Joined: October 18, 2002
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|Posted: Wed Feb 10, 2010 10:41 pm Post subject: Final Factory Dual Carb Adjustment Procedure FAQ
|1972-1974 Dual Solex PDSIT Carburetor Adjustment Procedure
The original factory dual carbs have been Great Mysteries to even some established long-standing Volkswagen mechanics.
They frequently suffer acerbic reviews by those who don't quite understand them, and they are often removed and replaced
by "progressive" centermount carburetors that lack the engineering of the originals that provide dependable service and
excellent driveability once you set them up correctly.
Here is what is going on with that weird left carburetor, and this will make it all crystal clear to you. Since the
beginning, VW controlled idle speed by a screw that positioned the throttle plate opening. Not much air is required to
idle a VW, and they had to dump a lot of gas into the mixture to make sure a combustible mix made it all the way to the
cylinders. It worked well, but it was pretty smoggy. These were the Pict 2 days where the idle speed screw was staring
right at you at the top of the throttle lever.
In 1970, VW discovered that they could make an engine idle with a lean fuel mixture to meet emissions standards, yet
still have decent driveability if you increased the air speed. Instead of a lazy trickle of air around a huge throttle
plate, they crammed the air through a high velocity passage and mixed the fuel into that airflow. The throttle bypass
carburetor was born. In a small tube, air could whiz right by the fuel metering orifice and make a decent lean mixture
that ran the engine. Thus began the Pict 3 days with that brass air screw on the side of the carburetor to adjust your
In 1971, VW discovered that they could meet even more stringent emissions standards by dumping even more air into the
engine at idle, as long as you then retarded the spark to bring the idle speed back down. That is why all 1971
carbureted engines had both a big bypass circuit in the carb and a vacuum retard diaphragm on the distributor to make
the spark AFTER the piston was already going down. Thus, the 5*ATDC timing specification that you must never time your
engine to unless you know the retard unit is working correctly.
So . . . whaddyadoo if the very next year you decide to introduce dual carbs which especially can't give a clean idle
when there are only two cylinders per carb drawing in emissions reduced fuel/air?
Right. You do a throttle bypass again.
On the 1972-1974 buses, this throttle bypass is called the Central Idling Circuit.
The central idling circuit allows us to make a good idle mixture and still have thrifty carburetors. Though some people
tell you that loose throttle bushings, will cause all kinds of symptoms and misbehavior, the central idling circuit can
actually compensate exceptionally easily for this condition . . . at least until the throttle levers are bobbing up and
down utterly sloppily. We can also complement the accelerator pumps by a judicious adjustment of the central idling
circuit since the fuel/air in the central idling circuit is already right there in the manifolds when we mash the
accelerator pedal. You might find that your hesitation comes from too rich of an adjustment of the carbs and the central
idling circuit. Then the accelerator pumps actually drown the engine when you need to blow away that 5.0 Mustang next to
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This set-up is a matrix. Each adjustment affects the others. Follow the sequence. If you decide to deviate, the
exploding variables may give you terminal confusion.
1. Adjust Valves Cold
Strive for .006" all intakes, .006" exhaust.
(1974 .008" sodium-filled exhaust valves have the 14mm locknuts and the big 8mm screws. As a general rule, rebuilders do
not install them due to expense, but if you have an un-touched garage find '74, see if there is a decal on the engine)
2. Adjust Points - .016"-.018" for a dwell of 44*-50* Don't be afraid to get the gap up towards 18"!! It will always
wear towards .016"
3. Inspect all hoses and connections for cracks, leaks, or omissions (Diagram 2). Look carefully at undersides,
particularly on those brake booster elbows. Your componentry and layout may vary from the diagram, but whatever, your
engine must be air-leak free. Anywhere you see little pink strips is a likely leak spot. If you download the diagram and
magnify it to 400 to 600 times, you will see the pink screws on the EGR valves that can allow air to leak into the
engine. Remove them and blob some silicone in the hole and put them back in. I removed my EGR and air pump. The
top-shots have no depth perception, so things like the E Preheater Valve and the C central idling plumbing are at
different elevations, and the left/right mixture screws and the air filter/air horns are at different elevations. Your
common sense and growing familiarity with all of these parts will make more sense over time.
