Fine tuning Weber IDFs

[Dingleweed]

So, I am trying to fine-tune my carb setup. First, let me say that the buggy runs smooth and has plenty of get up and go. Driving it around, someone would probably think that it is tuned just fine. I’m just trying to straighten out my air-fuel curve and squeeze out every last HP.

Here’s the engine setup:
2332cc
Engle W125 cam with 1.25 rockers (measure out to be ~1.2)
CB Wedgeport heads, 44 x 37 mm
Match-ported big-beef manifolds
Weber 48 IDF carbs
1-5/8 merged tri-mil exhaust with muffler
009 distributor with compufire set to 30* total advance
LM-1 wideband air-fuel system with in-dash analog gauge

Current fuel system and carb setup:
Stock pump and fuel lines with two in-line stock Bosch filters
Fuel pressure at carbs is just above 3.5 psi (3 gaskets under pump to set base fuel pressure)
Under full throttle the fuel pressure drops a bit in the upper rpms to just above 2.5 psi
48 IDF carbs, 40 mm vents
Floats set to 9.8 - 10 mm
50 idle jets
135 main jets
F2 emulsion tubes
150 air jets

Air-Fuel readings...
"Normal" driving:
12 to 12.5 idle
13.5 to 14.5 for most normal driving (light to 1/4 throttle, fairly rpm independent)
~14 cruise up to 55 mph in 4th (very light throttle)
~13.5 cruise up to 75 mph (still light throttle, maybe 1/4 throttle, at 4000 rpm with my gearing)
So, for normal everyday driving (light to ~1/2 throttle) I'm pretty happy with the A/F tune throughout the rpm range.

Full-throttle driving:
assume that I start at light throttle at ~2500 rpm and I gradually push the pedal down so that the accelerator pump squirters don't effect the A/F ratio.
~14 A/F starting point
drops from 14 A/F to ~13 A/F between light to 1/2 throttle
drops from 13 to 11 A/F between 1/2 throttle to full throttle at ~3000 rpm
readings stay around 11 (or just below) from 3000 rpm to 3500-3700 rpm
between 3500 and 4000 rpm the A/F starts to rise, and by 4500 to 5000 it reaches 14
Then it stays there ~14 up to 6000 rpm.
Between 6000 and 7000 rpm it drops from 14 to ~12.5

Here are the conditions I’m trying to cure for full-throttle driving:
(1) rich A/F readings from 2500 to 4500 rpm, and
(2) lean A/F readings above 5000 rpm.
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Dingleweed]

Oh yeah, and I have a pretty complete selection of main jets from 130s to 150s, and air jets from 150s to 200s. I've tried just about every combination. So, I'm starting to think that different emulsion tubes might be the solution to straightening out the A/F from the mid to upper rpms...
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[loudaddy]

try
40/F2/55/155/180
worked for me.
(thanks jc)

[Ken Taber]

with an engle 125 you are over carburated with 48's . 44s would be better. What with your combination why did you use a 125. I find on my dyno that your combo needs an fk-10 or fk-46 with 1.4 rockers. If you go to F-11s or F-15s that might help.

[74 Thing]

Read this thread...
http://www.thesamba.com/vw/forum/viewtopic.php?t=293837&postdays=0&postorder=asc&start=0

[Dingleweed]

I have read that thread a few times. Excellent thread with lots of good information!

One topic that isn't discussed in detail in that thread is one that I have been curious about recently (somewhat independent of my A/F curve at higher rpm) is that of emulsion tubes and just exactly how they work.

I have found a few threads on other forums discussing emulsion tube theory and how to use them to fine tune IDFs on other sites (one is a Cobra site and the other is for RX-7s).
http://www.clubcobra.com/forums/showthread.php?t=98360
http://www.rx7club.com/showthread.php?t=805950

But large V8s and rotary engines are quite different than ours. I have searched The Samba for similar threads, but haven't quite found any that discuss the details of emulsion tubes.

