Hi Zack,
Probably everyone will have advice for you here but
since I'm part of 'everybody', here is my contribution.
Silly things:
1) Look for vacuum leaks. It could be a missing gasket
or a warped carb flange or something as silly as a
disconnected or missing vacuum hose.
2) Check your fuel plumbing. I once had a nightmare
of a time with an MGB only to discover that in connecting
the new fuel line, I had plumbed it to only one carb! Yea,
it's stupid but things like that do happen.
2.5) Make sure that you have 'top-oil' in the carbs and
that the air pistons lift against resistance and fall freely
with a nice 'noise' when the reach the bridge. If they don't
you may have to recenter the jet.
Static adjustments
3) Check the settings of the floats in the float bowls.
They need to be right or the height of the jet is almost
meaningless.
4) Adjust the height of each jet to FLUSH with the top
of the venturi bridge. THEN, turn the mixture adjusting
nut down (read: lower the jet) two full turns. The nut
is a 32 tpi pitch so two complete turns lowers the jet
by 2/32" or 1/16". This is your starting place. From
here, always turn the mixture nuts by the same amount.
That is, do exactly to one carb what you do to the other.
5) Loosen all the linkage so the throttle butterflies can
move independently. Back the idle speed screws out
so that they don't touch anything. With both butterflies
held gently closed, snug the linkage clamps. Verify that
things move smoothly and both carbs throttles travel
fully from closed to open. Don't worry about flooding
the engine. SUs don't have accelerator pumps.
6) Screw in one of the idle speed screws until it just
touches the bracket. From there, tighten it one more turn.
7) Screw in the OTHER idle speed screw until it just
touches the bracket. This completes the static adjustments
and you are ready to begin "tuning."
Tuning:
8) Start the engine and allow it to warm up. If the idle
speed is excessive, back the idle speed screws out by
equal amounts until a safe idle speed is established.
Remember, you are only warming up the engine at
this stage.
9) Once at operating temperature and with the "choke"
fully OFF, synchronize the air flow by comparing the
UniSyn's readings and adjusting the idle speed screws.
Very little adjustement should be needed. This is the
FIRST adjustment. Later, a final tweak is applied.
10) Connect a vacuum gauge to the manifold. This
needs to be at a point PAST the throttles. You can
usually find a port somewhere to tap into. Measure
and record the vacuum. If your mixture is close, the
engine shouldn't be 'hunting' too much and the vacuum
reading should not be jumping around.
11) Manifold vacuum at idle is a good measure of
the combustion efficiency of an engine. The
vacuum is the HIGHEST when the engine is in the
best tune. Therefore, you need to tune for highest
vacuum.
12) Read the vacuum at a given RPM and record the
values. Adjust BOTH mixture nuts up or down by
about one 'flat' (one sixth turn). Reset the idle speed to
that of the first reading, if required. Read the vacuum.
Is it higher or lower? If the engine speeded up
when you adjusted the mixture, you probably
made it run more efficiently and the vacuum should
now be a little higher. Tweak the other way if needed
or add another 'flat' and measure things.
13) You get the picture. Keep the turns to the
mixture nuts synchronized and remember to compare
vacuum reading only when taken at the same engine
speed.
14) When you maximize the vacuum and reset
the idle to the reconmmended speed, you have
only one more step/tweak: Re-synch the airflow with
the UniSyn, if required.
If you have running problems after all that
setting and tuning, you don't have a tune-up
problem. Something is wrong. You are then
back to looking for vacuum leaks, misadjusted
floats, bent/damaged needles, wrong needles,
bad float bowl valves, miscenterd jets, bad
gasoline, etc....
I hope this helps. I certainly burned up some
electrons -- not to mention neurons! :o)
best,
rick / Currently MG-less.
Ps: I have a slow-running mixture problem on
my '88 BMW E30 M3. That guy is all Bosch
computer controlled and it STILL has a stumble
at 1300 rpm! Yet, I digress...
> Well I went ahead and tried to adjust the carbs. I
> thought that I understood the directions, but I guess
> not. I first tried adjusting the adjusting screws
> because the car was running rich. This was after I
> had loosened all the clamps for the throttle
> interconnection. I was going by the manual and by the
> instructions for the Unisyn. It didn't run very well
> after that, so I stopped by an import shop around here
> today. I wanted to find out if there was maybe a
> baseline for what the needles should be set at. He
> told me that they should be about 1/16" above the
> bottom of the opening of the intake on the carb (hope
> that makes sense). THen the car wouldn't run without
> the choke on. So I tried readjusting those back down
> to where they were, and now it backfires when I rev
> the engine. This is my first attempt at this.
>
> Zach
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