Weber DFEV carb vacuum signal at idle.

[VDub Campers Ltd]

Hi everyone. Professional Aircooled VW mechanic a business owner with 18yrs experience.
Im finding that that the weber 32/36 DFEV carb is a very common carb here in the U.K. They are used on both 1600 and 2.0l models alike.

Ive driven a fair few vehicles with these carbs on, and have been surprised at actually how good they can be.
Over the years i have been gradually building up my understanding of this carburettor, and i have spent literally hundreds of hrs thinking about them and troubleshooting them. Inspite of this effort, i still having difficulty finding a method that consistently provides good results at idle.

I have spent a lot of time searching online and i think there is very little in depth technical detail relating specifically to the aircooled VW engine application.
I am writing this post not about a specific engine or problem, but about my experiences trying to setup DFEV carbs in general on all aircooled engines whether it be 1600, 1776, type 1 engines or 1800 or 2000cc TypeIV engines.

The main issue that i am consistently struggling to overcome with these carbs is to do with getting a high enough idle (900 rpm ish) without having the primary throttle butterfly open so that exposes the hole on the inside of the 34m venturi, and thus triggers the main jet to start dripping fuel down the throat of the carb, therefore overiding the primary idle jet system.

The other problem that happens when the idle speed screw (throttle screw) is even just a fraction too much open, is that the ported vacuum can start to advance the timing at idle, which is obviously not what we want.

So how much ported vacuum is too much?
When i setup Solex 34 Pict carbs, im looking for zero ported vacuum at the vacuum hose at the dizzy at idle. This is always achievable. But im finding this is not the case with the DFEV progressive carb.

I have measured that on average the vacuum canister on an original Bosch 034 distributor will start to advance the timing via the vacuum can at approximately 4 In.Hg.

So in effect 3 In.Hg is the max vacuum signal that can be read at the distributor ported vacuum from the carb without changing the timing.

In real terms on an average aircooled T1 engine with a Weber DFEV carb and manifold setup, im finding that when following the manufacturers setup instructions, which state that as a baseline setup guide, that the idle speed screw is to be 1.5 turns out (off choke) and the idle mixture screw is to be 2 turns out (if the correct primary idle jet is fitted). Under these circumstances im only achieving on average a lumpy idle of about 780 rpm, and im reading about 3-4 In.HG. ported vacuum pressure at the hose that supplies the distributor advance can.

The only way to increase the idle speed to a satisfactory speed from there, is to wind in the idle speed adjustment screw further. Now we can achieve 900-950 rpm.

However....at this point we are now reading 5 In.HG ported vacuum at the distributor vac hose. Which is now moving the advance plate inside the Bosch 034 SVDA distributor.
Not only that, we are now getting an occasional drip of fuel inside the primary venturi, which gives an unsteady idle and is not how it is supposed to work (as far as my understanding goes)

Ive experimented with changing the primary idle jet at 5 point increments upward at a time, to try and increase the idle speed on the manufacturers base setting guidelines but to no affect.

So..........so what. What is the point of this post?
I guess im looking to understand if others have experienced the same issues as i have.
Im struggling to reconcile my good experiences driving customers vehicles with these carbs with the reality of trying to set them up correctly.
And i have unanswered questions.

1/ The carb is occasionally dripping fuel at 900 rpm described above. This cannot be correct. If its not correct then why does it happen at 900 rpm with brand new carbs, brand new needle float valves and float height set to manufacturers specification of 38mm to top of float and 3.5 psi fuel pressure?

2/ If there is 5 In.HG of ported vacuum present for the dizzy canister, when the idle speed screw is adjusted out to achieve 900 rpm. And when Genuine Bosch vacuum cans start advancing the timing at 4In.HG pressure, how can this possibly be a suitable carb setup for a VW with a SVDA genuine Bosch vacuum advance distributor?

As im writing this ive just thought of a possible theory.
Weber DFEV carbs come jetted for 1600 engines with primary idle jets sized at 40. This is much smaller than every 34 PICT carb and 34 ICT, jetted with 50 or 55 idles.
I have been re jetting these carbs up to 50 primary idle jets.
What if Weber is right, and they use a size 40 primary idle jet, but failed to tell us that we were supposed to create the correct warm idle mixture by a combination of setting the choke butterfly so that its permanently partially choked when warm and also obviously adjusting the mixture screw?


