Tuning Dual Weber IDF
Carbs on the VW Type 1 Engine
A fact - Weber carburetors never come out of the box clean or jetted
perfectly for your engine. If you order a set, they may come to you in the
ballpark, but every engine is different, even two engines built with the same
parts. You're going to be needing some different jet sizes. No way around it, if
you want to get it right.
Buying jets is just part of it and you have to be ok with that if you want to go to the
trouble to tune things well. Jets are expensive, so if you know someone that has
a stockpile of them to lend you, get them and start tuning away. If not, you'll
end up with a few sets that you can probably sell off for close to what you have
in them.
"Ok, well, I'm smart.. and I am handy with the drill bits.."
Well, good for you, sport. For the most part, stick to correctly sized and
calibrated idle jets. The holes are SMALL, and small defects in your
drillings will add up to big discrepancies in fuel delivery. If you want
to get handy with the main or air correction jets, go for it, they're much
bigger and will not suffer so badly because you sneezed.
First things first. If you do not have an understanding of the otto
cycle, go read about it so you know that what the engine does with intake and
exhaust are not separate events from each other. The exhaust cycle
contributes to the intake cycle. Given this fact and the carburetor's
dependency on the health of the otto cycle, you must make sure that your engine
is healthy before you spend any time tuning your carburetors.
Your engine's health - Are your valves
correctly adjusted? Is your timing set correctly? Are you SURE?
If not, find out. If you don't know what the right timing is, FIND OUT.
Don't guess.
Are your carbs CLEAN? Are your
floats set correctly? Clean the carbs and adjust the floats.
Here is
a cleaning procedure that should clean out most carbs, especially new or fairly
new ones. Older ones may need more attention.
If you slack on this step, the rest will be meaningless and frustrating.
After you adjust the floats, you need to make
sure your fuel pressure is about 3 to 3.5 psi. If it's too high or too low you
will have poor results. Don't ASSUME anything about the fuel pressure. A stock
pump under the wrong conditions can supply 10 psi of fuel pressure. I use an
internally regulated
rotary pump that runs about 3.5 psi. If your mechanical
pump is pumping too much pressure you can add gaskets under it to get it right,
or you can put a regulator in place.
You also have to make sure you have the right pump and fuel pump rod
combination. That is beyond the scope of this article. If you suspect there is
a problem, deal with it and come back to this. I like the rotary pumps because
you have instant fuel pressure, and your float bowls will be filled properly
when you turn the key on.
Venturi selection - For engines that rev pretty high (6000-7000 rpm), you are best off choosing
vents that are about 2 to 4mm smaller than your intake valve size.
For engines that are set up for torque, like bus engines, and other torque-happy
applications, you can go a little smaller on the vents and get better low end
response. For these, I'd recommend 4 to 6mm smaller than your intake valve size.
So what happens if your vents are too big? You'll have reduced air-speed at low
RPMs, which makes tuning harder. Air speed is one of the things that tells your
different fuel circuits when to start to come on. It will be soggy down low.
So what happens if your vents are too small? The undersized vents will restrict
your engine's ability to breathe at higher RPMs.
How critical is it? Not terribly, but if you're way off it will matter, and you
will feel it. Putting 40 vents in a set of 44 IDFs with 40mm intake valves will
be pretty soggy down low. Sticking with the 36's would be about right. Putting
28 vents into a set of 40 IDFs on an engine with 40mm intake valves will limit
your top end noticeably as well. A set of 44 IDFs, for example, can me made to
work on many engine sizes with the standard 36 vents, but you might find that
you could optimize them better if you are willing to spend the money and try
some different vents that would be ideal for your application.
A few notes about jets
More recently made jets are a little different in shape from the standard ones
made by Weber, and there are some characteristics of them that could use a
little attention. The key thing to watch out for when installing new jets is
blockage and burrs. Don't assume that they're clean out of the package. Many
have little bits of crap in them that will cause you misery if you just install
them. Clean them out with carb spray and look in them to make sure you have them
clean. Compressed air is very handy too for blowing them out after you spray
them. Get them clean and dry before you install them. Many have burrs along the
split areas where they compress to fit into your jet stack or idle jet holder.
