****The following is a copy of an old Monte Carlo Mailing List post.   ****

I have tuned many Q-jets by using a tailpipe mixture tester, tachometer
and changing jets or metering rods. They apear to do the same thing as far as
I can tell. They work hand in hand at all speeds above about 1000 rpm when it
stops using the idle circuit. Why would changing rods be more beneficial for
part throtle?

If there is a reason, maybe I need to change my strategy for tuning Q-jets.

Bob, you should probably change your strategy for tuning :)

First of all, I am assuming you have some type of emission setup/apparatus
with which you can find the exact air fuel ratio from the tailpipe. Note,
you can't do this with a regular oxygen sensor (EGO).

Second, I'm assuming that you are only tuning for idle. Or do you have
a chassis dynomometer?

I don't know how many Qjets you have tuned, but maybe it's possible that
all of the ones you used, in the same application, had the same jets/
metering rods, OR a metering combination which just so happened to
work for your engine. (I'm assuming you are not talking about an
electronic Qjet here).

You see, the idle mixture WILL change as you change the primary jets/rods,
but that is not how you should go about tuning. Here is my strategy in
Qjet tuning (choke tuning left out):
 

1) start Qjet off with an approximate combination of jets/rods for the
application. You need some experience, or hints from other people
to do this if you are a beginner. The book's hints are too rich IMO.

a) what rods/jets you start off with will also vary with what model of
Qjet you are using. For instance, if you are using a late 70's
Qjet with the primary fuel enrichment circuit, you would tune
the primary system leaner than you would if the Qjet did NOT have
the primary fuel enrichment system (this system I speak of consists
of two holes above the choke blade, which shoot out fuel at heavy
part throttle.

in order to find out what your Qjet does/doesn't have, you should
read that book I mentioned thoroughly.

2) When you first start up the engine, you need to try to get an
idle speed that will keep the engine running. Once you get
this accomplished, you should hook up a vacuum gauge to full
manifold vacuum --OR hook up a tach like you have. They will
both increase somewhat proportionally for idle tuning. You want
to adjust the idle mixture screws until you get the highest
vacuum/tach reading at the idle speed you set originally. Note,
this will increase your idle speed, so you will have to go back
and turn it down again. And possibly even re-adjust the mixture
screws. And do this over again, until you get a consistantly
smooth running engine at idle, with the highest vacuum.

3) Now it is time to adjust the primary metering. Hopefully
you will have selected the right jet size already, so that you
only have to play around with the metering rod size. Bob,
the power system is vacuum operated (unless you have an
electronic Qjet, in which you can just neglect all of what
I am saying). When vacuum is highest, the power piston is
pulled down against the force of a spring under it, and it
in turn pulls the metering rods down into their respective
jets. During cruising, and part throttle driving, this
tapered portion of the metering rods will greatly effect
the fuel mixture. The larger the size of the rod (the number
stamped on the rod indicates the size in .001" of the top
of the taper), the leaner the rod is, since it will allow
less fuel to flow through the jet.

I have found that part throttle tuning is best done on a flat
road, with no traffic, and regular, typical temperatures outside.
You should see how the engine runs/sounds at several part throttle
positions while traveling between 25-45 mph on a flat road,
and even at higher speeds.

Qjets can be tuned so that you get good gas milege, AND crisp
throttle acceleration, so you should not settle for less than
this. There should be no lean bogs, or stumbling due to an
over rich mixture, but if there is, you can correct for it.

4) If you notice problems when you leave from a dead stop, then
it is most likely due to the accelerator pump shot. Too lean
of a shot will give you a nice lean bog in which the engine will
sound and feel like it has suddenly died out. Then, if it isn't
too bad, the engine will suddenly kick back in, and you'll take
off.

If you have no 'bog' as above, but the engine runs rough and
stumbles right away, then you most likely have too much of
a pump shot. This shot can be adjusted as described in the book.
I won't go into it here.

5) primary metering rods and primary power piston springs pretty
much have to be tuned in unison. I think it is an aquired skill,
and I don't claim to be an expert, although I think I got
a very good combination for my application. The stiffer the
power piston spring you use, the faster the carb will activate
the power system in relation to how you press on the pedal. That's
the easiest way of thinking about it. But you should not confuse
the activity of the power system with that of the accelerator
pump shot. The power system doesn't do much when you start
off from a dead stop. Rather, it comes into play when you are
cruising around at part throttle.

You should be able to find some combination of power piston spring/
metering rods to suit your application.

I'll note here that certain power piston springs are rated
at different operating points, by vacuum. But when you are
out tuning on an engine, you will see that this will only get
you in the ball park (i.e. by looking at your vacuum gauge
during part throttle accel). You will ultimately have to
tune even the piston spring by feel.
 

6) If you find a combination that feels just right, and with which
you get great acceleration, and no bogging or rough running, then
install a set of new spark plugs, and go for a nice long drive,
part throttle. TAke out the plugs when the engine cools, and
inspect them for signs of a good mixture. This will just confirm
the mixture you have made, and is always a good step in tuning.

6.5) Before you get to this point, if you have made considerable
primary metering changes, you have probably noticed that your
idle quality went all to hell. You should readjust that as described
above, now.

7) Now you are ready for secondary tuning. You should make sure
that none of your primary tuning was done in the secondary range
(i.e., with your foot REALLY on the pedal). Now that you have
adjusted the primaries, you can press the pedal further down,
until the secondaries open up, and notice if there is any lean
bog. If not, then proceed with tuning: go to a drag strip,
or to a flat open road with no traffic (or cops:)), and do WOT
power runs. This is my favorite tuning :) (OF COURSE), and
I think it is pretty easy to tell how the engine is running
by listening to it, and feeling it as it revs up and you accelerate.
You can change the secondary metering rods for different
mixtures. Note that at WOT, only changing the power tip size
is really going to make a *BIG* difference. Changing the taper
only probably won't do much.

8) Advanced WOT tuning can include changing the hanger on which
the secondary metering rods hang, and the angle to which the
air blades can open -effectively adding or subtracting flow
capacity of the carburetor. When you do this, you can run
into additional transition-to-secondaries problems.
 

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