4. Test Vacuum Retard. (Diagram 1 #17 #18 ).
While you are here, look at where the adjustments are and familiarize yourself with the locations of things)
With a warm engine at a correct idle speed of 850-950 rpm, it should speed up when you pull off the 5mm green vacuum
retard hose from the distributor's dual vacuum advance unit. If it does not speed up:
a) leaky hose test, put a finger on one end and apply vacuum to the other. You should not be able to draw air.
b) leaky diaphragm test, engine idling, apply vacuum to #17 end of hose while still attached to #18 vacuum unit, you
should feel vacuum and the rpms should drop.
c) no vacuum signal from left carburetor, go to INITIAL ADJUSTMENTS Step One.
5. Set Timing
a) to 25-28* BTDC at 3,200-3,400 rpm with both hoses off the distributor (Diagram 2 H and I).
Do not bother plugging them. The factory mark on the scale actually covers a range of 25-28*.
Now let engine idle. It might run fast with your vacuum hoses off, so remove the central idling circuit solenoid wire
(Diagram 1 #13) to get the idle below 1,000 rpm. If your engine refuses to idle low enough because everything is
bulloxed on your engine, temporarily block the central idling air intake with your finger, it's the brass tube under the
elbow on the left carburetor (blue tube with pink socks in Diagram 2). We do NOT want the centrifugal advance skewing
your results! Do what you have to do to get the engine speed down below 1,000 rpm. Look for anywhere between 0* and 7.5*
BTDC. If timing is above 7.5* at less than 1,000 rpm, adjust your above timing (step a) down towards 25* BTDC.
b) check idle timing with H & I vacuum hoses back on, central idling solenoid wire #13 re-energized. After putting the
wire back on, you have to kick the central idling circuit back to life with a blip of the "Only Push Here (Diagram 1 #10) short and hard, just the nature of the beast:
5* ATDC (1972 and all automatics)
10* ATDC (1973-4)
If you cannot get the idle timing down to 5-10* ATDC with the vacuum retard hose good, and the vacuum retard unit
holding vacuum, you will need to check for vacuum signal from the left carb in the below INITIAL ADJUSTMENTS Step One.
6. Clean and gap plugs to .028" and re-install with a light film of oil on threads. If you happen to want to do a
*compression test*, you may use the results in INITIAL ADJUSTMENTS Step Two
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Take off left and right air horns. So we can properly calibrate the central idling circuit mixture, we want the central
idling tube (Diagram 2 blue tube with pink "socks") to remain on the brass intake tube. Turn it so that it points to the
left somewhere, out of the way. If you only have some funky piece of hose instead of the correct tube-with-elbows, keep
it on and position it away from where we are working. Likewise, the preheater circuit (Diagram 2 E) vacuum hoses must
remain attached to the right air horn thermostatic switch (the asterik at the blue/orange stripe) even when it is off
the right carburetor.
1. Set Throttle Stop Screws (Diagram 1 #5)
We are not going to do the Bentley-suggested Rip The Carburetor Off And Stick A Feeler Gauge Up There Deal. Back out the
screw until it no longer touches the carburetor boss. Turn the screw back in until it just touches the carburetor. Go an
additional 1/4 (90*) turn after contact. Did you have to push hard to keep the screwdriver in the slot? Take an
additional 1/16 (45*) turn. Start engine and pull the vacuum retard hose #17 off the left carb. Check for vacuum signal
coming from the little brass nipple. If you have it, good. If you were missing the vacuum signal during the above timing
adjustment, go reset timing to proper value now. If you have no vacuum signal after this critical throttle stop screw
adjustment, you need to remove the carb and check orifice, passages and carb body-to-throttle body gasket for an
obstruction. I had to drill a new vacuum retard passage once . . .
2. Synchronize High Speed Air Flow
The 1972 bus has it easy, your adjustment is on the left side (Diagram 1 #8 ), a little spring-loaded screw that shifts
the position of left and right levers on the cross shaft. Beautiful. Easy. Too expensive to keep. 1973-74 have to loosen
the upper and lower locknuts at both ball joint ends on the right throttle link (Diagram 1 #9) before starting the warm
chokes-off engine. Please get your pair of small needle-nose grips and lightly clamp the very center of the throttle
link where the nut is welded to the link. This will prevent it from spinning horribly out of adjustment on you while the
engine is vibrating. The air horns are still off,yaah? Get your Uni-Syn approach plan figured out before you start the
engine. I like coming through from the rear deck lid as it is the only way you can comfortably adjust the right throttle
link. Open the Uni-Syn a good amount. Now get your tach-dwell meter set to rpm and hook its red or green alligator clip
to Coil(-) Terminal #1 (usually a green wire that leads directly to the condensor/distributor). Hook the black alligator
clip to a ground (I use the coil mounting 10mm bolt).