Its a feature of the DFEV carb that when the throttle lever is operated, there is a pushrod which comes up through to the top of the carb and interacts with the choke butterflys, further pushing them open.
Maybe this partial closed at idle choke butterfly idea would reduce the airflow through the carb and thus reduce vacuum pressure at the ported vacuum hose for the dizzy, so that it does not advance the timing at idle?
Maybe the airflow reduction by the partially closed chokes also mitigates the tendency for fuel to be dripped through the primary main jet,due to strong airflow at idle in the venturi?
When setting up these carbs ive always set the choke so that it fully opens after about 4 minutes. The choke butterflies end up fully vertical on a warm engine. Maybe this is not the way its supposed to be.
I will run experiments and report back.

If anyone has any thoughts on this i'd be interested to hear them.
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[SGKent]

some late weber of that style have an air bleed screw that is a bypass like a 34-pict-3, where the idle speed is set using the air bleed. The throttle plate must remain closed such that the transition ports are not exposed or the main will leak fuel - worse it causes the transition from idle to speed to fall lean as the well is partially drained, and the full function of the transition holes are defeated.

Just thinking out of the box, but if there truly is no idle air bleed adjustment, either thru a screw or the changing of a jet, then the next stop would be to drill a small hole in the butterfly of one (assuming you have spare parts to replace it if it fails) to see if increasing the air a little might improve the idle.

Last - the type of linkage does have an effect on how these carbs behave, I used to turn them over and make sure both butterflies were properly closing and covering the holes. It can take almost the patience of a surgeon to get them properly adjusted. Using thin carbon paper, or holding them up to the light to be sure they are properly centered as well, so they do not dig into the body on one side and a big gap on the other side, then make sure the linkage is properly opening the primary and secondary. It is preferable for a street car that the primary is well on it way to being open before the secondary is engaged. If all that is done and one cannot get enough air in to idle at a reasonable idle speed, then as I said the only other option may be either a similar model with an air bypass screw, or drill a small hole in the butterfly to let more air in at idle, and adjust with the mixture screw.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[W1K1]

have you seen this article?
https://www.aircooled.net/making-weber-progressive-dfev-work-aircooled-vw-engine/

I don't know why people try to run those progressive carbs anymore???
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1973 super
1965 squareback 1500E
1971 bay window westy- subi swap

[Wildthings]

I have not experienced the problems you are talking about with the Holley clones I have run; about 100,000 miles on a Type 1 and over 400,000 miles on a Type 4.

Have you tried lowering the float level by a MM or so to prevent the dribbling?

In my tuning endeavors with these carbs I have tried drilling a hole in the primary throttle plate so that the throttle plate could be adjusted a bit more closed, but it didn't fix the problem I was trying to fix so I soldered it back up. The main thing I have found is that you don't need or necessarily even want a functioning power valve on the primary side or high speed enrichment on the secondary side. I run with both disabled and then enlarged both main jet sizes to make up for it. This makes the carbs pull strongly from a stop with no hiccups and transition very smoothly onto the secondary.

If you do decide to try drilling a hole in the primary throttle plate start small, maybe 3/32" and see what that size does for you.

[SGKent]

Mike - there are a series of small holes below the venturi where the throttle plate is. Their purpose is to flow out fuel to enrichen the mixture as they are exposed. This is in addition to the accelerator jet. They pull fuel from wells that are part of the main jet system. When the throttle plate is closed there is no vacuum at the holes so no fuel comes out. in this condition the idle mixture circuit totally controls the mixture at idle. As the plates open the holes are exposed and fuel pours out of them in addition to fuel provided by the main jet. It helps transition the carb from idle to speed. Sometimes the idle circuit cannot provide ample air for a given engine size so the idle speed is too low. If the screw is turned to open the throttle plates, it exposes one or more of the holes, and they start to flow fuel. That means now the idle mixture is a combination of the enrichment ports and the idle circuit. It makes for a poorly running carb. Idle control is poor because the control is diluted due to the fuel flowing from the holes, and there is a flat spot because the extra fuel needed to transition is already being used. The only solution is to keep those holes covered until the throttle is opened. Weber dealt with this in later progressives by adding an air bypass like what is in the 34-Pict-3. Idle speed then becomes a balance between the air bypass and mixture - and the throttle plate(s) remain properly adjusted closed. In fact Weber specifies in some of their publications that you use a piece of thin carbon paper, or a specific feeler gauge wire to set the placement of the throttle plates, then leave them alone. Idle speed is then controlled thru the bypass. On occasion I have had to file or even tweak throttle plates to make them perform as the engineers designed them. Worn throttle bushings make the matter even worse.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[VDub Campers Ltd]

[Wildthings]

[Wildthings]

[VDub Campers Ltd]

[VDub Campers Ltd]

Yes, but as you yourself mentioned one common method to do this is to drill a hole in the throttle plate. One can not add a bypass circuit to a carb that didn't come with one, but they can easily drill a small hole in the throttle plate.[/quote]


I very much do not like the idea of this. I just cannot believe that weber make a carb that will not run on the idle circuit and also provide ported vacuum at idle. And the only way around it involves drilling air holes in the butterfly.
To me the issues im seeing absolutely has to be something to do with incorrect jetting setup out of the box or poor advice online.

