Carb Help...

WildWilly

WildWilly

True Classic
We're getting closer, it looks like we have the timing under control and a more permanent solution to install when the new Marelli dizzy arrives. But now we are having issues with the carb. Its a Weber 36 DCNVA. Here are the comments from Butch.

Pulled the carb apart everything appears to be fine, no gunk, but I cannot get it to idle correctly. As the air passes through the venturi it is suppose to draw the gas out in a atomized kind of mist, this one just dribbles in as large drops, not sure why it is doing that. Also, when the carb came here it came with 2 base gaskets, one with a tiny hole between the carb throats, and one with no hole. I do not know what the difference is. It just seems to be lean all the time.

Does anyone have any idea what might be going on?? Which gasket should we be using? I think the stock jets are 40s, so we're looking to get some 45s and 50s, but that will take a while to get them here. As always, thanks...
 
faulty????

We're starting to think the carb is faulty. Butch has totally disassembled the carb and made sure that everything is nice and clean. We get fuel when the accelerator pump cuts in and shoots fuel straight down the carb, but when we try and maintain it, the fuel coming out the venturi is dribbling out, hanging off the bottom of the "pipe" and dropping into the carb. We have changed jets with ones from my other 2 carbs just to see if that made a difference and no go. No matter what we do the fuel in the venturi never comes out in a nice spray pattern to be mixed with the air. any ideas out there???
 
More info....

Ok, so we took the carb apart again and learned something that we think is the issue. The "jet" is made of 3 parts, the air corrector, the emulsion tube and the main jet. The jets are 132, which we suspect are far too small. The older carbs we have are 110, so even smaller. So I guess we have to get something larger. When we here folks talk about 40, 45 or 50, am I correct in thinking that's really 140, 145 and 150?

How do you determine the correct combination of the 3 parts? How do I know if I even have the correct parts with a small jet? Pictures will come later. Thanks...
 
Dribbling fuel

Sounds like a leaking needle and seat or floats set too high.

A needle and seat can look fine but actually leak.

Keep at it, you're getting closer.

Cheers,

Rob
 
Weber Carbs

Most of the Weber down draft carbs use interchangeable parts.

The emulsion tube has a main jet on the bottom and an air correction jet on the top of it. You should also before getting started have the correct float level as this can change things drasiticly as well. In the centre of the barrel is a venturi or choke where the air is accelerated, see picture. It is at this location that the fuel is sprayed into the air stream. The main jet allows or restricts the fuel, it meters it into the emulsion tube. Remove it and you will have a flood of fuel entering the Carb. On the top of the emulsion tube is the air correction jet. The size of this jet affects the amount of fuel being drawn from the bottom main jet. All of this affects the mixing of fuel air vapour in the emulsion tube that now flows to the centre of the barrel.

So if you have a 150 main jet and a 200 air correction on the top you will get a certain amount of fuel going into the carb at the venturi. If you put a larger air correction on the top of the emulsion tube you will get less fuel entering the emulsion tube because you have now increased the air in the tube but have increased the static pressure in teh tube. Please remember that the carb is working in a vacuum. If you decrease the air correction size this will allow more vacuum in the emulsion tube and will cause a richer mixture to be made. You can also accomplish this by installing a larger main jet. What you are doing is changing the amount of fuel vapour concentration being delivered into the air stream of the carb barrel.

I have some spare jets that I was going to put on ebay, I am just about to go out for the evening, but can look tomorrow what I have to offer along with venturi's as well.

Now look down the barrel of this carb see the centre ring where the fuel vapour is to come out. This part goes in both ways but only allows fuel to flow if it is installed correctly. The fuel vapours go up the one arm to the centre of tube. If you have it in backwards it will not flow fuel to that barrel. You can see that it has the connecting arm running toward the bowl of the carb.

TonyK.

