Healthy IDF carburetors on the VW engine
Bolting IDFs on a VW engine is more common now than ever. Many kits
are bought and slapped on, and fellow hobbyists are so thrilled with the look
and the apparent performance change that they get in and drive it and don't do
anything else to them.
They're missing out. Often, there is a lot more to be had.
This article is a continuation of my article on how to clean and adjust a new
set of IDFs out of the box, as well as my tuning guide. This article fits in
where the other questions aren't answered. It is a work in progress.
Choosing your Webers
It's not hard to mismatch carbs to your application, as there are many
combinations of bodies and vents available. Choosing the right carbs is almost a
matter of taste, as there is the psychology of having monster carb bodies
looming over your intakes. Many times people choose larger or smaller carbs than
what really makes sense for their engine.
Solex published a book years ago that has some formulas for sizing carbs, and
although it is solex, I have looked over the formulas and they are really pretty
close for choosing the right carbs for your engine. The formulas are shown
below.
Where:
d = the carburetor diameter (barrel size)
Vh = the capacity of a cylinder in cc's
n = max RPM/1000
K = size of the venturi in mm
Compelling information, and handy, yes, if you intend to stick with what you
have for an engine. But a lot of us get the fever for more more more, after a
short time and then they take those 40 IDFs that they bought for their 1600, and
put them on their 2 liter engine with a big cam and heads, and suddenly they are
trying to run a marathon breathing through a straw.
If you get pretty wild with the engine, the Solex formulas start to wane in
effectiveness. Another method for sizing carbs on performance engines makes some
sense too. You can somewhat size according to valve train. If you use the size
of the intake valve, and apply some rules accordingly, you can come up with carb
sizes that make sense.
Some rules of thumb:
For hot engines, vent size=(intake valve-2mm)
For moderate street engines, vent size=(intake valve size-4mm)
For mild engines, vent size=(intake valve size-6mm)
Now if you are into building wild drag engines, you're probably not reading this
article, but those combos often have BIG carbs on them, with vent sizes equal to
the intake valve size or more. The rules change.
For mild to moderate/hot street engines, you can pick a carb based on Solex's
formulae, or intake valves. For instance:
Pick an engine of 2 liters. Single cylinder volume is 500cc. Suppose your choice
of cam is going to put the powerband in the 3000-6000 range. Now do the math,
and you will come up with a throttle plate of about 44.9. Pick the 44's then.
Now your vent will ideally be about 36mm. NOW, using the intake valves, you
could be reasonable about your combo. A good set of ported heads with 40x35
valves will scream to 6000. You've picked a good combo, and want to size your
carbs. This seems like a moderate engine to me.. and I would pick a 36mm vent
based on that.
Size your carbs to make sense. Be realistic about it. If you think you will want
to use them on a different sized engine in the future, hedge your bets. In my
opinion, going a little larger on the carb bodies than what is perfect for your
engine is a good idea, if you know that the future holds upgrades.
Fortunately for us, there are lots of venturi sizes available. Here is a list of
the venturi sizes I have seen available for different IDF bodies:
40 IDF - 28, 30, 32, 34
44 IDF - 28, 30, 32, 34, 36, 38, 40
48 IDF - 36, 38, 40, 42
As you can see there is some overlap, which is helpful. A set of 40 IDFs will
work well on many engines, up to about a 2 liter, depending on what you do with
the valve train. In addition, a 44 IDF could be a good match for a 1776 with the
right combo, especially if it wraps pretty high or you intend to upsize later.
Vent size choice should be reasonable for the body you choose, especially if you
intend to to stick with what you have. Larger vents in the same carb body will
get a little harder to tune, but using the tuning rules and bending them a
little here and there you can come up with some good results no matter what you
choose, within reason. Will it be perfect? Well, are any of these carbs perfect?
Not really, but you can get a good fit if you do your homework. Carbs just
aren't as tuneable as something like Fuel Injection, which is about as close to
perfect as you're going to get on these engines, but they can be tuned to
perform well and not suck the fuel like a junkie.
