Weber Emulsion Tube Tuning - Page 2 (2025)

swatson454 wrote:I've tested all mannor of main jet/air corrector combinations and am pretty darn certain that my combination is good. Like I said, the fuel curve stays flat at 12.9:1 at WOT from 2,500 rpm up to redline. I'd like it to be a touch richer but I'd have to enlarge a 175 air corrector to something smaller than the 180 I have now and I just haven't bothered to do it yet.

The emulsion tube is the only thing I haven't touched and it's because I don't know what to do with the damn thing. I'd sure like to be able to pull some of the excess fuel out of it at part-throttle cruise. For all I know, the emulsion tube isn't what I need to be focusing on and I'm overlooking something else. That's why I'm asking.

Things seem backwards. I have a rich part-throttle cruise but it goes lean when I go to WOT regardless of accelerator jet size. I'm missing something here.

Yes, you are missing something here hence, my questions.

You change the emulsion tube and your a / f ratio will likely change also no longer being flat. How do you know your engine performs best with a perfectly linear fuel curve?

It's rich at part throttle. I will assume the main circuit is activated. Drop the main jet size and the main air corrector size a little bit less. Reducing the main jet and main air corrector will lean the entry point a bit but should maintain the overall air / fuel ratio. You likely have the wrong emulsion tubes.

What size engine, is it cammed, ported, headers, multiple carbs, IR system, what weight vehicle, it's use, etc.????????

Troy Patterson TMPCarbs.net TMP Carbs

If this is a four cylinder, modded with a synchronous 38 DG(x)S, go back to a progressive 32/36 DG(x)V. They always run better, with what you're trying to achieve.

If this is a V6, time spent reading the late John Passini's writings is always time well spent.

Also, have you blanked off the power valve, or are you still running it? This will have a big affect on you're WOT readings.

Swatson, we need more specific info regarding your vehicle and engine specs. Also..... Are you running one of the newer DGAS /DGES carbs built after the Weber plants came off strike a few years back? One problem we have found with these carburettors is that the early DGs had 27mm chokes, whereas the new DGs have 30mm chokes, but retain the same out of the box baseline jetting.

So assuming the original DG was jetted correctly for the application out of the box (which few are)...... We are finding that the newer version with 30mm chokes requires one step increase in main jet and 2 step increase in idle (slow running) jet to bring fuel delivery back to that of the earlier version having smaller 27mm chokes. That's just a rough rule of thumb, though. We will need more info to be of any real assistance.

I might add that we have not found a need to change emulsion tubes, nor float settings, nor alter out of box jetting dramatically except in the case of highly modified engines of 2.5 to 3.5 litre displacement.

Your 38 DGAS / DGES should have the following out of the box (generic) jetting......

27mm - choke (main venturi) - Note: New DG carb has 30mm choke
4.0 - aux. venturi
F50 - emulsion tube
145 - main jet - Note: Use 150 mains with new 30mm choke
185 - air corrector
45 - idle (slow running) jet - Note: Use 55 idles with new 30mm choke
70 - accelerator pump jet
30 - pump bleed (back to fuel well)
2.50 - needle valve
40 - 52.5 float setting

Hope this helps,

Harry

I bought it in December. It's a Redline Weber conversion carb for the slower running and larger displacement (than it was intended for) Jeep 258 ci, inline 6 and has '27' cast into the barrels on the back of the carb.

Redline said it was supposed to have been shipped with 55 idle jets but 45s were installed and it wouldn't idle without exposing the progression holes. It has 55s in it now.
It shipped with 145 mains and 170 air-correctors. That combo started out at 12.0:1 at 2,500 rpm but had leaned out to 12.9:1 by 4,000.
I ended up with 150 mains and 180 air correctors and things stay pretty flat at 12.7:1 now.

F50 Emulsion Tubes
70 pump jet
They also call for a 18mm float drop. 11/16ths... ish

swatson454 wrote: They also call for a 18mm float drop. 11/16ths... ish

I saw the float drop reference in a earlier post and again above. Sounds like terms are being mixed. Are your posts running through a language translator program?

Float drop is measured with the carb in the running position but no fuel. This is adjusted so the needle does not fall out when the bowl is empty

Float level is measured with the carb inverted so the needle is against the seat. This sets you running fuel level.

You should be primarily on the idle circuit at cruise, drop to a 50 or if you have idle air jets on that model, go up a size.

Where's your idle fuel screw adjusted to? (how far out from seated?)

Mark Whitener
www.racingfuelsystems.com
____

Good work isn't cheap and cheap work can't be good.

Troy Patterson wrote:What size engine, is it cammed, ported, headers, multiple carbs, IR system, what weight vehicle, it's use, etc.????????

Troy Patterson TMPCarbs.net TMP Carbs

As other posters have noted, we really need more specific info regarding your engine, vehicle, use, etc. to be of further help. And also the fuel formulation regarding any ethanol content, etc. Don't get too fixated on 14.7 being the holy grail or 'stoich'. Today's pump gas is much lighter and more volatile than were older fuel formulations. Just find the air / fuel ratio that performs best with your particular engine and the fuel you have available and dial your carb in as best you can.

Good to know that you have the 27mm choke model. That pins things down a bit.