Grimsby Ontario Canada.
 
wrong jets....

me thinks. Pictures attached, not that you can really see much. The jets seem to be a 132, best as these old eyes can see. We couldn't see any numbers on the air corrector, so I guess we will have to try harder to see something. My understanding is that the 36 DCNVA should have 140 main jets, no idea what size air corrector and no idea which emulsion tube. It would be good to know that info. We have another carb that we took the jets from and can see that the emulsion tube is different in this one, more holes and larger holes but the jet was a 110 (we think). Anyway, so I guess the first thing to determine is what combination of air corrector, emulsion tube and jet should we be running. You can see from the pics which emulsion tube we have. Our thinking is that the 132 jet is just too small. Butch says this will be why the timing is so far out, to get her to run when she is this lean, he needs to push the timing. The timing is currently set to about 18 degrees, his hope is that with the correct jets, we can get the timing back down to the 8-10 degree range, where it ought to be. As FiatFactory and Dom have experience with this carb, I'm hoping they chime in on the combination we need to get this right.

Tony, once you know what jets, air correctors and tubes you might have, let me know. If its what we need, I'll drive down to pick them up. I'll also try and remember to bring along what we have, so you can have a look.

We have checked the float and it is in the correct place. Getting the right parts to get the correct air/fuel mixture is the magic formula that I'm hoping we can find.

You mention that you might also have venturis available, just to help me understand, does this mean that these can be changed?? So we now have 4 parts involved in the combination?? I'll pass this along to Butch, as well. I'm sure the venturis are installed correctly as they are getting smaller as you go down the throat of the carb. Does that sound right? Once we get this carb fixed up, we'll get the timing bang on as well. Thanks.....



 
I think you're still missing something

Lean main fuel jets won't have any effect on your idle speed or the amount of ignition advance you're running at idle.
 
Fuel dribbling out of the aux venturi is symptomatic of a blocked idle circuit. The jets you are pulling are the mains, not the idles. The idle jets are in separate little holders on each SIDE of the carb, just below the level of the carb top cover. Pull those and you'll see that they are probably 40's... try some 50's (which you should have in one of your dmtr/datr carbs)...

you are looking at the MAIN jets, not the IDLE jets... the main circuit sizing has nothing to do with getting the carb to idle. Mains are 132, Emulsion tubes are F36 (marked on the side right up near the top), air correctors are 175 (marked on top).

double check the float level and needle and seat... float level can get bumped around in transit.

Phenolic base plate needs the extra small hole in the middle to provide manifold vacuum to the choke pull off diaphragm... I sent you two spacers cos I think the first one I sent you by mistake is my drilling template that has a small crack in it... that's why I sent you a second one. Check your emails and you'll find one that tells you to make sure that a hole gets drilled in the phenolic spacer to match the small hole in the manifold, that so the choke pull off can get vacuum to work.

DCNF /DCNVA also has a separate idle mix adjustment for each throat.

Remove the idle jets and holders, remove the idle mixture screws and blow thru these passages with compressed air...these are the smallest passages and metered orifices in the carb, so any crud is likely to get caught in them... it sounds like your not even running on the idle circuit.

reinstall the idle mix screws all the way in until they lightly seat and back out each an equal amount, about 1.5 turns. Set the idle speed screw that actuates against the main throttle lever so it is JUST touching the lever, and then give it another 1/2 a turn in. Fit some 50 idle jets.

Set the static ignition timing to 10 degrees.

Start the car and adjust the idle speed screw so that it holds a steady 900rpm idle.

