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Weber DGV
Carb Hesititation Expose
As
of: 9 February 2004
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.
John Dandy
(theAutoist
NOTE: John Weimer's new "nom de plume")
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