To help start off the discussion, here are a few pics of Weber emulsion tubes and their specs (since the F#s are not very informative):

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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Dingleweed]

The main circuit and emulsion tube (ETube) parameters are:
Head (pressure difference between atmospheric and the center of the auxiliary venturi)
Float level
Air jet
ETube holes
ETube outer diameter
ETube inner bore
Main jet

Here’s a pic of the carb and main circuit with no air flow through it illustrating the fuel level within the main circuit and within the ETube inner bore:

Now for a pic when the throttle is wide open and the main circuit is in full operation:

Here’s a summary of my understanding of what happens inbetween and what controls each parameter… I am assuming that we just stay wide-open throttle from a lower rpm to when the main circuit is in full operation and we’ll ignore the idle and progression circuit.
As the volume of air through the carb increases with rpm, the speed of the charge through the main and auxiliary venturi increases. The increased flow through the aux vent increases the head.

As the head increases, it pulls on the main circuit. The float level is below the first set of holes in the ETube. So, at first the head is drawing air through the air jet and upper holes and on the fuel level in the main jet well (which starts at the float level). Eventually, the head draws the fuel up to the nozzle level where the fuel and air from the upper ETubes mixes and is drawn into the aux vent. At this point the main circuit has started, and everything in the main circuit is pretty much pressure driven as the fuel and ETube air flow rates are very small.

As the rpms increase so does the charge speed through the vent, the head pulling on the main circuit, the flow and speed of the fuel in the main circuit well. As the fuel flow around the ETube increases, the fuel stuck inside the ETube inner bore is drawn out by the Bernoulli effect. This lowers the fuel level inside the ETube which exposes more holes. The more holes that are exposed to the fuel flow, the more air is mixed in. Eventually, the speed of the fuel around the ETube pulls out all of the fuel from the inner bore and all holes are exposed. At this point the main circuit is in full operation.

Summary of parameters and what starts their operation:
“Head” – absolute value of the pressure drop in the aux vent
Air speed through the auxiliary venturi
“Float Level”
Determines what head is necessary to start the fuel flow into the nozzle
“ETube outer diameter”
Determines the fuel flow rate that starts to draw fuel out of the ETube inner bore (exposing ETube holes)
“ETube inner diameter/bore”
Determines how much fuel is inside the ETube which needs to be drawn out to expose more holes
“Etube holes”
Emulsify air and fuel in the main circuit if they are exposed. The higher the holes on the ETube, the sooner in the process they begin to work.
“Main jet”
Determines how much fuel flows into the main circuit for a given head.

I think those are the basics. But, of course, there are always complications and just about all of these are inter dependent. For example:
* Once the air starts flowing through the air jet and inner bore, it creates its own venturi effect and lowers the pressure in the inner bore changing the atmospheric level and actually drawing fuel into the inner bore.
* Another is that once significant fuel flow starts, the float drops which lowers the float level (atmospheric pressure level) inside the main circuit which changes just about everything.
* The air jet has an effect on many processes: it controls the air flow into the circuit and air-fuel mix once the circuit is fully on, and on the other end, it also helps determine when the fuel in the main circuit starts to flow due to the ratio of the air jet to main jet size and how many holes are exposed.

Hopefully some of the more experienced tuners can chime in here with their experience with what exactly different emulsions tubes due to the full tuning rpm range; i.e., changing to a tube with a larger diameter does… or changing to a tube with fewer holes does…

For example, Ken’s suggestion to go from my F2 to an F15, these tubes are exactly the same except their outer diameters. The F2 is a 7.5 mm outer diameter and the F15 has an outer diameter of 8 mm. Would this change increase the fuel speed around the ETube turning on the main air metering circuit sooner? So, instead of my main circuit reaching full operation at 4500 to 5000 rpm, it might reach it at say 4000 rpm?
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Tbirdusa]

[Ken Taber]

I have dyno'd FK'8 vs fk-10 , vs fk 46 on 2332's and by far the fk-46 has more low end torgue than the others. I don't know why but the dyno does not lie. You need 42 or 44 intake valves and heads that flow . The max hp is 6500 but the torque starts at 2500. The 40 series Engle cams is far about anything out there.