a) Start the engine, and grab the stubby little vise-grip. You will clamp it only on the accelerator cable end where it
comes through the cross shaft " U". Now, Only Push Here until you get 2,500 to 3,000 rpm. Hold the vise-grip against
the cross shaft "U" and lock the vise-grips on the accelerator cable end. Make sure the visegrip is hanging down
already, or you'll get rpm creep as gravity does it for you. Wiggle it and let it stabilize your engine speed to between
2,500 rpm and no more than 3,000 rpm.
b) Obtain your reference airflow on the left carb. Move the little wheel on the Uni-Syn until the red slug reads on the
lower mark ( if you have three marks) on the sight glass. If it changes the engine speed dramatically when you place the
Uni-Syn on top of the left carb, your float may be a little low, the Uni-Syn may be restricting airflow, or the engine
may be running lean. Try to get a lower reading on the sight glass by opening the little wheel some more, that'll give
your engine a little more breathing. Catch the reading before the engine speed changes, just whip it on, press down, read.
This is your reference reading! The left carb is the master! Not because it has the weird central idling circuit carburetor built into it,
but because it has . . . the non-adjustable throttle link.
c) Now Uni-Syn read the right carb. If it changes the engine speed due to mixture issues, make short sharp readings.
Shorten the right throttle link if your Uni-Syn red slug is reading low (actually, "dancing" is the better word) over
here on the right than it was on the left side. Lengthen the link if the Uni-Syn red slug is dancing higher on the right
than it was on the left. Turn the right throttle link to match the Uni-Syn reading with that of the left carb. 1972
people just have to turn the 10mm nut on the left crossbar end (Diagram 1 #8 ) and see which way makes your UniSyn
reading change. We 1973-4 bus owners have to suffer (but we have a big inside hatch up top, so there) with little
locknuts and ball joint ends. Release your needle-nose visegrip on the throttle link, rotate it to synchronize airflows,
hand-tighten the little locknuts that have probably vibrated far away from where they need to be, then re-clamp the
needle-nose visegrip to help hold things together. Give the cross shaft a little tap-n-pull at the Push Only Here visegrips, to either help drop the rpms or maybe nudge them up after your right link adjustment skewed our reference rpm. So you have to go back to the left carb to see what the change of engine speed did to your original reference
Uni-Syn reading. Read the new left airflow. Now go back to the right and adjust the throttle link to make it the same as
the new left reading. Did the engine speed change? Go back to the left.
d) When they are both the same, carefully lock the locknuts on the right link. Rotate the upper and lower ball joints
with the throttle link until they are all as far rotated as they will go in the direction you need to tighten the
locknut. If you do not move them all as a unit before the nuts are locked down, you will change the synchronization. I have little ignition wrenches that work well on these little nuts. Make sure that you "backwrench" the ball joint as you tighten the locknuts. Rev engine a little, double-check your work with locknuts secure. I hope your readings are
synchronized. If you find instead that, for example,the right side is actually drawing slightly less air than the left
(like the width of the line on your sight glass), and you know *from your recent compression test* that the left side has less overall compression than right side . . . leave it be. Let the weak side of the engine have a little more air if there is a hairsbreadth of difference in airflow. Remove vise-grips. Shut off engine. Now your ears are ringing.
3. Synchronize Idle Air Flow
This will be on an idling engine, so you won't go deaf again. You should have just enough slop in your throttle link
ball joints to allow us to fine-tune idle air flow passing through the carburetors. The throttle links are out of the
picture at idle, it's just the throttle stop screws. You only get about 3/4 of a turn adjustment range, so you may not
achieve synchronous delite. Close down the Uni-Syn pretty far. Warm engine only. Adjust Uni-Syn to that preferred 1/3
mark on the sight glass on the left carburetor. Read it quick if the engine speed changes because the Uni-Syn is acting
like a choke, then go to the right carb. If, for example, the right carb is reading higher, back out its throttle stop
screw 1/8 (45*) turn, and turn in left throttle stop screw 1/8 of a turn. Did that help? If not, leave it where you just
adjusted it anyway. Check vacuum retard for signal, it is unlikely that 1/8 of a turn is going to suddenly block the retard port, but check anyway.