Inspect for this and get it cleaned off before you install them, or the burrs
can chip off and fall into the stack or holder, causing blockage. Check it all.
I like to clean them, install them in the holders or stacks, and then pull them
out and clean it again, to be sure.. Then you know if anything was going to chip
off it did, and has been cleaned out.
It's wise to invest in a set of jet gauges to inspect the jets you use, to make
sure they have not been changed. If you trust the stamped or screened writing,
you might end up with results you do not expect. I've had them come to me sized
wrong a couple times. While I don't like to ream jets, I think having a reamer/sizer
is handy for checking them, and sizing them in a pinch. I personally think it is
a wise (albeit expensive) choice to have an arsenal of correctly sized jets of
all increments around the range that you need to use, but that's something you
will likely accrue if you build a few different sized engines with some
different combinations of carbs.
Idle jetting
Once you've settled on your vents and your carbs are adjusted and cleaned up,
you can choose your idle jet size. I've noted a formula in the books (basically
1.6 * vent size) for idle jet sizes, and I have to say that vent size has little
to do with idle jets, on the vw engine at least. More efficient engines can
benefit from leaner idle jets, and less efficient engines probably need a little
more. 50-60 is the range, and there is a trend
toward larger jets for larger carb bodies. The throttle plate size is a key
factor.
Idle jets are usually 45-65 in size, mostly
in the 55 range. The rule is typically to multiply the vent size x 1.6 to
arrive at your rough idle jet size, but what I've seen is that 50 to 55 is
mainstream for about any 40 or 44 IDF. 55's will make sure you're not leaning
out when you are tuning, so I start with those.
Before you do any more, MAKE SURE the air bypass screws are CLOSED. These
are the little screws and lock nuts that are next to the mixture screws. If
they are not closed, you'll probably never get it jetted right.
Now. This article assumes that your
engine is in top notch condition before you go about tuning the carbs. That
means general engine health, timing, dwell, fuel delivery, fuel quality, all the
way down the list. And all THAT's another subject.
After installing your idle jets, you close
the idle mixture screws and unscrew them all about the same amount, maybe about
1.5 turns out. Then, with your linkage disconnected, unscrew the idle speed
screws (the ones the throttle arms rest on) until they are no longer touching
the arms, and then turn them back until they are JUST touching. Now turn them in
another half turn or so. Start the engine, and after you get it warmed up a
little, see
if it will idle ok. Now get your synchrometer (Unisyns suck, get a snail if you
don't have one) and use the idle speed screws to even it out, and get you to a
reasonable idle speed. Not too fast, keep the idle speed down for now, and get
it even. If not then turn each mixture screw out another turn. If it STILL will
not idle well, then your idle jets could be too small. Now get your synchrometer
and use the idle speed screws to even it out, and get you to a reasonable idle
speed. Not too fast, keep the idle speed down for now, and get it even
using the snail. The snail readings at a proper idle should be between 4
and 6 or so.
Now go to each mixture screw one at a time
and slowly turn it in (by slowly, turn it about 1/8 to 1/4 turn at a time and
stop and wait for the response of the engine for a few seconds) until the engine
begins to slow down (you'll be leaning out a cylinder). Now slowly back the
screw back out (same method you used to screw it in) until that cylinder starts
to pick up again and run smooth. This is the critical point in the mixture
adjustment. Slowly open the screw a little more at a time until you
reach a maximum idle speed. Don't go any further! If you're not
sure, stop there, screw in back in until the idle drops, and then start to back
it out again until you feel confident you have unscrewed the minimum amount that
you have to in order to get max idle speed from that screw. Move on to
the next cylinder. Do this until you have done all 4 cylinders.
