Changing Weber DCNF venturis: photo how-to

This is probably the most complicated thing you can do with your carb still in the car, and it's still very easy. In the next two posts, we'll disassemble the carb and remove the primary venturis; reassembly is the opposite of taking apart. A couple of words of pre-advice: keep everything VERY clean and VERY organized. The whole thing should take about an hour or so.

1. I set up a little operating area with a clean cloth. Get your tools ready - I like a ratcheting 8mm socket and a wide blade screwdriver. You will also need a 5/8 (or 16mm) wrench. Also, a small tackle box or something to hold all the nuts, washers, screws and parts you will find really speeds things along.

2. Remove the 8mm nuts and washers, 4 per air horn, careful, VERY careful, not to drop them in a carb throat. Put them away in a bin so they do not get dirty or lost. Remove the air horns.

3. Loosed the hose connections to the airbox at the breather and scoop ends. Then carefully lift the airbox up and away. Be wary that each of the four rubber spacers in the airbox have four small metal inserts that the bolts go through - sometimes they may try to fall out. Be careful that you get them all and do not let them fall into a carb (see a theme developing here?)

4. Using the 5/8 wrench, loosen the connections for the fuel hose, and be careful of the gaskets and brass fittings. These should just be snug, NOT tight, so be careful here. Place the brass fittings and gaskets in a clean place. You will need to take off the smaller fuel return hose if you have one (you should). Lay the fuel hose off to the side out of the way.

We'll continue in the next post.
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CONTINUED FROM POST #1

5. Loosen the screws holding the top of the carb onto the body. There will be either four or five. Place them away in a clean bin with their lock washers. Gently lift the carb top and the float assembly up and out - wou may have to slightly jiggle it as it comes out. Remove the gaskets. You may can re-use them, but they are dirt cheap so just use new ones going back in. Lay these gently in a VERY clean place for storage.

6. THIS IS THE MOST DANGEROUS STEP. Unscrew the centrally mounted accellerator pump jet which sticks into the barrell. THERE ARE SMALL COPPER WASHERS ON BOTH SIDES. Some folks leave the bottom one in place in its well, some take them both out. IT IS A WILEY BASTARD AND WILL TRY TO JUMP INTO THE CARB. Be careful. Lay the accel pump jet in a very clean bin for storage.

7. Now, the aux venturi is held in by a small spring tab, so pull it straight up and out. This piece is directional, so when you put it back in, be sure the hole side goes toward the brass jets and the spring clip toward the outside. Lay aside in clean area.

8. The venturi is a light metal ring that just is lightly pressed into the bottom of the carb throat. I use two fingers, and while slightly rotating slide the venturi up and out (note the fat side with the size number is on top). You may need to spray some lubricant and let it sit if too tight. A little wiggle might help as well, but they usually are easy to coax out with two fingers.

You can now see down the open barrel to the throttle butterfly plates. Re-assembly is the opposite of this. BUT, before you put the airbox back on, power up the fuel pump to ensure no gas leaks at the brass fittings that you removed. You may need new gaskets - Do not overtighten - it just makes it worse. You can start it without the air horns and airbox to likewise check for leaks, ensuring no overly dusty or dirty particulate air is around. It's a nice sound if you haven't heard it, and it gives another chance to inspect.
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Very nice thread, Russ. More good stuff from the DCNF Maestro.
Thanks.

Ciao,
George

 

Nice post! Did you blast your manifold with a certain media? If so what media did you use? Also your airfilter looks great. Is it refurbished? If so who or how did you do it? thanks

 

Great post and pics.

My back aches just thinking about doing them in situ, though.
Also like to check all is well with carb body to manifold. Working with Alfas and weber installations on small british motors one often found Gorilla Joe has bolted it down.

Do you find shaft or mounting flange air leaks very often with these older 308s?

 

Thanks!
The manifolds are just cleaned. The airfilter, though got bead blasted and powder coated.

You are obviously quite correct that one of the main things about older Webers is that they start to leak around the shaft flanges if worn out. I have heard about it but never seen it personally. I may be wrong, but I understand that all newer replacement carbs (of which I am using) and recent rebuilds now utilize an upgraded shaft bearing seal less prone to leak.

Also agree is a terrific time to check the carb body. The big check I do is when the top and fuel line goes on, pressureize the system and look at every connection and jet for leakage or dribble - especially accel pumps and the air vent to the float chamber.

Here are a couple of photos showing the longer 4.5 aux venturi which you should have. The smaller 3.5 is commonly used in high reving motorcycles. The second photo is a comparison of venturis, and these are sizes 34mm, 36mm and 38mm. The stock size for a carb 308 is a 32mm venturi. The sizes make a noticible difference in characteristics. I am discussing that a little in the Weberology 501 thread as well.
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Would it not also be advisable to first disconnect the battery before starting this procedure? (what with the gas lines disconnected and all)
If you or anyone else were to turn the key on during this process, there could be a potential for fire hazard when the electric fuel pump starts doing its job.
This is great information. Thanks for sharing your tips.

 

I guess it depends on your shop; I am usually by myself and do not stop until this job is finished, taking only an hour. For sure, if I had to stop inbetween and leave it undone I would disconnect the battery as well as cover the carb throats. Someone would have to open the door, insert and turn the key after finding and turning on the hidden anti-theft kill switch that I only know where it is.

 

I like the hidden kill switch idea. I've got a hidden fuel pump kill switch on my 62 Jaguar XKE coupe.
You can get about 1 block before running out of gas if you forget to turn it on.
It's a simple push on push off button hidden inside the foam on one of the seats. You need to push on a certain part of the upholstery and it's 100% hidden.