4. Synchronize carburetor mixtures:
With pocket screwdriver (Diagram 1 #2 & #3 ), seat the left and right carburetor mixture screws gently. Back out 2 1/2
turns each (3 turns if you already know that your engine has been running lean). Count with the pocket clip. Start
engine, pull off retard hose (Diagram 1 @ #18 or Diagram 2 H) from the vacuum can end, and kill the central idling
circuit by pulling Central Cut-Off #13. I do so recommend that you rewire as per ReWire Digram. Your engine should run
without too much complaint at about 6-800 rpm. Your tach-dwell meter is still with us, no?
1972, please find your Mixture Reference Hose out of the tool box and put it in the right booster elbow at the red-tipped asterik in Diagram 2. Let the hose that normally goes to the thermostatic vacuum switch just dangle.
1973 and 1974 find your Mixture Reference Hose on the underside of right air horn at the black tipped asterik. It is
the hose that has the vacuum, not the one that goes forward to the Diagram 2 E preheater diaphragm. and pull it off the
With a warm engine, should be hissing. Put your finger over it. That is called Finger On. Take your finger off the end
of the hose. That is called Finger Off. We can adjust your engine as good as any CO tester with this hose.
Here's the drill:
Finger On to Stabilize,
Finger Off to Measure.
Finger on, stabilize.
Now off, MEASURE:
a) Does idle go up when you take your finger OFF? . . . . . Rich.
b) Does idle go down when you take your finger off? . . . . Lean.
c) Does idle remain the same when you take your finger off? Amazing.
You are now going to turn each screw 1/4 turn in (as in CW facing the screw), test mixture, 1/4 turn in, test mixture,
1/4 turn in, test mixture, until you have the amazing no change.
You are now going to turn each screw 1/4 turn out (as in CCW facing the screw), test mixture, 1/4 turn out, test mixture, 1/4 turn out, test mixture, until you have the amazing no change.
If c), go buy a lottery ticket
Now we shall fine-tune with Central Idling and Retard still off.
Follow closely. You are measuring the side opposite of the pulled wire.
If you pull the right cut-off, it is no longer participating. Therefore, the rpms you measure have to be the left side's contribution.
Example at an initial idle of 650 rpm.
If you pull the right cut-off, and the idle drops by 150 rpms to 500,
your measurement is Left-500.
If you then pull the left cut-off and the idle drops only 100 rpm to 550, that is Right-550.
Left side in this example is the leaner or weaker side.
Right appears to be the richer stronger side.
Since we want a slightly lean final mixture, we are going to reduce the right mixture another 1/4 turn. Test mixture at
mixture reference hose right after you adjust any screw. It should slightly break up, finger off, and recover, finger on.
Pull cut-offs one at a time and measure rpm drop again. Maybe now pulling the right cut-off reduces the idle to 500 rpm
but pulling the left cut-off drops the idle to 525 rpm.
OK, Left-500 and Right-525, we're getting there, but now let's say your mixture test shows a big drop in idle rpm when we perform the
Finger Off Mixture Check.
Well, richen the left carb 1/4 turn richer let's try.
Now we pull right cut-off and the idle drops 125 rpm for a Left-525.
We pull left cut-off for Right-525 also. We then do the mixture test again, finger off, and the idle drops a bit but recovers nicely. See? It's
easy. Now turn both mixture screws CW another 1/8 (45*) turn in because we really do want it running a bit lean on the
dual carbs only, the Central Idling Circuit will save the day. Put Retard hose back on (idle will drop precipitously),
put Central Cut-Off wire back on, and "Only Push Here" the throttles to kick on the central idling circuit (idle
5. Set Central Idling Speed/Mixture:
Central Idling Speed Screw (D1 #4) must be turned out enough to hear air sucking through the central idling air intake.
You can pull the elbow briefly make sure you hear it working, then put it back on. Set Central Idling Speed Screw to
1,000 rpm or so. Pull your mixture reference hose, finger OFF, and turn the Central Mixture Screw (D1#2) out CCW until
idle speed will not change as you turn. Now turn it in slowly until idle begins to drop. When you put your finger on the
end of the mixture reference hose, the idle should go down a bit. Keep turning the central mixture screw in slowly as
you on/off/on/off the reference hose until this is no change in idle speed between on and off. Now reset Central Idling
Speed Screw for an idle of 1,000 rpm.