Now back to the synchrometer. Get them as close to the same as you can, and at
the target idle speed you want. 800-900 is a reasonable range. Lower you can
get and live with it, the better IMO.
You're getting close now. Repeat the idle
mixture procedure and when that is done, if your idle is right, you're done with
the idle settings for now. If your idle is now off, go back to the synchrometer
and idle speed screws.
These iterations help you trim it out - you
start low on idle, get your mixture right, and then move the idle up,
and check mixture again, and if you have to adjust your mixture to the point
that it messes up your idle, then you adjust that, and mixtures one more time
and you should have it. If you don't have it by now, you are probably doing
something wrong.
So what do you do if two barrels on one of
your carbs are not flowing the same? If it's only about quarter point or less,
don't sweat it - it's most likely to affect your idle more than anything, maybe
a weaker or more eager barrel at idle. If it is more than that, your carbs
need attention, namely that the throttle shaft may be bent, or there may be a
problem with the butterfly plate or its alignment, or something else causing
different air flow. Some people recommend using the air bypass screws to
adjust it, and this does help, but if it's more than a point off, it will
cause problems that the air bypass can't account for. They need to be FIXED if
this is the case. I never use the air bypass screws for more than about a point
of difference.
Main jetting
Now with the idle jetting done, main jets are next in line. Assuming F11
emulsion tubes, your starting point for main jets is about 4.1 to 4.3 x the vent
size. So for a 36 vent, your main jet should be roughly 145 to 155 to start.
150's a good choice to start with unless you're feeling lucky, in which case you
can start with a 145. With a 145-150 main jet, you'll be pretty close with a 36
vent. Similar rules work for smaller and larger vents, though once you get past
40mm vents, the rules don't seem to apply so well.
Air jetting
Air jets are more empirical, but a good starting point for them is about 200 if
you have 145 mains. If your mains are considerably smaller, like 135 or 115 or
something, a 160 to 180 air jet is a better place to start. Start there and
don't mess with them until you know the other jets are right.
Synchronization
This is a topic that does not get enough attention. Synchronization is
EVERYTHING after the jetting is done. Synchronization will such a difference in
the way it runs you just can't believe it when they are right. Unfortunately all
the linkage that is out there has certain characteristics that keep your
synchronization from being perfect all the time, so all you can do is get it
close and keep after it every once in a while.
The biggest problem with synchronization is keeping the geometry correct. This
is hard to understand without illustrations of why, but if you just trust that
it's important, you can avoid all the rest.
Linkage
There are different kinds of linkage out there, and a lot of people use
crossbar linkage, while some others use bellcrank type linkage. Each one has
its application, but I prefer bellcrank when I can use it and can get good
linkage.
Crossbar linkage: How to ensure that your
downrods have the right geometry
With your downrods, if you can position the crossbar arms so that your downrods
are vertical (left to right), that will help matters, and complicate the
situation less. Once you have achieved that, you can work on making sure that
they are both at the same angle (leaning from front to back of the car). Use an
angle finder, and read what the rods are, and use washers to get them the same.
There shouldn't be a lot of difference to start with, if you have the right
linkage for your setup. Different intake manifold types (offset versus straight)
will use different linkage setups. If your crossbar is sitting at an angle with
respect to your fanshroud, you have the wrong linkage.
Once your downrods are at the same angle side for side, then your
synchronization will be easier, because the throttles should now be offsetting
the same amount on each side throughout the full range.
Now, to synch your carbs, loosen the nuts on one downrod, and use the rod's
opposed threads to set the carbs so they are opening at the same time. Be
careful when doing this (I do it with the engine off) so that you don't open
them a lot too many times.. you'll dump a bunch of fuel into the engine if you
do. When you test, make sure that you are using the point where the cable
connects to the throttle arm in the middle of the crossbar. If you twist at one
of the outer arms, you will not get accurate results. Just push on the arm at
the point where the cable meets it, so that you are applying the same force that
the cable does when it pulls. If you put your hand on the crossbar and twist it
using your thumb or something on the middle arm, you are still applying a force
that the crossbar will not experience under normal conditions.
