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| Author |
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gull
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Carburators
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Mar 23 00:51 UTC 1999 |
This item is about something that's becoming a lost art; the carburator.
There are those of us who still mess around with these things, even though
fuel injection has made them obsolete; either because we drive a car old
enough to have been fitted with one from the factory, or because
retrofitting one was the only economic solution to an expensive FI failure.
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| 21 responses total. |
gull
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response 1 of 21:
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Mar 23 00:56 UTC 1999 |
I'm leading off with a question I've never gotten a good answer to. Maybe
someone who fiddles with carbs can answer it.
The Weber 34ICT carbs on my van use a simple arrangement to set the idle
speed; an adjustable stop that props the throttle plate open slightly. I've
noticed that some other carburators let the throttle plate close fully, and
instead bypass air around it with a passage in the carburator body. I'm
wondering why there are two approaches for this; since merely holding the
throttle plate open slightly seems simpler, I assume there's a reason why
this is inferior. Anyone know the answer?
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scott
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response 2 of 21:
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Mar 23 12:10 UTC 1999 |
Personal style of the designer?
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gull
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response 3 of 21:
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Mar 24 02:05 UTC 1999 |
Re #2: Maybe, but it seems like there must be a better reason than that.
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happyboy
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response 4 of 21:
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Mar 25 01:12 UTC 1999 |
a few weeks ago i rebuilt a single barrel for a
81 f-100 (straight 6) three on the tree pickup...
the idle speed screw i didn't mess around with at all...
hmmm, this was my first time.
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n8nxf
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response 5 of 21:
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Mar 25 11:24 UTC 1999 |
r.e. #1. I think it gas to do with proper air / fuel mixture. When the
throttle butterfly is most of the way closed you are no longer using the
main jets as there simply is not enough air flow for them to work. Instead
there is a little hole drilled into the side of the body in close proximity
to the outside edge of the throttle plate. (When the plate moves off this
position no more fuel is drawn from this hole as the air velocity is not
great enough.) Moving the plate to adjust the idle speed would also change
the air / fuel mixture. By having the air screw, you get less change in
the mixture over the same idle speed range. Idle speed is changed by
the air screw and the mixture can be tweaked with the idle mixture screw.
There might be some patent infringement issues involved too.
I can't see converting a fuel injected car to carbureted as being cost
effective unless you do away with the engine control computer and are
willing to violate all emission laws. Getting it to run decently would
also be difficult.
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gull
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response 6 of 21:
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Mar 26 02:38 UTC 1999 |
Depends on the car. It's reasonably common to do so on fuel-injected
Volkswagens made between 1975 and 1980. The Bosch L-Jetronic system used on
these is great when it runs, but since it has no built-in diagnostic or
limp-home modes repairing it is very trying. Parts are also expensive and
sometimes hard to find. Since these models all (except 1979 California
models) used old-fashioned Kettering points-and-condenser ignition, the
engine "computer" only gets involved in the fuel injection. Conversion is
simple, though it never results in an engine that's quite as smooth as it
was under FI.
While the resulting vehicle would be illegal in California and other states
that have anti-old-car laws, most of the time it wouldn't have been legal
anyway. California requires 100% original equipment to pass. In other
words, if you don't have the exact original distributor, air cleaner,
muffler, etc. on the vehicle, you can't drive it. It doesn't matter if the
change is one that'd affect emmissions or not. (The motivation behind these
laws seems to be that sales of new cars generate tax revenue. If they were
really just concerned about emissions, passing a tailpipe sniff test would
be enough.)
In the case of my Volkswagen, I tracked the problem (after about ten hours
of troubleshooting) down to the ECU, the engine "computer". A rebuilt ECU
is $250 and carries no warranty. A conversion to Weber 34ICT carbs was also
$250, and pretty much guaranteed to get it back on the road. You choose.
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scg
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response 7 of 21:
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Mar 26 04:45 UTC 1999 |
Hmm... My car (which, judging by the trouble the dealer (in Michigan) had
transferring the California title to me, must have originally belonged to
somebody in California) has the California emmissions package on it, and given
the industry I work in it's somewhat likely that I'd end up in California at
some point. What do I need to do when getting repairs done to make sure it
stays California-legal?
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gull
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response 8 of 21:
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Mar 26 06:08 UTC 1999 |
Mostly, just make sure they don't take "short cuts" by disabling equipment
that isn't required here, instead of repairing it. For example, when the
EGR valve failed on my van, someone removed it and plugged off the pipe to
it, instead of spending money and time trying to find a replacement. This
is becoming less of a problem nowadays, because modern cars need this stuff
to run right. Mine was made in that awkward stage when they were still
trying to tack emissions controls on old designs.