Then once again turn the Central Mixture Screw whichever direction to get a 25-50 rpm increase in idle speed with your finger OFF the reference hose. That's right, a little rich on the central idling circuit while
we're a little lean on the dual carburetors' contribution to the idle.
Now put the mixture reference hose back on the pre-heater thermostatic valve. Idle should drop a bit to let's say, 950-975-1,000 rpm. So go take a test drive and see how smoothly your bus runs . . . there's more tomorrow with a stone cold engine once again to check the chokes.
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We need a stone cold engine to see how your chokes are doing.
Just like your warm adjustments, we need to take off the left and
right air horns with the central idling tube remaining on the brass intake tube (Diagram 2 blue tube with pink "socks").
Keep the preheater circuit (Diagram 2 E) vacuum hoses attached to the right air horn thermostatic switch (the asterik at the blue/orange stripe) even when it is off the right carburetor.
Drill into your brain the following factoid:
The Choke Index Adjustment is not to be tampered with to try to cure a fast idle. Just because your bus is "back on choke" after only five minutes inside the store and the engine is revving away with your foot off the gas, does not mean that the chokes are adjusted "too far on". They are *supposed to* choke back on within *minutes* on a cold day. The choke index adjustment is to have the correct interval from cold start to all the way open. Period.
The Fast Idle Link Adjustment determines the engine speed while the chokes are on. The last little notch on the fast idle cams barely have an effect on the idle speed, maybe just enough to knock out the vacuum retard, and it is HERE that you are supposed to be able to come out of the store after five minutes and NOT have a racing engine like 99.782% of all factory dual carb engines since 1973.
People are not sensitive to the fact that they have no idea, no idea, no idea, what little step the choke has the fast idle link stepping on! You can't see it! You don't know if the fast idle link is the cause of engine racing/stalling, or if it is the choke index adjustment (and those things have wide variability out there, let me tell you). We shall escape the rulebook here, and ask your specific engine what it would like.
1. Choke Index Adjustment
For this one time, please remove the Pertronix red wire from the coil #15(+) terminal, or remove the breaker point green wire from the #1 (-) terminal. Lightly loosen the three ring screws around the choke covers and try to get some movement now while they are cold. You then tighten the screws only enough to be able to turn the choke covers with some resistance. Now look at your chokes with a cold engine. Push Only Here and see which choke springs shut better than the other. Flip the plate back and forth and make sure it is free while you Push Only Here. Make a note of the difference between the two. Now turn on the ignition and watch your chokes open, it does not matter if the cross bar is pushed. Lightly touch the choke covers to make sure they are getting warm. If one is cold and you have determined that 12 volts is present at the choke spade, you must replace it (this is important, don't cheap out. There are right and left choke covers because they wind up the bimetals in opposite directions)
Within about three to five minutes, one of your chokes is going to reach open.
a) you can make the other one match it.
b) or you can make it match the other.
c) if both were perfectly synchronized, well, you really are special.
Shut off ignition and hook up your points wire or Pertronix.
Use your keen analytic mind here. If quicky was on the index mark, and the other wasn't, match to quicky. If quicky was too far rich (see diagram) and slowpoke was the one closest to the index mark, match to slow poke. We don't care really! We just want them both to hit "open" at the same time. Later, when you are free to futz around and experiment, you can adjust both simultaneously to maintain this open synchronicity.
Apply two or three pumps of the accelerator then start the engine without touching the accelerator. Hopefully, your
engine will burst into song at a fast idle. Kick it down after a couple of seconds and get to the back to watch your
choke plates open. You're looking for relatively even opening times.
The engine might:
not want to go down,
get shaky and slower,
or get smoother if it was shaky.
It all depends on how synchronized your chokes are (Diagram 1 #6),and how synchronized your fast idle links (#7) are.
2. Adjust and Synchronize Fast Idle Links:
The fast idle links are the adjustment that makes life worth living. They can give you that satisfying two steps on the accelerator and the engine bursts into life at a reasonable rpm (1,500-1,800 immediately, then as the engine warms in the first several seconds, you tap the accelerator, and it drops back down to 1,300-1,400, and two minutes later as the engine warms still further, it will drop to 1,200-1,300, then off the chokes with the retard now able to drop the idle down to the final 950-1,000.