Watch the throttles, and compare what both sides are doing. They should be
opening at the same exact moment. Realize that as the engine warms up the
geometry changes slightly so there may be some difference between hot and cold
engine. I like to warm mine up before I do this. After you get them synched
right, tighten down the nuts so that both the heim joints are centered to their
positions. I rotate the joints so that they are both resting against the
position they would be pulled to when you tighten the nuts, and then I tighten
them. Be sure to not let the rod twist when you position or tighten them. You
may find that tightening the nuts changes the geometry slightly, so you may have
to compensate a little bit for this on the rod.
After you get the nuts tight, check it again, and repeat if necessary.
Why this matters so much: Once you get them synched perfect, you will FEEL how
much better it runs, especially on low throttle lower RPM. If you have a head
temperature gauge, you will see that when the linkage is not right, one side
will run warmer than the other. If the left side throttle opens first, the left
side will run warmer, especially at low throttle low RPMs.
The last thing you should do if you have never done it: Loosen the throttle
cable from the center arm and have someone push the gas pedal to the floor. Pull
the cable tight, and then snug the connector. This will ensure that you do not
put undue stress on the throttle shafts on the carbs at full throttle. If you
put too much pressure on them you can twist the throttle shafts too much and
bend them. This is especially true of setups that have the return spring and
stop on the FRONT of one of the carbs, which is the way most Weber setups are
out of the box. I use the CB Weblink kit to put the springs both on the throttle
linkage side, to help with synchronization.
This with some practice and a good feel for what you are doing will help a lot
with making sure your engine runs smooth.
Drive it
Something you should understand about tuning from this point. Your accelerator
pumps are going to try to fool you unless you understand their purpose and
function.
Accelerator pumps are there to compensate for some physics. The fact is that
fuel is heavier than air, and it takes longer for the fuel to pick up speed in
the circuits than it does for the air to pick up speed in the throats. So when
you stomp the gas, the air starts moving faster a lot sooner than the fuel does.
The accelerator pumps are there to provide a little extra fuel during that short
time it takes for the fuel to catch up. Keep this in mind when you are tuning
the jets, and avoid rapid pedal movements for now.
It really helps to have a tach, and an air fuel gauge, and I would say that a
tach is next to necessary, while an air fuel gauge is a luxury that you can do
without unless you are FINE tuning, and going for near perfect in terms of
mileage and power.
Take the car out and drive it. Pay attention to what's going on at about 1500 to
2000 RPM, and at 2000 to 3000 RPM, and from 3000 to 4000 RPM. While the RPM
bands will vary depending on what size carbs, what vents, and what engine combo
you have, these are pretty safe ranges to look at.
1500 to 2000 RPM is almost purely idle jets in action. Keep the pedal steady
here, in 3rd gear so you can see what it does with a slight load. If this area
is running ok, move on. If it feels weak or soggy, you might need different idle
jets. Unfortunately, it's sometimes hard to tell if it's rich or lean unless you
are experienced here. This is where an air fuel meter will help you out. What
you can try is opening up your mixture screws about half turn and see if it
feels better. If it doesn't, then it might be too rich. If it does, then your
idle jets may be too lean. For those of you with air-fuel gauges, about 13-13.5
or so is what I like. Move on to the next range.
2000 to 3000 RPM is the "transition" stage. This is the part where the main
circuit starts to take over. Your idle jets start to matter less here, and your
air corrector jets are the progressively deciding factor for why your engine is
running the way it is. With the pedal steady, make note of how it feels in here,
and move on.
3000 to 4000 RPM is where your main jets are really doing their job. With the
pedal steady, if it feels lean here, (sort of like it is running out of fuel)
then you might want to try going up a main jet size. If not, try going down one
and see how it runs. Go down until it starts to feel lean (weaker with maybe
some popping). When you feel it go lean, move back up a size and you are done
with the mains. For those of you with air-fuel gauges, about 13.0 - 13.5 or so
is what I like.