[modok]

Now I could be wrong, but I think the f2 tubes have the step lower on the tube, or at least mine do.
The chart does not show this, but it does not say where the step is located either.
Try f-15, (or get some f-11s and solder the four lower holes shut, same thing)
Also could try a little smaller vent, could have the same effect.

[Ken Taber]

48 idf's with a 125 cam will run better with 38 vent than it will with 40 vents. Also the f'11s are better.

[Dingleweed]

Thanks for the replies. Ken, before I put the 48 IDFs on there I had 44 IDFs with 36 vents. Once I got the 48s tuned in (similar A/F ratio behavior for most of the rpm range) I did notice (on the butt dyno) a big difference between the 44s and 48s. Up to about 5000 rpm, there really wasn't much difference between the two. But after 5000 rpm it was night and day.

With the 44s the power just dropped off a cliff above 5500 rpm. So, I hooked up a vacuum meter to measure the manifold pressure. Above 5000 rpm with the 44s I was starting to pull a vacuum. By 6500 rpm I was measuring 2-3 inches of vacuum (and there was no point to going above 6500 rpm).

After installing and tuning the 48 IDFs with 40 vents, there was so much more power above 5500 rpm. Where the power with the 44s would just die away, the engine just keeps going with the 48s. It keeps pulling up to the high 6000s. Now the power doesn't noticably start to drop until 6700 to 6800 rpm, and the vacuum gauge just stays pegged at atmosphere. I am guessing now (as you suspect) that power above 6500 rpm is probably limited by my cam and lift.

I think I'll put an order in with my local VW shop (Joe's Buggy Haus) for some different emulsion tubes and test them out for experimentation sake.
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Ken Taber]

I built a 2387 with clyde berg 44x37.5 great heads with an Fk -10. It had the same power with 44s with 40 vents as it did with 48s and 40 vents. but more drivable. Your butt dyno it not showing you hp, only that it will keep on winding good . If you had a real dyno you would see the power is dropping off above 6000. even thow you think it is making hp because it is reving.. My original drag motor made its max hp at 7000 rpm even though it when to 9000 . It had an fk-91 in it. Butt dynos are not real , sorry

[Ken Taber]

I forgot to mention that the Engle cam series W-100, W-110, W-120 , W 125- are obsolite. The new FK-40 series it the best, does requier 1.4 rockers though.

[Dingleweed]

Putting aside the topic of my particular cam, heads and carbs, I'm really curious of hearing about people's experience with different emulsion tubes.

How exactly would F11 and F15 emulsion tubes perform differently than F2s?

Does anyone have actual pictures of the different tubes so we can see the differences not included in their spec sheets? (i.e., as Modok mentioned that their steps might be in different locations.)
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Ken Taber]

Quit wasting your time . What you want is not going to be fixed with carb tuning. Make a cam change and see whats new and eye opening.

[Dingleweed]

What I want is an informative discussion on exactly how emulsion tubes work and to hear about people's experience with using different emulsion tubes to fine tune their carburetors.
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1962 Dune Buggy, 2332cc, W125, 44x37 wedgeports, 48 IDFs, Rancho Pro Street transaxle

[Ken Taber]

You are not going to get what you want, I have been at this since 1974 and what you want is a waste of time. That's why no one else is responding.

[modok]

I put a lot of thought into it once, and I still can't figure it out. Fact is they don't all work according to some universal principle. Look at a DRLA tube, it has four huge holes in it!
John C's discriptions are as good as anything, go with that.

I would say overall the tube controls where and how much the air jet kicks in.
You want it to kick in sooner, lean out the low end and let you run a bigger main jet. So I figure you need the holes higher, or maybe drilled straight instead of at an angle, or maybe you need to solder the lower four holes closed and re-locate them to the top?? I don't know, try it and see!!
But if you want to find out you need to get at least one other set of tubes so you can frankenstien the other set.
F-11 is common.

[Ragman]