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Just as I predicted a couple of decades ago (if I do say so myself), fuel
injection has taken over. So much so in fact that you can't buy a new car today
equipped with the traditional means of mixing air and fuel. So why a carb
section at this late date?
Simple: If you were to limit your fuel system knowledge to FI only, you'd be
uninformed about half the cars out there. Because the average age of the
automobiles on the road is eight years or so, there are still many millions of
carbureted vehicles in our highway population. A large percentage of these aging
conveyances have fallen out of the professional repair realm into the care of
do-it-yourselfers, and I figured a review would help you cope during this period
of transition and upheaval in fuel systems.
Leery of theory?
I fondly remember my experience a couple of decades ago when I first learned
carburetion theory. Plenty of lights went on and connections were made to
problems I'd seen, but hadn't really understood. Thereafter, I had a great deal
more success troubleshooting drivability and performance complaints than before
my enlightenment. I'm a firm believer in the value of scientific knowledge in
automotive diagnosis, so I strongly suggest you go back and read the preceding
section on carb theory for a quick, painless review of carb principles that you
can apply to any specimen, regardless of who made it or when.
Big bucks and false witness
A couple of general observations are appropriate here. First off, it doesn't
seem like that many years ago when a brand new carburetor was reasonably
affordable. If you ran into a specimen that had serious problems, you could
simply take this replacement route and feel you'd done the most practical thing.
No more. It's not uncommon to see a price of $800 today, with quite a few
examples up over a grand.
As you should know by now, carburetors are often mistakenly blamed for
various troubles. I remember a slant six showing up in my shop that had had four
different carbs installed by others, yet continued to exhibit starting and drivability
problems (the real culprit was a combination of an ignition glitch
and a vacuum leak to #6). Examples of this sort of thing abound: The mix master
is wrongly accused when coked-up heat risers, a malfunctioning heated air intake
system, bad plugs, etc. are actually causing the symptoms. One quality
remanufacturer claims that 70% of the carburetors it gets back as returns are
actually okay.
I believe the reason is that a carb is an intricate, complex mechanism that's
not always well understood. It's sort of like the situation with computerized
engine controls: Because they're present and require considerable mental
discipline to diagnose, people tend to jump to the conclusion that they're the
source of the complaint, which results in perfectly good (and expensive) modules
and sensors being replaced when the fault is really something much more basic.
On the other hand, air/fuel mixers are indeed prone to many ailments, some
terminal. So, you've got to maintain a balance of mind while troubleshooting.
The old shotgun approach of replacing things until the problem goes away has
never been acceptable, but is much less so today at these prices.
Unless the problem is obvious (an overflowing bowl, perhaps, or a lack of
accelerator pump output), you've got to eliminate the other possibilities before
you condemn a carb. Otherwise, you're shot gunning, which is an expensive and
frustrating sport.
Floats that won't
Maybe you remember the plague of gasoline-soaked foam floats that caused
seriously over-rich running back in the seventies. Apparently, it's not entirely
over. A tech expert with a company that supplies replacements told me,
"Heavy floats are still occurring, and new float sales are still high. It
seems that as soon as the carburetor makers change the type of foam, there's a
new gasoline additive that defeats it."
When you encounter a heavy float, you often have a choice of replacements.
You can buy nitrophyl (the generic term for the closed-cell foam used in floats),
or the brass pontoon-type. Technicians frequently request the latter, quite
logically assuming that there's no chance they'll soak up fuel. But they aren't
necessarily invulnerable -- anything that's sealed can leak. As one parts maker
said, "We carry both brass and foam. When brass fails, it's catastrophic --
gas flowing out of the carburetor, maybe a fire -- whereas with foam it's
gradual degradation of how the car runs." My opinion? Use what's readily
available from a reputable source.
Feedback
The feedback carburetor was never all that satisfactory -- a relic of the
mechanical age trying to make it in the electronic era. It has all the frailties
and drawbacks of a regular carburetor, with the additional complications of a
remotely-controlled mixture system. The number of connections to the carb can
make you crazy -- a typical specimen has eight vacuum nipples and four
electrical terminals. There's a good chance you'll be seeing problems with such
an animal, so read the next section on feedback carbs.
VV land
The Motorcraft Variable Venturi carb, which was used on various '77 through
'82 Fords, gives great response and efficiency. That is, when it's working.