Its so crazy to me that every other 34mm venturi carb (Solex, Weber ICT, ICH) all run nice with 50 or 55 idles. But im not seeing that with this carb.
And i cant say catagorically yet, but it looks like it wants to run on a 40 or even 35 idle, which in all these years ive never picked up on. And ive never seen any advice online recommending it.
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[SGKent]

once the carb basics are set up, everything clean, no vacuum leaks, correct float height, lower butter fly plates properly adjusted (they normally require it) and all the linkages properly set, then one can look to jetting because it will reveal itself. If the engines these carbs are mounted on have a low vacuum signal due to being over cammed, then that is another issue. I have only twice failed to solve a problem with a carb and both time t was on a cheap lawnmower carb that the body had become warped and worn out. The Weber carbs generally just need a little TLC to get them running right. But then when the are on a T4 bus, there is no manifold heat so they tend to ice up sometimes, and fuel droplet size is poor under those conditions.
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[Wildthings]

What happens on these carbs is that at idle because of the high vacuum the power valve is closed, and as you open the throttle it takes a quarter of a second or so for the power valve to open causing a lag. Then at cruise because you have a tiny engine pushing a big box down the road you have low vacuum and thus the power valve is open causing a rich condition. The long runners also likely cause air to pass both ways through the carb also adding to the engine running rich at cruise, I believe this is why most carbs sold for both Type 1 and Type 2 engine so not have power valve as the mixture naturally enriches due to the fluctuation when near full throttle.

Everyone always screams that the problems with these carbs is caused by icing, which I myself believed likely 35 years ago when I first bought a Type 4 with a progressive on it, so I added both an intake air preheat and a heated manifold setup and saw close to zero change in how the engine ran. I tolerated a minor lag coming off idle for years, even with the two forms of heat and then once I got a wide spectrum A/F analyzer I began to see what was actually happening and ditched the power valve and rejetted, leaving a smooth running rig once the engine warms at all. At sea level my AFR runs about 15:1 at light cruise and about 12:1 at full throttle.

As for the effects of the throttle valve size on these engines a 32mm throttle bore is pretty small for a 1700-2000cc engine, so needing some bypass air at idle is not unexpected.

IIRC I am running either a 50 or 55 idle jet. The idle AFR is about 12:1 as well.

[VDub Campers Ltd]

Wild things.
I must admit I don’t really understand the purpose of the ‘power valve’
I thought that thing was just a device to open the choke butterflies just a crack at idle. Same as a 34 Pict 3 carb.
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[Wildthings]

[mikedjames]

I let my progressive idle down to 800 rpm or lower if it feels like it, with the throttle plate open a tiny fraction. I use the cold idle throttle screw to hold the idle higher on a cold engine, which turns out to be about 1600rpm when first started, dropping back as it warms up. Thats with the throttle plate well off closed, so the distributor is advancing a lot.

Timing does not change with the vacuum hose on or off once it has warmed up.

I am just about to fit a digital vacuum sensor onto the vacuum port of my progressive, so I will be able to actually measure the ported vacuum at idle,
hopefully in a few days time.
This is because I am building up a small digital controller for an EDIS-4 module for crank timed ignition, and it will have a display on the box showing the vacuum, plus feeding the dashboard multifunction displays with the vacuum and timing advance value.

I do use a stock oil bath air cleaner setup, I measure 36C/97F air at the top of the carburettor air box.

The jetting is something like
P main 125 P idle 45 P air 160
S main 132 S idle 80 S air 180

The choke is like the 34PICT3 - if you increase the vacuum due to air flow in the venturi, it pulls on the lever to open the flaps, so choke is on at idle, off at WOT, and then the flaps open as the bimetallic strip warms up.

I can just about get the power valve to cut in - it needs a fair bit of air flow at over 4000 rpm on my 1641, then the engine runs rich, theres a vacuum pickup in the venturi that feeds a long passage to the "power valve" diaphragm, that pushes open another valve that dumps fuel through another jet ..
So with a bit of throttle pedal waggling I can adjust the AFR at 65mph from 14:1 down to 11:1.. But if I run a bit slower, it tends to run a bit leaner.

https://www.thelatebay.com/index.php?threads/making-the-weber-dfav-32-36-carburettor-work.19121/
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1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[SGKent]

[VDub Campers Ltd]

[Wildthings]

[SGKent]

[Wildthings]