FE.

http://www.clubcobra.com/forums/show...t=74594&page=7

Stefan's settings seem a bit leaner than what you are showing, plus he changed to 40 mm chokes. From what I've been reading, almost a must for BB engines. But I'm just learning.

my setup right now is 40mm chokes,140 main jet, F15 emultube ( a modified F11 tube it is. I Plugged the lower row of holes with tin solder and drilled 4 new ones between the top two rows of holes), 100 air corrector, 65-70 idle with 100 holder, no 0.00 bypass for pump.
The air screw is at 1 turn out to prevent popping and sneezing.
I ended up removing the bypass for the pump because when stomping on the gas it would bog even with a 0.35 bypass.
The 100 air corrector was needed to prevent leanness at WOT.
I guess that many are running too large main jets and making do with large air correctors. That produces richness all over the band and those black plugs.
The new KC engine is running just fine and with light gray plugs. But there is a hesitation at part throttle at around 3000-4000 rpm which I think is the ignition, because it is missing at that speed.
I have just received a new distributor cap with drilled holes to prevent corrosion inside and I have had to change the distributor gear because it was eaten up by the cam gear.
The car goes like a rocket, but at part gas it hesitates. I have tried everything with the carburetors until I discovered the timing to start getting erratic at above 2500 rpm. So I had to put a new cap, new weights, new gear, new springs (one was broken).
If tomorrow the weather is as good as today I will take the car out and see how it runs and let you know.

Stefan