NOTE: The following was posted by “Docmirror” on the VLG
forum. It contains his observations on setting up the three barrel carbs used
on the Miuras. Good Reading!
Hi guys, I've been time challenged recently but I wanted
to jump in
about the engine popping. I'm afraid it's likely caused
by
carburation misadjustment. The only way to have a
"pop" sound is if
there's fuel in the combustion system. Regardless of the
advance
curve of the dizzy, there will be no pop without fuel,
and of course
valve overlap.
The Muira has those quad IDF 40mm 3 barrel which makes it
different
than the Espada/Jarama/400GT etc. I have literally NO
experience with
the three barrel IDF, but gobs of experience with the IDF
two barrel.
I am guessing that the configuration and setup is very
similar. As
you certainly know, the 2500-3200RPM band is the
transition from the
idle metering circuit the main circuit. The crossover
isn't exact
and varies primarily with jetting and vacuum across the
venturi area.
In any one-on-one barrel to valve system there is
something
called "reversion" that is significant
depending on the length of the
intake, and the RPM. When the intake valve closes, there
is a push-
back pressurization that causes the mixture to actually
back up in
the intake runner. With the throttle nearly closed, this
backup is
fairly well blocked, with the throttle partially open,
the reversion
is actually visible at the velocity stack in an
over-carbed system
such as the Muira. I am guessing that this is the primary
reason of
some of the fires in Muira's, as a bad case or reversion
can lead to
fuel soaking in the air cleaner. Then, one backfire up
the intake
will ignite the mixture on the air cleaner as it is
drawing a nice
steady stream of fresh air to help burn.
Avoidance: I doubt the car was originally designed to pop
or backfire
into the carbs. It is a matter of tuning and adjustment
of the carbs
to minimize rich fuel conditions. Fuel in the '60s was
thicker than
it is today. That means that most carb cars are too rich
for today's
polyglot mixture of fuels. Partially offsetting this is
the reduced
volatility of modern fuels, making them rich on fuel, but
still able
to burn fairly well.
First: Remove the carbs from the car, disassemble them,
and clean,
clean, clean. Check for warpage of the baseplate, and
reassemble with
a new needle and seat, and check the float level
carefully. Make the
level slightly too low, rather than too high. Very
general
instructions ar found at:
http://www.carbs.net/Weber/Tech/idf_adjustment_controls.htm
Next, you will need a Uni-syn or other sync tool and
preferably a
color-tune. If you have someone with an exhaust gas
analyzer, that
would be ideal. Start each barrel lean, rather than rich
so that you
don't foul all the plugs. Have the carbs sync-ed
preliminarily, and
then go to the colortune to get the idle mixture set.
Bunsen-blue is
ideal for mixture, but here's the rub; At transition RPM,
this will
make the carb too rich as the main jet comes online!
leave it a bit
lean, so when you reach bunsen-blue on the colortune, go
in maybe 1/8
of a turn(depending on sensitivity of adjustment). Once
all the
barrels are set, go back and finish the sync. Then, move
on to the
main circuit.
I would recommend going down one size on the main jet,
but be careful
with this, as too lean can lead to detonation,
overheating and
destruction. So, too rich is giving popping at decel, and
too lean is
detonation and destruction. If you found the EGA, you can
use it to
sniff the mixture on main circuit. don't leave the
colortune in while
above about 2500RPM, it might also self destruct.
Different emulsion
tubes can also affect mixture at higher RPM, but when you
start doing
this, you need to consult a Weber guide to determine
tube+main jet
combos. Generally speaking, the more holes in the side of
the
emulsion tube, the leaner the mixture.
Diagnosis: If the car is popping at 3000RPM on
deceleration, there is
fuel getting into the venturi, where no fuel should be.
This is going
to be a leak in the float chamber over to the venturi, or
fuel
bypassing the needle and seat and getting to the venturi
that way.
These carbs are old, and they do leak under high vacuum.
If they
don't pop during smooth throttle operation around 3000RPM
partial
throttle, then I would suspect a leak.
Let's see, the rebuild kits are available from carbs.com,
parts too.
Don't forget the accelerator pump ball check and jet
cleaning. The
plugs will tell if it's rich, but it's so hard to read,
because you
can't keep it at the same RPM all the time until removing
the plugs.
A carb tuner will get it really nice, but that's pricey.
I would
start with a good cleaning and baseline setup before
hiring it out.
I've found all kinds of things wacky just by disasembly
and cleaning.
Like sand in the float chamber, a cracked seat valve, air
bypass screw
(side of carb) without a ring gasket, venturi locknut
missing, cat
hair in the idle circuit, etc.
Don't forget the baseplate surface check, as it's a
common problem.
Also check the mainfold flatness, it can't be made up
with thicker
gaskets or silicone. The carb may be 'setup' for a
venturi with a
vacuum leak, thus throwing off all the calibration for a
normal
venturi. If you don't want to buy a whole pile of main
jets, I have
had good luck using a soldering iron to close off the jet
hole and a
jet bore(drill) to make a new size jet. I've had to go
down two sizes
due to the altitude, and my idle screws are nearly all
the way in.
The car stumbles around 3000RPM, but no lean above that,
and I don't
pop at all.
Taller velocity stacks will reduce, but not eliminate the
reversion.
I don't know what kind of air cleaners it uses. If you
don't mind me
going bonkers here, you could get smaller inner venturis
for the
carbs, thus increasing the speed of the air through the
venturi, and
improving all kinds of things at mid range, with a
concomitant
reduction in top RPM/speed. It's a darn shame that the
RPM we want to
use so often is the band where the Webers get hinky.
There is a
Delorto carb that has three circuits and they have a
noticable stumbe
at 1800-2200RPM where idle to mid crosses.
Hope any of this helps. It takes a bunch of time to get
this right.
I've spent hours fiddling with them on VW, Lambo, Datsun,
BMW. Add a
turbo, or change the cams, and then you start over!
Doc