All that being said. I have bolted a set of 44 IDFs on my mild 1848cc bus engine
with 35x32 valves and I have gotten fair to good results in air-fuel ratio, as
well as decent driveability. It was done as a test for a friend of mine who came
over to play with his problematic carbs. After we got them worked out, we tested
them on that engine and they actually did pretty well. They were slightly soggy
down low, but good in the midrange and no end in sight to the top end, at least
as far as the carbs were concerned. The Webcam 119 grind and the heads were the
obvious power limits.
Should you feel that you are not sure you are sizing your carbs correctly or
feel your application does not follow these rules, you can always seek the
advice of someone that does a lot of combos. Most people don't mind talking to
you about it if you can answer their questions with more than ambiguous
information.
About symptoms
Many times, symptoms can tell you what is wrong, but
sometimes it can throw you, and get you looking in the wrong direction. I can
think of a lot of times that I was *sure* that certain parts of my engine were
healthy, so I ignored them as possibilities.
What's crucial to tuning your carbs? Well, it may be
obvious when you hear it, but make sure everything else besides your carbs is
doing well. Your ignition must be faithful, your valves adjusted correctly, and
no leaks on the intake and exhaust. With these things in place, you can tune
your carbs a lot better. If your valves are out of adjustment, or your ignition
is missing, you can easily blame symptoms on carb tuning.
Now with that said, here are some common symptoms and
causes (if it can be sure it is carb related):
Backfiring through the exhaust
First you need to know WHY this happens. Exhaust
backfires are from unburnt fuel that ignites in there, making it like a cannon.
There are different reasons why this can be the case, so you need to make sure
the things NOT related to the carbs are not causing it. Those causes are
ignition misses and exhaust leaks. Ignition misses cause the fuel charge to get
forced out to the exhaust without actually igniting. That added fuel in the
exhaust WILL ignite in there, since it mixes with a lot of hot gas, goes through
the exhaust transformations and eventually pops. Exhaust leaks facilitate this
by sucking raw air in between exhaust pulses, but it's not necessary in order
for it to happen. As soon as the raw fuel meets any raw air, it's going to
ignite.
Now, related to the carbs, a rich condition will cause
it. If your ignition is working perfectly, and you have a rich condition and
big exhaust, it will happen. It can also happen with a smaller exhaust system,
but it takes a little more richness to make it likely. Something is too rich.
Even if your jets are right, when you decelerate you might just be getting a
rich condition from your mixture screws.
Keep in mind that in a highly tuned VW, some popping on
deceleration is just likely to happen. When the throttle valve is in the idle
position, fuel does not flow out of the main circuit, just the idle circuit.
The combined effect of the closed throttle and elevated engine rpm is to create
a high vacuum in the intake manifold. This vacuum causes a high air flow rate
through the small gap formed by the throttle plate and carburetor throat. and
the idle circuit will not deliver enough fuel to create a normal, combustible
air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion
chamber. It gets sent into the exhaust system unburned and collects there. When
the fuel in the combustion chamber DOES fire, it's not fast, and the combustion
still occurs as it is shoved out the pipe, where it sometimes ignites the raw
mixture that has collected and the exhaust then pops or backfires. Mine does
it, and it's normal to a degree, as long as it's not shotgun type fire that
sends everyone around you hitting the deck and yelling "GET LOW! GET LOW!".
Those types of backfires are accumulations after the collector that are caused
by rich conditions and ignition misses, as well as leaky exhausts.
Backfiring through the carburetors (popping or
huffing)
This happens when fuel manages to ignite in the intake
runner. The most common causes for it are timing and lean conditions. Lean
conditions happen if jetting is not right or you have a vacuum leak. You can
check for a vacuum leak with starter fluid spray or carb cleaner. If you spray
it at a joint, and things change in any way, you have a leak.
Other causes include ignition problems, retarded timing,
intake valve leaks from failure of valve, seat, or spring, or even a foreign
object wedged into the seat. An engine that has been sitting could have
corrosion on the seat. Tight valves will leak too. Even a bad set of points or
condenser can cause problems like this. As mentioned above, make sure
everything else is healthy.
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