(NOTE: We are not doing a from-scratch adjustment here. We are only going a half turn either way on these links. If yours are missing nuts, completely loose, or grossly out, you will need to remove carburetor, close choke fully, and stick an .024" (1700) or an .028 (1800) feeler blade between the throttle plate and housing and tighten up your fast idle link nuts in this mandated position. You can still fine-tune airflow with below procedure )
There are two locknuts per link to trap the link in the choke operating levers. (Diagram 3) Loosen the upper nuts, then re-secure finger tight. Push left choke plate gently towards closed. It will happen across a resistance. Stop right there. Push right choke plate gently towards closed. It will happen across a resistance as well. Stop right there.
(if they don't find that resistance in the same spot in their respective locations, you may have the loose fast idle cam syndrome. . . check by Push Only Here and close the choke plates tight, both of them, and look at the distance between the carb boss and the throttle stop screws, if they are the same distance from the boss, you will have to synchronize your fast idle links at the top step of the fast idle cams with a stone cold engine only)
What we want to do is get both chokes on the first step of the
fast idle cam then leave them there as we start your now-warm engine. You can tickle each individual throttle lever open just enough to engage the first step of the fast idle cam.
Don't use the cross shaft with this step because it will keep unloading the choke you set first.
Just as choke plate meets resistance, open its individual throttle lever just a bit until resistance disappears. The throttle lever will hold at that slightly open spot even as the choke plate springs back to open. Then don't touch anything, because any movement on the throttle cross shaft will release the fast idle cam. Start engine and you should have a fast idle. It *will be faster* than the actual cold engine idle tomorrow morning! so don't worry about it right now with your warm engine.
Reference the Uni-Syn to mark on left carb, then check right carb. Now you need to make a judgment call:
a) if your warm up has always seemed to be too fast while on choke,
you will want to reduce the fast idle on the side that is drawing more air.
b) if your engine doesn't seem to want to catch and hold an idle when you start it first thing, you will want to increase fast idle on side that is drawing less air.
As per Diagram 3, to decrease airflow, turn the lower nut back 1/2 turn, so the fast idle link is allowed to move upwards a bit. Then re-secure upper nut finger tight. Compare airflow.
(if the fast idle cam drops out, just close the choke plate again until the resistance point, tickle open the throttle lever, set it back down)
To increase the airflow, through whichever carb you're using to synchronize, loosen the upper nut a half turn, *push down on the link* you are opening the throttle, and take up the slack with the lower nut, finger tight. Make sure both carbs are on first steps of fast idle cams and check airflow one last time.
You can do this whole procedure again after you have finished setting up carbs and run a few days of First Cold Start Of The Day Investigations. You should have an immediate start of 2,000 rpm when it is below freezing, an immediate tap on the accelerator should knock it down to
1,600 rpm, then another tap in a minute should knock it down to 1,300 or so. A regular idle should occur by the five minute point. All idles should be smooth, no engine rocking. Secure the locknuts, dab with paint when you are satisfied for good, put the air horns on, with the central idling tube back on the left carb, and put away the Uni-Syn.
If your choke is still going on a fast idle after a short visit inside the store, please please please do me this favor. Do not adjust the choke index just yet. Time how long it takes for the engine to come back to a correct idle. If it is a minute or less, just drop down the fast idle link adjustments a smidgeon on the left carb! That will bring you the vacuum retard a little sooner. If it is more than a minute, feel free to go through the procedure again and move the choke that finds resistance too quickly opposite the the arrow drawn on the diagram. Move the index mark 1/32- 1/16" at a time. Write down where you started and where you ended up in case you have to un-do your adjustment. This whole deal takes five minutes when you're familiar with it. Really.
** Final Notes.
*The chokes cannot move into position until the throttles are opened.
That is the way the fast idle cams work. Once the chokes are freed by your stepping on the gas, they spring shut and the throttles are now taking orders from the chokes. Chokes will be totally shut when the engine is off. But your vacuum break diaphrams are designed to open the chokes a set amount once the engine fires so you won't flood. Make sure those things are replaced at carb overhaul time.
*If you ever do flood your engine by pumping the gas and not getting it to start, there is a device called the unloader that mechanically does exactly what the vacuum break does, it opens the chokes when you floor the gas pedal. You are allowed to floor the gas pedal while cranking! This clears out flooding. But please release the gas when the engine catches, and don't "punish" the poor thing by revving it stupidly just because you almost drowned it.