That should be all it needs... they are actually pretty simple.

metered parts for the dcnva/dcnf series are:
idle jet
main jet
emulsion tube
air corrector
aux venturi
main venturi
pump jet

and then there is a selection of pump cams too

that's why they are such a great carb, they can be altered to suit any application.

dcnva carb with top removed
dcnva 002.jpg


the pen is pointing to the MAIN jet assemblies (air corrector on top that you can see, emulsion tube and main jet at the bottom not in view)
dcnva 004.jpg


IDLE jet and holder in position (linkage side shown) Pen pointing to idle jet holder in position on linkage side
dcnva 009.jpg
dcnva 008.jpg


Pen pointing to idle jet holder on accelerator pump side
dcnva 012.jpg


about half way between the idle jet holder and the main jet holder you'll see a small brass bush fitted on each side. This is the idle air supply(corrector) make sure these are not blocked.

Idle jet position with jet and holder removed (linkage side)
dcnva 028.jpg


idle jet and holder
dcnva 023.jpg


idle jet, holder and O ring
dcnva 025.jpg


pen pointing to Idle mixture screw
dcnva 014.jpg


pen pointing to second idle mixture screw position (mixture screw removed)
dcnva 016.jpg


the brass tube fitting between the two idle mixture screws is where the rubber pipe that supplies the choke pull off diaphragm connects to.

idle mixture screw components
dcnva 022.jpg


Pen pointing to the Idle speed set screw. The brass pipe to the left of the idle speed set screw is for vacuum advance distributor supply on the original application, you need to cap this.
dcnva 018.jpg


36 dcnva metered components. Starting at 12 o clock and going clockwise.
pump jet, auxiliary venturi (4.5), main jet components(main/e tube/air corrector), idle jet components(idle jet /jet holder / O ring), idle mixture screw,main venturi (28mm)
dcnva 038.jpg


SteveC
 
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Now you don't see that level of support every day, got'a love this message board.


There's plenty of Weber books Willy.

I think you might see if you can borrow or amazon them if you can. Even once you have her running they can be of invaluable assistance in fine tuning. The lads on here will know the target sizes for each but knowing how these superior carbs work will keep you away from the Holley carbs.

Edoardo Weber understood Fluid Dymanic better than many of his contempories but what sets a Weber apart is the accuracy and tight tollerances that make swapping components around to suit the application possible. They are a complex instrument but very reliable once set up.

And there's the rub its hellish easy to chase your backside around the block when you go in blind and make little mistakes here and there. I remember the fuel circuits were confusing to begin with but once you had it clear in you mind what they did it will 'click' and you can visualise in your head what's going on.


Weber Carburators by Pat Braden

An oldie but a goodie
Weber Tech Manual by Bob Tomlinson


For me the holy grail, if you can find them a pair of booklets I archived by the 'Weber Wizard', I treasure them because of the trouble they got me out of in the 90s.

Weber Carburettors by John Passini Book 1. Theory
Weber Carburettors by John Passini Book 2. Tuning & Maintenance


Bottom of the pile but useful none the less was the Haynes book.

I used to make a cheat sheet and tick the specs off on what I was working on.

Be slow and methodical and cut like a carpenter - measure twice cut once.
 
carb base plate

Just in case it wasn't addressed in one of the posts, make sure that the base of the carb isn't warped as most of the 2bbl webers are. Take a straight edge to the bottom of the carb and see just how much gap there is by checking diagonally and straight across the bottom. Air can be pulled in at that point and lean the idle mix considerably.

Just realized it was a new unit that Steve supplied so this won't apply! As others stated check your spacer block and manifold mount surface. Also manifold to cylinder head for leaks.
 
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Idle jets

WildWilly said:
We're getting closer, it looks like we have the timing under control and a more permanent solution to install when the new Marelli dizzy arrives. But now we are having issues with the carb. Its a Weber 36 DCNVA. Here are the comments from Butch.

Pulled the carb apart everything appears to be fine, no gunk, but I cannot get it to idle correctly. As the air passes through the venturi it is suppose to draw the gas out in a atomized kind of mist, this one just dribbles in as large drops, not sure why it is doing that. Also, when the carb came here it came with 2 base gaskets, one with a tiny hole between the carb throats, and one with no hole. I do not know what the difference is. It just seems to be lean all the time.