Now, back to the transition stage. If it felt lean there before you did
anything, and the main circuit felt lean, determine of moving the main jet up
helped the transition stage. When the idles and mains are both right, the
transition should be pretty good too. If your mains and your idles feel good,
but your transition does not, try going up a couple sizes on the air jet and see
if that helps. If not, try to go up a couple sizes more and see. By now you
should have it cleaned up. If you STILL have problems with transition, something
else may be wrong. Read the article to the left about transition.
Accelerator pump settings
Once everything else feels right at steady state, then you can start playing
with the accelerator pumps. While there are different pump jets and bypass
valves, you can usually get what you need out of the ones that come on the carbs
out of the box. First, make sure that they are adjusted the same. Drive it, and
see what happens when you give it gas more quickly. If when you are in second or
third gear, and you push the pedal from cruise to WOT over the course of about a
half second, and it bogs, try unscrewing each accelerator pump nut about 3 turns
and try it again. If it's worse, then they were too lean, and you should go back
the other way. Try 3 turns at a time until they are right. You shouldn't need
more than about 1/2" of rod sticking out of the nut. If you do, you might have
some timing issues you need to deal with. For those of you with an air-fuel
meter, your meter's response to mashing the pedal should be as close to steady
as possible, but that's not realistic to expect. If your engine falls on its
face, and the meter goes lean, screw the nuts in about three turns on each side,
and try again. Same rules for if it goes rich for any period of time, like two
seconds or so. What I like to do is back off the screws until it falls on its
face, and then start screwing them back in until it doesn't anymore. Too much
fuel from the accelerator pumps is going to cause the excess fuel to wash oil
off the cylinders, and cause you poor mileage.
Common mistakes
Lots of folks are driving around thinking their state of tune is good, when in
fact, they've covered up symptoms with incorrect jetting. The only ONLY way to
know for sure is to have an accurate air/fuel meter, but you can avoid a lot of
it by just paying close attention to what the different circuits are doing. For
dialing the jets, paying attention to what happens as you mash the pedal or
right afterwards is wrong. The accelerator pump is screwing with things when you
do that. Jetting requires paying attention to what's happening at steady state,
during fixed RPM ranges, under different conditions.
What happens under WOT is not an ideal way to assess your jetting, at least
under heavy load and at lower RPM ranges. Things will go rich there, and should
due to what your engine needs. If you have an Air/Fuel meter and it is dropping
down to the 11's or something when you nail it, worry about some of that when
you have the rest tuned. Don't shoot for getting that right until you get the
rest right. That part may never be perfect, but you can get it closer after you
do the rest.
If you think you have it tuned, drive it for a while, a distance, a few days,
whatever, and see how you feel about it. If you try to jet an engine that has
been running really rich for a while, it may take a few stages of leaning it
before you get it right, due to carbon buildup, and may take some new plugs to
be sure. Plugs are another factor that has to be paid sufficient heed to get
things right. That's another topic, but about 0.025" for stock, more like 0.040"
for CDI and such.
New engines: Never try to tune a new engine perfectly. It's going to change. I
tend to suggest running them a little rich in the beginning to keep temps down
and make sure you don't lean it out while your rings are trying to seat.
What kind of mileage should you expect? Depends on how you drive. If you
drive it like you're sick of the high gas prices you should get close to 20 if
everything is right, maybe even more. I get about 25. You can get very good
mileage if you lean things out close to 14 across the board, reduce your
accelerator pump action to the very minimum, with just a hint of hesitation when
you punch it, and use a vacuum advance distributor (SVDA) with the ports on the
carbs (many have vacuum ports you can attach to a T fitting and then to an SVDA
distributor that is tuned right for use with dual carbs. Not all cam/head setups
are going to work well with an SVDA distributor, but many will do fine.
If you drive with a heavy foot, then face facts. Your mileage is going to suck.
Hope this helps you.
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