If your car is fairly new and/or common, parts availibility shouldn't be
much of a problem. It's more of a problem on old vehicles. A topic similar
to this came up on the VW Type 2 mailing list, recently, with regards to
(yup) EGR valves. As one list member put it, "They don't make them anymore.
We can't all have one."
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keesan
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response 9 of 21:
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Apr 1 01:06 UTC 1999 |
Jim likes a carbureted stick-shift car because he knows how to start it by
pushing or popping the clutch. A hill helps, depends on the size of the car.
Seems like that option is gone with fuel injected cars.
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drew
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response 10 of 21:
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Apr 1 02:14 UTC 1999 |
Also enough strength to actually push the vehicle.
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n8nxf
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response 11 of 21:
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Apr 1 11:47 UTC 1999 |
r.e. #9 Oh? I frequently start our 95 FI Subaru by rolling down the hill
in front of our house and letting the clutch out in 2nd gear. I like FI.
Our Nova is carbureted and has always had hesitation problems. You just
don't have enough control over the precise air/fuel mixture with a
carburetor to keep emissions down while maintaining good throttle response
in all situations. FI is much better in this regard. The Nova will give
a surge of power (enough to get you into the intersection) and then the
power suddenly drops to almost nothing, then it catches after a second
or two. It's more of a carnival ride with all the thrills ;-)
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gull
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response 12 of 21:
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Apr 1 22:44 UTC 1999 |
I've never denied that fuel injection is better. I loved the way my van ran
with FI. It'd start first try in the winter, and run silky-smooth anywhere
in the RPM range. And that wasn't even a very sophisticated system.
Problem is, analog low-voltage systems being what they are, 23 years of
corrosion and of somewhat inattentive maintenance by the previous owner had
taken their toll. If I'd been sure I could make it work again, I would
have ordered the parts without hesitation. I couldn't afford to start
throwing expensive parts at it, though, when I wasn't sure of the diagnosis.
Given what I knew, there were almost certainly multiple problems, and I was
tired of it dying on me on the road.
And yes, I could roll-start it. There had to be at least 9.6 volts left in
the battery to power the fuel system, though. It'd usually fire on the
second or third compression stroke. (With a carburated engine you still need
some battery voltage, to power the spark. And electric fuel pump, if so
equipped, though if it hasn't sat long it'll start on the fuel in the float
bowl.)
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n8nxf
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response 13 of 21:
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Apr 2 12:05 UTC 1999 |
I wouldn't spend much on an old car either. I suspect that I will be learning
more about FI as the Subaru ages. I hope I will have the time to get to know
FI inside out when that does happen.
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gull
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response 14 of 21:
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Apr 4 03:55 UTC 1999 |
Best suggestion I can have is get a good service manual. One that covers
the FI system, instead of just assuming it's out of your grasp. It isn't.
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happyboy
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response 15 of 21:
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Apr 4 18:33 UTC 1999 |
has anybody here ever replaced an electric choke
with a manual choke?
what kind of tools will i need?
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n8nxf
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response 16 of 21:
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Apr 5 10:29 UTC 1999 |
Several times. The conversion kits I've seen require only basic tools
like a crescent wrench and a screwdriver.
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happyboy
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response 17 of 21:
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Apr 5 12:54 UTC 1999 |
cool...will i need to borrow a drill?
what are they, about $20?
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n8nxf
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response 18 of 21:
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Apr 6 10:39 UTC 1999 |
You can usually snake the cable through an existing hole in the firewall
but you may want the drill to drill a hole in the dash for the choke
plunger. They are about $7 at Murray's.
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mdw
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response 19 of 21:
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May 29 07:57 UTC 1999 |
Hm. I guess my Volvo isn't california legal anymore then. The original
distributor developed an intermittent in the pickup coil, so the engine
would mysteriously die at random intervals on the road. (The pickup
coil is used by the ECU to tell it the engine crank position.) I still
have the original distributor, although I'd hate to think of the labour
it would take to find and fix the break in the coil.
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gull
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response 20 of 21:
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Jun 1 02:43 UTC 1999 |
You're okay if it was replaced with one of the same type.
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mdw
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response 21 of 21:
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Jun 2 08:20 UTC 1999 |
Ah. Maybe it's legal then.
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