Does anyone have any idea what might be going on?? Which gasket should we be using? I think the stock jets are 40s, so we're looking to get some 45s and 50s, but that will take a while to get them here. As always, thanks...
Click to expand...

Hi Bill,

yes, this is because with the 40s that the carb comes with you need to screw the idle adjustment screws out about 4 turns (to get enough fuel), which is way too much. You need to go to something between 45 and 50. With 50s I only needed to screw the idle adjustment out 1 turn (too little) so 47s are probably perfect.

As Steve mentions, the idle jets are on the side of the carb (very easy to access).

Cheers,
Dom.
 
info is great...

thanks for this Steve, we'll have another go today. The carb has been completely disassembled and cleaned and the floats set. So we'll try some of this new info and see where it gets us. Will update later. Thanks again...
 
are you still

using the 36 DCNVA? In another post you mentioned that you had issues when stopped at lights, etc where it would be difficult to keep her going and you were going to go back to a different carb. Just wondering if you were able to sort it all out. The problem in my case, is that every time we fiddle, it costs money.

One more question, did you have to change the main jets at all??
 
Hey Bill,

Yep, I'm running a dcnva. I did experiment a lot, but in the end my issues were simply the idle jets were too small. I was running 50s this summer, and the car ran really well, just a bit on the rich side. I'm on 45s now but I think 47s are spot on for my engine.

I am running different main jets than the ones that came with the carb, but forget about these: they are not your issue as they have little impact below 3000 rpm.

Swapping out the idle jets is a 2 min job. After you do that, simply screw the idle screws all the way in and then back them out until the idle picks up and is most stable. Then another half a turn (out) from this point and you are done.

Cheers,
Dom.
 
No dice...

we have cleaned everything out and had the idle jets as much as 8 turns out, anymore and they will fall out and we're nowhere near where we need to be. The issue for me is that the labour cost to get this carb set up will far exceed the cost of the carb or another one, its starting to get silly. We've been fooling around with this for days. We are going to call Holly tomorrow and see if they have a dual that will mount to this intake. It will be much easier to tune and much easier to get parts for a Holly. This thing is just not working. We have a few things to try today and will have to give up at some point very soon. Is it possible that a vacuum leak would lead to lower vacuum in the venturi and therefore its not sucking in the fuel as it should? Its just dribbling in, at idle or at open throttle, when we blip the throttle, the accelerator pump gets us going, but there is just nothing coming out the venturi at idle or any other RPM. The carb has been totally disassembled and everything cleaned, we've even run paperclips through all the lines to make sure they are clear. A question about the spacer. We are using the one with the hole drilled in it, which is your template and you think it might be cracked. I would describe it as weeping around the spacer, not really wet, but there is definitely fuel there. Is it possible the bottom of the carb is not flat as mentioned by others? We will drill the other spacer today and try that.
 
good to hear

that you're still running this carb. We'll see what we can do, just can't believe with the 40s installed, the fuel is dribbling out the venturi. I would have thought that it would at least show a spray of fuel, just not enough of it, but nothing like that at all, just dribbling off the bottom of the pipe. I'm wondering if there isn't enough vacuum to pull the fuel in.
 
Idle jets

Bill,

I have 2x Weber 50 idle jets and 2x 55s. They're yours, just PM me your address.

Now, you will never, never, never see air/fuel mix discharge from the auxiliary venturi at idle. The idle discharge port is below the throttle plates and out of sight until the throttle is opened. There is one, two or sometimes even three transition ports above the idle port that are "uncovered" and exposed to manifold vacuum as the throttle plate opens. These are there to reduce any hesitation as the carb transitions from idle to open throttle conditions.

If you are turning the idle mix screws out that many turns and not seeing any benefit, then you idle jets are TOO SMALL or you have a blockage. It indicates there is not enough fuel in the mix and turning out the screw on a jet that is too small won't magically produce more fuel. Instead you need a jet that is bigger than you really need and then reduce it's flow with the mixture screw.

