How to tune a Weber DGV Carb!!
Its actually really simple,
assuming that your carb is in good shape otherwise.
This is the procedure to be used on ANY DGV carb
once it's installed on the engine, correctly "plumbed" and the linkage is connected.. Essentially the final tuning
after the carb has been rebuilt and you know that it
has the correct, or "close enough" jetting for your
1. Gently bottom mixture adjusting screw (the one at
the base) and back out 1.25-1.5 turns.
2. Turn idle speed adjusting screw so that it just
hits the throttle bellcrank.
Now turn another .5
3. Start engine using whatever means necessary-choke,
It shouldn't be very hard to start,
but every once in a while...
4. Nurse the car with the throttle/choke/whatever
until it warms up and you can completely open the
choke. Note, you may need to bump the idle up to get
it to idle without any throttle.
Ok, now it's idling on its own without the throttle.
5. Now SLOWLY adjust the idle mixture screw (the one at
the base) and find the spot where you get the FASTEST
idle speed. On a properly rebuilt/jetted carb, you
shouldn't have to go too far, probably less than a
full turn in either direction.
i.e., the setting
should ultimately be somewhere between 0.5-2.5 turns
6. Now that you have the fastest idle speed
attainable, back the idle speed adjustor down until
you get the idle to 1000 rpm or less. If you can't do
that, then you might want to make sure you don't have
too much ignition advance. You basically want to get
the idle to spec AND have the throttle as closed as
possible. (The reason for this is if you have to have
the throttle open any, you begin to bring the
progression ports into play which suggests that you
don't have the correct jetting for you application.
In this case, your idle jet would be too lean and
you'd need to go to a richer idle jet).
7. Now, you have got a relatively smooth idle with
the idle screw turned out as far as possible, i.e.,
the throttle is as closed as possible. Now, very
slowly, tweak the mixture screw to again find the
fastest/smoothest idle. The changes made at this
point will be subtle. When you find the fastest idle,
then once again, adjust your idle speed screw so that
the idle is where you want it (typically 900-1000).
Again,, at this point you should be screwing the idle
speed screw OUT to reduce idle, the idea being to have
the throttle as closed as possible and still have a
stable, smooth idle around 900 rpm (or whatever you're
You're 9/10's finished fine-tuning
8. The last thing I do after this is I make sure that
there is little to no off-idle hesitation which would
indicate a lean mixture. The accel pump in the Weber
should prevent that, but sometimes, its possible that
your final mixture setting is on the lean side and so
you might get some hesitation. If you do, turn the
mixture screw out just a hair, like 1/16th of a turn
and then run the engine at 2000rpm for about 10
seconds and then let it idle. Now snap the throttle
open quickly. Hesitation? No, great, you're done.
Still some hesitation? Make another 1/16th turn out
9. Now when you're doing this, its important that you
are able to tell if you are hesitating due to a lean
mixture, or stumbling due to an overly rich mixture.
There are several clues that will help you out here.
The first is if its leaning out, you may get some
backfiring through the carb. The second is if its
over-rich, you'll see lots of black smoke when you
snap the throttle(s) open. The third is if turning
the idle mixture screw out a sixteenth of a turn makes
it worse, then its over-rich. Ultimately, what you
>want to see when you snap the throttle open is a hint
of black smoke out of the tailpipe. No smoke, you're
too lean. Just a little smoke is what you want.
Now, if you've got a nice, perhaps *slightly* burbly
idle around 900rpm, and an engine that snaps
enthusiastically off idle, you're done! Put the air
cleaner on, and take a spirited drive around the block
and enjoy your properly set-up and tuned Weber!
And, like the DGV, this procedure works for any
Weber application. The main key to remember is that
you want your idle screw having to move the throttle
as little as possible, since, again, if you have to
open the throttle very far, it will uncover the
transition ports, and that suggests that your idle jet
is too lean.
One other consideration is that if you have the
throttle just about closed and the mixture adjusting
screw pretty far in, then your idle jet may be too
Another note: I keep saying that you want the
throttle as closed as possible, but in actuality,
having the idle speed screw turned in about a half to
one turn (after making contact with the bellcrank) is
about right. Any more than that and your idle jet is
too lean, any less than that and your idle jet maybe
So, basically, set to initial settings (mixture screw
1.25-1.5 turns from bottom, idle speed screw 1 turn in
once contacting bellcrank), install. Start, warm-up
engine, make sure choke is fully open, then adjust
mixture screw to get fastest idle, bring idle speed
back to spec with idle speed screw, tweak mixture
screw to get fastest/smoothest idle, adjust as
necessary with idle speed screw. Now check for
off-idle lean-ness/richness and adjust accordingly.
Not too hard, and no special tools are necessary,
just a tuned ear! Or a hand-held tach or dwell meter
with tach function. And, I'd be ashamed if I didn't
tell you to go through your ignition system once you
get things running with the carb. Set ignition
timing, etc. Once you do that, you may need to go
back and make some final adjustments on the carb, but
that should be minor, unless your ignition system is
Hope you enjoyed this! Now, go get your hands dirty!
President, North Alabama British Motoring Society
Owner, Ben Pender's Vintage Imported Auto Repair