*As the chokes warm up, they are allowed to open at will. But if you do not press the gas, the engine will sit on those fast idle cams til the cows come home, and your engine can get revving pretty high if you go back in the house after starting. (BAD! YOU!) You are expected and encouraged to get moving.
*As long as the left fast idle link is holding the left throttle open, your vacuum retard orifice #17 is going to be blocked and your timing will be advanced! As soon as the engine is warm and off the fast idle, then the retard kicks in. Your chokes must remain on long enough for your engine to warm up enough to transition to the retarded
idle. Otherwise, it is stall city.
This transition is what screws up 99% of all mechanics. You MUST NOT ever screw around with choke adjustments and fast idle link adjustments and throttle stop adjustments and idle speed screws or mixture screws either, or timing adjustments, in the effort to cure weird fast idles or stalling while warming up! That will guarantee you a visit from the . . . Matrix Devil whereupon you are sentenced to a hell of exploding variables.
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This is a sophisticated system with interactions between different circuits. When this engine comes off the idle, to
transfer over to the main carburetor circuits it has to transition from a rich small high velocity circuit dependent
upon a retarded timing map, over to two lean circuits with advanced timing within a narrow band of rpms. The accelerator
pumps serve as a bridge. (check your accelerator links to make sure the little cotterpins are in the middle holes on
each link. . the '74s are adjustable nuts) When it works, it works beautifully.
In most instances, I have seen the two carburetors set way too rich and the central idling circuit rarely contributing enough. The accelerator pumps will drown the engine when you hit the gas in this scenario, and it seems just like a flat spot! So, hapless mechanics everywhere
advance the timing and richen the carbs some more to get rid of the "flat spot" and then have to reduce the central idling circuit even more to get the idle down, and the whole thing gets wacky and they hate these carbs. Nope.
Make sure the retard retards, retards, then get that central idling circuit hissing big-time, lean out the dual carbs so the accelerator pumps give you the punch you need, and you have a smooth economical engine that lights right up in the morning.
All right, all is not paradise, now that we're here I should tell you that there was one little problem. The problem was that VW never quite fixed these carburetors' inability to master ambient temperature swings.
They suffer from hot engine idle deterioration ('74 automatics had a hot idle compensator on the rears of the throttle bodies to help keep the engine idling in hot ambients- that provides additional idle air to the dual carbs when extra hot,
they suffer from transient cold engine lean outs (mitigated by a cold enrichment device bolted up to the central idling circuit that serves as the central idling circuit's very own "choke" (Diagram 1 #16).
These adjunct devices are both simple bimetals that have a little brass plug or cut-off that "un-plugs" when needed. They only need to be clean to work. The cold enrichment device has a resistance heater with a wire attached to the box (the central idling circuit's very own "choke heater"). The hot idle compensator is purely mechanical.
Even when you have your engine dialed-in per this procedure, it will sometimes get away from you during cold or hot spells. I have reached a point where I almost unconsciously compensate for chilly days or hot hot days by absent-mindedly going to the back and giving each mixture screw a 1/4 turn rich when it's cold, and a 1/4 turn lean when it's hot, or I will turn each screw in 1/4-1/2 turn at 5,000 feet and bump the timing 3* advanced to compensate for thinner air. And I do remember to re-adjust when I come down from the mountains. So shall you.
(P.S. when above 6,000 up to 15,000 feet, take the retard hose off the distributor full time and yes this time please block it off. You will get better take-offs.)
If you leave your engine alone after this adjustment procedure, it may give a little balky behavior as per 1970's emissions hell that we all had to endure, whenever the weather goes astray. This procedure is aimed specifically for driveability (but you will pass your emissions tests with flying colors if you do your mixture reference hose to "no
change finger off" on the central idling adjustment).
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
Most of these systems are no longer mandated. You want to disable some that pose a risk to your engine's health, and you
want to keep those that work. This diagram will help you identify any unknown wiring and switches and plumbing that have
managed to hang through the years with your engine. Perhaps it will help answer what that strange sensor is languishing
over the battery (yes, that would be the 4th gear only vacuum advance temperature sensor #25), or the switch on the '74
cooling flap shaft that shuts off the EGR when the engine is not fully warmed up (#16)
Evaporative Charcoal Canister and hoses (Diagram 4 #4, #8, #11, #12) and particularly the tee #11, should be intact and functional.