If it is leaning out on top, you can drop your air corrector sizing in steps until the lean condition resolves and see if that takes care of it without throwing the remainder of your jetting off. Don't be hesitant to drop air corrector size below 170 if need be. Generally, one step in main jetting equates to 3 steps in air corrector sizing. For instance, the Peugot export model for Africa (2.9 V-6) used a DG carb having 27mm chokes, F80 e-tubes, 55 idles, 150 mains and 150 air correctors.

You can also switch from F50 to F6 emulsion tubes...... But that will affect most of your jetting and you will then have to sort everything out from scratch again.

Regarding idle (slow running) jet sizing with the DG...... A rough rule of thumb is that if you have to open the mixture screw more than 2 turns, your idle jets are too small...... And if you have to close them to less than 1/2 turn out, your idle jets are too large.

Your intake manifold design will have a bearing on jetting and mixture adjustment as well (open or divided plenum, plenum depth, runner pairing if divided, etc.). Give us a a bit more information and perhaps we can give you some things to try that you haven't already done.

Hope this helps,

Harry

Delayed response.

It's a bone-stock 258 ci inline six Jeep engine. It has 165k miles on it but runs really well. Apparently, these things are nearly impossible to kill. The Jeep has piss-poor gears at 2.73 with 33 inch tires but load is load, right?

Anyway, the log-type intake manifold sucks but I'd still like to get a/f numbers that are at least close. I was just out driving it around and a 2,000 rpm lean-cruise showed a rich 13.0:1 ratio. Ease into the throttle and it jumps up to 14.5:1 until I was deep enough in the throttle for the power valve to activate. That seems completely bass-ackwards to me. It's rich when I'm light on the throttle and lean when I get heavy. I'm talking about lower to moderate acceleration here. WFO usually holds at 12.5:1.

Last night I took it out and loaded it up to WOT while holding the foot brake, keeping the engine at 2,000 rpm. The results were 13.5:1 @ 2,000 rpm, 12.5:1 @ 2,500 rpm and 12.5:1 @ 3,000 rpm. Does that mean anything?

I have 55 size idle jets installed now and the lean-best mixture screw setting is right at 1.5 turns out. The throttle plate is most definitely not uncovering the first progression hole. It's a good, solid lean-best idle.

My jetting attempts have done nothing to correct this. All that's left is the emulsion tubes.

To recap:
Idle Jets 55
Main Jets 150
Air Correctors 180
Emulsion Tubes F50
Power Valve opens at 6 inches
Any thoughts?

You may have said and I overlooked it, but what are the driving symptoms? Forget the numbers for now. Specifically, what are you looking to improve or change with the way it drives?

I've leaned you can not always trust the numbers, especially if I'm not the one in control of all the variables.

Troy Patterson TMPCarbs.net TMP Carbs

Troy Patterson wrote:You may have said and I overlooked it, but what are the driving symptoms? Forget the numbers for now. Specifically, what are you looking to improve or change with the way it drives?

I've leaned you can not always trust the numbers, especially if I'm not the one in control of all the variables.

Troy Patterson TMPCarbs.net TMP Carbs

With the exception of a header, D.U.I. distributor and the Weber, the long block is bone stock and old. I did remove the water lines from the aluminum, stock manifold.

My problem mostly lies in the a/f ratio that I'm seeing. Having a rich lean-cruise that goes lean as I accelerate seems totally backwards to me. It runs pretty well with the current jet combo but it does lean out enough just over 2,000 rpm under a moderate, but not WOT throttle, to knock.

Like I said, I just don't know enough about emulsion tubes to know if that's what's causing the backwards readings and, if so, which one to select to fix it. It's bugging the crap out of me and I want to learn it. The way my little brain sees it is that I should be in the 14:1s under a light load, lean-cruise scenario and it should get richer as I get deeper into the throttle. It does the opposite now until WOT.

Plus, WOT at any rpm shouldn't yield 13.5:1, right? That's what it gave me lastnight at 2,000 rpm and the engine knocked.

Yes, I'm fixated on the numbers, lol. I want to be able to read the monitor and know if something needs to be changed. I can do that with WOT but this part throttle on the progression circuit stuff has me wanting to drink mid-day

I appreciate your help. If I'm leaving out important info, let me know.

Thanks

Okay, if yo are too lean at the entry point and beyond in the main circuit's fuel curve, I'd recommend either:

1) an F-15 or F-11 emulsion tube to lean the entry point some. How much leaner you need to go is unknown at this point, but if you need to go ricer still, you can go to an F-8 and richer still F-7 emulsion tube.

2) a smaller main air corrector and smaller main jet, but drop the main air corrector more than the main jet. A rough rule of thumb with Weber tuning is every three sizes in main air correctors is equivalent to one step in main jet size.

By reducing the main air corrector you are reducing the amount of air available to lean the entry point of the main circuit, but by reducing the main air corrector you are also enriching the entire fuel curve which is why I say to also reduce the main jet. I'd drop the main air corrector by twice the main jet size and drop the main jet one size and see what you get. I'd change the emulsion tube first.

I am assuming the idle / off-idle jets are sized well with these recommendations. It is likely or possible you will need to both change the emulsion tubes and main jet and air correctors to get the right balance, but then again, maybe not. Trial and error is the only way to determine exactly the right combination.

Hope it helps. If you need a source of emulsion tubes, you can contact me.

Troy Patterson TMPCarbs.net TMP Carbs

Mark Whitener
www.racingfuelsystems.com
____

Good work isn't cheap and cheap work can't be good.