Its main weakness is that big vacuum diaphragm, which tends to rupture. That
results in ultra-rich running, a fouled O2 sensor, and perhaps even an
over-filled crankcase
from gasoline dilution. An approved modification that'll make the diaphragm last
longer is to drill a drain hole in the bottom of the cavity 1/8 in from the
gasket surface with a #46 (.081 in.) bit. This eliminates the nasty fluids that
eat up the rubber.
Another common problem with the 7200 version is a jammed stepper motor that
won't respond to computer commands. You may be able to free it up with
electrical contact cleaner.
An R&D engineer at a company that makes carburetor repair components gave
me a good tip: "When you pull the top off a VV, don't disassemble it and
throw it into the solvent. Instead, use spray cleaner so you don't have to
remove the enrichment rod levers, etc. That way, there'll be much less to adjust."
Actually, the idea of doing no more disassembly than necessary can be applied to
any carb.
FUBAR and reman
We've all seen carburetors that fit the acronym "FUBAR" (Fouled Up
Beyond All Repair). Serious corrosion, throttle shaft bore wear, broken castings,
etc. mean they can't be fixed with ordinary parts or tools.
A trip to the wrecking yard is a possibility, but it's often fruitless and
always time-consuming. A new unit can be unbelievably expensive. So, how about a
remanufactured replacement?
If you're like most of us who've been working on cars for a while, you've had
traumatic experiences with rebuilt carbs sometime in the distant past. And maybe
you still avoid them. I understand. But that's really an outmoded attitude today
where quality brand-name remans are concerned. I'm not talking about one guy
working in a tumble-down shop with a screwdriver and buckets of spare parts. I
mean responsible companies with national distribution. For example, one I know
of has 550 employees, including full-time carburetion engineers, 200,000 sq.ft.
of space, and the capacity to turn out 4,000 units daily. It also has a free
tech hotline that handles 100-200 calls every working day. The technical
services manager of this outfit told me, "With all we do to the cores we
get, we should actually be considered a manufacturer. Our raw material is old,
screwed-up carburetors." They wet-flow test every unit before it goes out,
too, so quality control is excellent, and the settings are already pretty close
when you bolt it on.
Reman carbs definitely reduce financial shock, and as long as you go brand
name you don't have to be afraid of them anymore.
Oops
As you may have learned the hard way, it's easy to strip fuel inlet threads.
Fortunately, repair fittings and kits are available to rectify the situation.
There's the self-tapping type that's been around for years. It works by cutting
new threads and going deeper into the housing. If you've got a case that's
beyond that, there's also the type that uses O-rings that seal against the
inside of the inlet bore. It's held in place by a set screw.
On Quadrajets, the threads that hold the fuel inlet valve are sometimes
ruined, but there's an aftermarket valve assembly with O-rings that'll take the
sting out of this tragedy.
Those fine threads for the lean mixture adjusting screw (at the feedback M/C
solenoid) in Rochester Dual Jets and Quadrajets are frequently destroyed in
service, too, but there's an easy fix that includes a knurled repair stud that's
driven into the carb body and an adjusting cap with female threads.
Speaking of mixture control solenoids, you may be familiar with the tiny
O-ring and its brass retainer that reside at the end of the plunger on a typical
E2SE. They have to be removed to vacuum test the unit, and they can come off and
stop up the orifice. What to do? Well, instead of buying a new solenoid (over
$70), you can get a repair kit.
You know those little brass cup plugs in the secondary metering well of a
Quadrajet? They tend to leak. Machined aluminum replacements with o-rings are
available, and there are also thick rubber-like seals that fit tightly into the
cavity in the carb base and press against the underside of the well.
While I'm on the subject of GM carbs, I should mention one successful shop
owner's approach to poor drivability in late models. He told me, "Since a
high percentage of our customer base is GM, we've been hearing plenty of C3
carburetor complaints, such as poor cold running, surging, and lack of power --
maybe two per week. It's almost standard for us to clean them out and do all the
internal adjustments from float level and metering rods to knocking out the
plugs and resetting the mixture. Then they run like a charm." Unfortunately,
everybody hasn't had such pleasing results. Some technicians I've talked to say
the only remedy for a lean fuel curve in certain cases is a switch to different
rods or jets.