This system serves up some nice fuel fumes when starting off after a sit. Better that the engine enjoy them, than the cabin full of your friends and the dogs.
Crankcase Ventilation is a good system to have. The crankcase breather hose should be inspected for kinks and deterioration. If you have lousy rubber it will sag shut. Then someone will tell you that you need an overhaul because oil is blowing out your dipstick. The breather box should be cleaned at least once every ten years or so. The '72-'74 system is passive but effective.
Preheat Intake Air (Diagram 2 E), keep! Can be checked by applying vacuum to the end of the hose at the thermostatic switch on the underside of the right air horn. Follow this hose to the valve itself on the air cleaner/filter intake. It should close off the fresh air under vacuum. The thermostatic switch in the right air horn should "sing" quietly when
warm. Your mixture reference hose's day job is to provide vacuum to the preheater when intake air is under about 55*.
Your air filter intake fresh air supply should be hosed in from just above the battery. It is a big diameter (38mm?) pleated black paper/aluminum hose.
Thermostat and flaps, keep. These are engineered to work with your chokes, enrichment device, fast idle time, and EGR temperature switch, and they really help the oil from getting contaminated with cold combustion byproducts that encourage sludge. Make it all happen as the factory designed it. As thermostat supply dries up, we need to create a
robust idiot-proof alternative.
The EGR is a potentially dangerous system if it develops fresh air leaks. Exhaust is inert and "cool" to the engine. It has been asked to recirculate to cool down combustion, but if there are any leaks in the EGR plumbing or valves, oxygen gets in there and heats up the combustion due a lean mixture that burns your exhaust valves. I disabled my EGR in 1980,
lugged the inlet to the balance pipe right at that "Vac Valve" (for '74 only) on the Diagram 4, sealed the EGR valves at the vacuum nipples (Diagram 4 #21) in case the diaphragms should ever leak, and blocked off the muffler's EGR flange.
My last California smog test NOX emissions were well-within specs without it. If your system is intact all the way to the muffler, you can test the EGR valves for proper closing by pulling a hose and applying vacuum to the end of the valve. It should slow the engine down, then recover when you stop applying vacuum. Make sure your EGR plumbing is air-tight from the muffler take-off all the way to the balance pipe.
The Air-Injection system is an accessibility obstacle course (#22) and I deleted it as well, plugged the heads where the injection pipes (#19) thread in, and I blocked off the central idling anti-backfire valve port at 3, and kept the pump as a momento only. Air injection has a major downside to it, it raises exhaust temperatures through the roof and
promotes cracking in the valve port/seat area. These air-cooled engines are pushed to their limits with air-injection and, much like a mis-fire that overheats a catalytic converter, a rich main or a mal-functioning choke because the wire fell off or something, will heat those exhaust ports as detrimentally as a leak in the EGR will heat the combustion chambers,
in both instances, the heads suffer. The anti-backfire valve (#23) has to work if you're keeping it. The control hose (it tees into the "Mixture Reference Hose") opens the air pump outlet to the central idling circuit every time there is high vacuum, i.e. when you shift, so that the afterburners don't backfire like mad. If any of these connections leak, you are introducing air that will burn up your engine under load. Pull the hose off the air pump-to-check valve (#1) also. It won't give you vacuum leaks, but it will give you noise, soot, unwanted heat, and exhaust fumes waiting to get sucked into the cabin through the heater blower.
This may seem like a fabulous amount of gobbledygook, but like everything else, a little ponder time will help your brain assimilate.
Communicate with some of the IAC dual carb adjustment survivors here and elsewhere. They can give you real world feedback on life with these carburetors. I unfortunately lost the finest example of these carbs that I knew when my car got wrecked in August 2009. Those who drove my bus will tell you that these carbs can rival fuel injection for smoothness and beat FI fuel economy. Expect a solid 18-22 mpg, slightly less for the the '74 1800 carbs.
If you have questions or comments, please email me at [email protected]. I am happy to pay you a summertime Itinerant Air-Cooled visit anywhere in the country to help you set up and learn how to maintain your dual carbs. Colin
Link to article and missing pix.
Last edited by Amskeptic on Sat Feb 13, 2010 12:12 am; edited 4 times in total