Carb Hesititation Expose
Let's try and expose some of the deep dark mysteries of the Weber DGV carburetor:
The most common problem you may possibly find with a Weber DGV is hesitation upon acceleration, but I have never experienced it in 24 years of combined use of 2 vehicles equipped with them. Most people experiencing hesitation think, "put in bigger jets" right off the bat. Well, forget that silly notion, the jets have nothing to do with it.
At this point, before we get further into the subject, you may want to print the exploded view of the carburetor at http://www.piercemanifolds.com/DGV5A.pdf to make it a little easier to keep up. You'll probably also want to print out the parts list at http://www.piercemanifolds.com/dgvpl.pdf to help you understand what's what on the diagram. Heck, you probably want both of these in your car's log book anyway!
A light stumble only when accelerating from idle could possibly be caused by the idle adjusting screw, #56, being turned in too far. Back it out 1/8 to 1/4 turn and try it again. If backing it out as much as 1/4 turn doesn't cure it the problem is in the accelerator pump circuit. This circuit is made up of parts #34 through #37 and parts #80 through #82 in the exploded view with passages drilled through the carburetor body, and I'll explain how it works.
Gas is admitted from the float bowl into the chamber under the accelerator pump diaphragm #36. *Note: I noticed something missing from the illustration. There has to be a check valve between the float bowl and that chamber or the pump can't work, but evidently the artist missed that part. When the accelerator is depressed the linkage acts against the lever in part #34 which in turn pushes against the pin on part #36, the pump diaphragm. This action forces gas through a passage into the bottom of the metering well which is the passage part #82 screws into. Part #82 is not only a screw securing part #81, the accelerator jets, but has a check valve built into it. That check valve keeps the metering well full of gas at a level above that in the float bowl and ready for an immediate squirt through the jets upon acceleration. OK, here we go. The pump forces gas through a passage to the metering well, up through the check valve, and out the jets. When the accelerator is released the pump diaphragm sucks more gas into the pump area and the check valve in #82 keeps the metering well full and ready for the next shot. Now to explain how the check valve works we'll use a simple analogy. Say you have a drinking straw standing in a glass of water. If you suck water up that straw and hold your tongue against the end the straw will stay full, but when you pull your tongue away the water will fall to the level of that in the glass. That check valve in #82 works the same way. If it doesn't hold, the level of gas in the metering well will fall. Now, say we punch a pin hole in the straw. suck the water up, and try to hold it, We can't because of the air leak and that same thing happens in the carburetor when the check valve in #82 is working properly, but #82 isn't tight or the gaskets #80 are leaking for any reason. So, A bad or fouled check valve in #82 or any leakage around the gasket area will allow the gas in the metering well to fall to the level of that in the float bowl leaving a void at the top that must be filled by the accelerator pump before gas can exit the accelerator jets. While this void is being filled the throttle butterfly(s) is/are opening, but without an immediate extra shot of gas and the engine hesitates until gas reaches the accelerator jets or the venturi starts sucking gas through the main jet(s).
It's as simple as that.
Any problem with the accelerator pump diaphragm itself will show as a gas leak at the cover #34 and the diaphragm will have to be replaced. Don't be concerned about that, I ran one for 14 years and it never leaked.
Like any carburetor there are several other problems that can occur with, age, wear & tear, and trash in the gas. Most of those can be overcome by rebuilding the carburetor, cleaning out the trash, or minor adjustments.
The most popular type on small English cars is the DGV 32/36 with manual, electric, or water chokes designated by a suffix number such as 5A for a manual choke. This would be listed, Type 32/36 DGV 5A. Another type that is becoming more popular is the DGV 38/38. To learn the differences put "Weber Carburetor" in a search engine, find some sites, read about them, and phone the vendor if you need to.
When buying a Weber kit it is best to buy a new one complete with everything you'll need to complete the job, and to get the support of the vendor. Many times second hand ones have been messed with somewhere down the line until they are, well, a mess. Internet auctions are the best place to find a screwed up carburetor. If they won't sell it to a friend or somebody they're liable to meet just leave it where it is.
In order to install any Weber kit you'll have to also have an early exhaust manifold or an exhaust header. If you're not modifying and tuning the engine to the max, like in serious racing, it doesn't make any difference which you use. The first thing you want to check is the thickness of the securing flanges on the intake manifold and exhaust manifold/header. They should be within 1/32" of being the same thickness. Don't cut the flanges thinner on the aluminum intake to get a match, you could weaken them to the point of causing them to break, grind the outside of the exhaust flanges if they are to thick. Most likely they will be too thin to match the intake and in that case you'll have to weld on some tabs or make up some step washers to even up the clamping pressures of the original flange washers and nuts. Now for the gasket. If you're using an early exhaust manifold the regular stock manifold gasket will work fine, just be sure to install it with the metal side facing the manifolds. When using a header you need to also use a header gasket. They are normally a little thicker than the stock gasket and look like heavy gray paper. They are also more susceptible to compression and leakage, but this is easy to control and eliminate. Just tighten them normally during installation and again after the engine has ran to operating temperature and cooled, it doesn't have to be dead cold. Thereafter tighten them every time you drive the car until that time they are tight at the start. At that point the gasket is completely compressed and you don't have to worry about it leaking. Otherwise just follow the instructions in the kit.
A couple of things you may want to do before bolting the carburetor on are: Set the idle adjusting screw, #56, by turning it in until it seats and backing it out 1 1/2 turns. Further adjustment is usually required when the engine is running, but that will get you started. The other thing is to adjust the automatic choke when so equipped. Loosen the choke housing screws and set the carburetor on a flat surface so the throttle linkage is off the edge of the table or whatever. Open the throttle and turn the choke housing until the choke butterflies, #5, close. let the throttle close and don't touch it until you're finished setting the choke. Now, open the butterflies with your finger and pull it away quickly. The butterflies should close completely, but rather softly with only a soft click, not slam shut like a rat trap. Adjust the choke housing until you achieve this effect and tighten the housing screws. This is often the only choke adjustment you'll have to make, but each car is an individual and you may have to re-adjust in actual use. A lean choke is indicated by coughing and sputtering on acceleration when the engine is cold. A rich choke is indicated by blubbering and lumpy idle when the engine is cold. To adjust, loosen the choke housing screws only enough to allow you to turn the choke housing and move the index mark on the housing 1/32 to 1/16 of and inch richer or leaner as you see fit and try it again when the engine is cold.
In addition to the above and kit instructions go to
http://www.mgbexperience.com/service/weber-hose.html and install the hoses and vacuum lines as depicted in the last photo and text. Do it as shown and you'll have it right. If your car doesn't have carbon canisters simply eliminate that part of the instructions.