Legal aliens
Now for some specific info on imports you won't find elsewhere. If you ever
get a carbureted Toyota that runs way too rich while cold, it's not necessarily
due to an incorrect choke setting or a faulty vacuum pull-off. Ken Creel, owner
of an independent shop in Florida that specializes in Toyota only, told me,
"Something I see often, and make quite a bit of money on, is a bad AAP (Auxiliary
Acceleration
Pump). We just had one today, and another shop had replaced the EGR valve and
was talking about modifying the carburetor. The symptoms are black smoke while
the engine's warming up and a #1 plug that's a lot darker than the others. The
exhaust clears up once normal operating temperature is reached. What happens is
the vacuum-operated diaphragm for the pump starts to leak, and the vacuum hose,
which is connected to #1 cylinder's intake manifold runner, fills up with raw
gas. When the temperature/vacuum switch on the engine closes, the gas can only
go as far as the switch, so the smoke stops even though the diaphragm is still
leaking. We used to buy whole carb rebuilding kits to fix this, but now you can
just get the diaphragm from Toyota. It's part number 21667-38011. Just remember
to always check that hose for the presence of gasoline."
Nissans seem to be especially prone to lean hesitation problems. In cases
where there's a bulletin, substituting the slightly richer "countermeasure"
components will often mitigate matters, but there are also bootleg cures such as
drilling jets to a modest degree. As one expert, who shall be nameless, said,
"Oftentimes, a small modification will change a car that's a pain to drive
and that the owner is ready to get rid of into a fine-running vehicle."
The opposite condition in Nissans, richness that causes rough running, can
show up when gasoline formulated for winter use is still sold in hot weather (this
is especially critical at high altitudes). For '80-'82 210's and 310's and other,
earlier models, a needle and seat kit and, in some cases, a new fuel
pump and/or pump cam are available to cure this unpleasantness.
One shop owner mentioned a simple thing that's sometimes overlooked: "Maybe
six times this month with Nissans we've seen the little filter screen under the
needle valve get clogged, and after five or ten miles it's like you ran out of
gas."
On most Nissan models with feedback carburetors, there's an easy way to
determine if stumble or hesitation comes from the electronics or the carb's
innards. Simply disconnect the wire to the duty solenoid. If the problem
vanishes, the brain or its sensors or wiring is at fault. If the same condition
continues, you'll have to get inside the air/fuel mixer.
There are a couple of important cautions to observe when rebuilding Nissan
carburetors. First, on some models there's a plastic restrictor in the
accelerator pump inlet hole. If your solvent is too caustic, it'll melt the
restrictor into a blob that'll stop flow entirely, and you'll get the symptoms
associated with a bad pump. You can remove the welch plug and drill out the
passage -- you'll find plastic on the bit. Also, there are O-rings sealing the
main circuit on certain designs, and if you soak the unit long enough to damage
these circular seals, the result will be an extra air bleed and lean running.
Getting the right replacement O-rings can be difficult.
SOP for CVCC
How about Honda? Well, that unusual CVCC carb has inspired plenty of
trepidation among technicians. Remember the tip that was going around for years
about never turning a Honda carb upside down? It's true. As Scott Lightsey,
owner of a Florida shop that specializes in Japanese cars, told me, "They
do not like trash, period. They have very small orifices throughout." He
recommends replacement of both the regular fuel filter (by the tank) and the
"separator" (what Honda calls its ordinary in-line filter) at every
tune-up. On older models without the separator, he installs a universal
auxiliary filter.
There was a plague of sinking floats when Honda went to hollow plastic units
in the early eighties. You couldn't see any cracks, but they were full of gas
nevertheless. Lightsey told me he had up to 10 cases a day in his shop. Now, he
rarely encounters them. The company established an extended warranty to cover
this situation (7 years or 75K miles), but it ran out way back in '89 for the
last cars covered.
Whenever you have occasion to rebuild a Honda carburetor, note that the idle
mixture screw carries a tiny o-ring, which is commonly overlooked. It can get
jammed in the hole and shut off fuel flow. Lightsey says he reuses the old
o-rings if they're in good shape because the new ones included in most
rebuilding kits aren't exactly the right size. Ditto for those that seal the
power valve and the accelerator pump cover. Coat them with Vaseline or
equivalent. |