I'm still concerned that you are describing dribbling fuel drops. That is NOT RIGHT. It's screaming at you that there is a blockage, float levels are wrong or the needle and seat are worn or any combination of those. At idle there is not enough vacuum in the auxiliary venturis to draw anything out of them. Therefore there is something definitely wrong. I don't recall, do you have a return line to the fuel tank? If not your fuel pump may be overwhelming your needle and seat pressure at idle and overflowing your carb. I repeat, dribbling fuel visibly at any throttle opening is wrong.

And yes, an air leak at the base gasket will make all this work a waste.

One final point, be careful about sticking paper clips or similar into small galleries in carbs. Carbs are precision instruments and some parts can be easily damaged. Compressed air is better. Soaking in a solution to dissolve any deposits or blocks is better. And finally, if you have access to a sonic bath, try that. I've gone that route with success in the past.

Good luck,

Rob
 
Spray of fuel from the aux venturi aperture does not happen until there is sufficient air flow into the engine. This does not happen with the engine at idle and simply running up the RPMs. The engine must be under significant load with equally significant air-fuel mixture required to produce the required energy from the engine.

All weber carb passages and jets must be clear including the fuel feed inlet parts.

If the needle-seat is not sealing with the float at the high set level, fuel in the bowl will be too high and could cause a problem like this.

What about the float level?

Are the emulsion tubes clean and clear?

What is the fuel pressure? Much higher than 3-4 Psi is going to cause problems.

How well does the throttle plates seal against the side of the carb barrel? Could the throttle plates be out of alignment, worn or worn throttle shaft bushing-bearings? It is also possible the throttle shaft is bent by previous owner abuse.

Weber carbs are well made precision devices that are problem free provided they have clean fuel and proper care.

Holly is not likely going to have a suitable replacement carb, even if yes, the fuel performance is not going to match the Weber carb.

Check this,

Bernice





WildWilly said:
the fuel is dribbling out the venturi. I would have thought that it would at least show a spray of fuel, just not enough of it, but nothing like that at all, just dribbling off the bottom of the pipe. I'm wondering if there isn't enough vacuum to pull the fuel in.
Click to expand...
 
Idle screw too far out.

Hi Burnice,

Bill told me in a PM that the mechanic has the idle screws 8 turns out from base, which is way too far (2.5 is considered max). This is a typical symptom of undersized idle jets. With the idle screws outr that far, my suspicion is that fuel is bypassing the idle circuit and flowing into the primary circuit.

Bigger idle jets will put the issue to bed, one way or another.

Dom.

Rupunzell said:
Spray of fuel from the aux venturi aperture does not happen until there is sufficient air flow into the engine. This does not happen with the engine at idle and simply running up the RPMs. The engine must be under significant load with equally significant air-fuel mixture required to produce the required energy from the engine.

All weber carb passages and jets must be clear including the fuel feed inlet parts.

If the needle-seat is not sealing with the float at the high set level, fuel in the bowl will be too high and could cause a problem like this.

What about the float level?

Are the emulsion tubes clean and clear?

What is the fuel pressure? Much higher than 3-4 Psi is going to cause problems.

How well does the throttle plates seal against the side of the carb barrel? Could the throttle plates be out of alignment, worn or worn throttle shaft bushing-bearings? It is also possible the throttle shaft is bent by previous owner abuse.

Weber carbs are well made precision devices that are problem free provided they have clean fuel and proper care.

Holly is not likely going to have a suitable replacement carb, even if yes, the fuel performance is not going to match the Weber carb.

Check this,

Bernice
Click to expand...
 
Floats should be at 42.5 / 52, refer to this post here for more info.

Bernice, the carburettor is NOS, so shaft wear, bent base, previous abuse etc isn't the issue here...it's an adjustment thing...

SteveC
 
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