Adjusting 34-PDSIT-2/3 carbs
by Richard Atwell
As part of the rebuild of a 72 engine I had to make sure the carbs were operating correctly. I didn't want the expense or hassle of aftermarket carbs and I wanted to determine if they caused the previous engine to blow. Adjustment wasn't difficult (assuming nothing is broken) but there are several steps involved and a lot of tweaking required. Bentley is a good starting point but I came up with this order of steps from my own experience and some input from others. I could find very little information about adjustment and most of it was in the form of quoting specs from Bentley.
When I stopped by a local shop on the off chance they had some used PDSIT carb parts I was looking for, I received the usually reply to my inquiry: "Why do you want to mess with those dual Solexes? Why not put in a 009 and Kadrons". As luck would have it, they had those very parts for sale! Who knew a new distributor could fix carb problems? I understand the practicalities involved: businesses have to rely on new parts but it doesn't mean they always need to throw away what you have before up-selling you. Maybe they should inspect what you have first.
"Of course the Kadrons don't have any chokes but hey it's California right?", he replied. Wrong. Why are people so afraid to try and adjust the stock carbs? I thought the bigotry started with FI but it seems certain carbs can't escape the big bore beetle conformist mentality either. Certainly there is a attractiveness to simplicity and availability but if we threw away every NLA part restoration wouldn't be as interesting.
I decided to attempt to adjust them to see just hard hard the process was and see how the bus drives. These carbs worked for 50k miles...why not another 50k? The results speak for themselves.
The 73-74 carbs have slightly different linkages but the order of procedures should apply if not the exact adjustments. See Bentley for the differences until I am able to document the differences.
Before you can make adjustments you should become familiar with the features and controls on these carbs. 34-PDSIT-2/3 describes both left and right carburetors:
PDSIT style dual carbs are still available from Brazil under the Brosol brand name. Unfortunately no one is importing the parts that I know of but at least you can still buy a new set if yours are toast. With the sky high price of Webers since the factory in Spain was shut down after a union strike, these carbs have become attractive again.
If the carbs have never been touched since they left the factory or some bonehead mechanic screwed them up then tuning them is going to be difficult at best especially if you had serious engine performance issues to begin with. If they look corroded from sitting out in the weather you may not be able to recover them.
Removing both carbs isn't a five minute job but ensuring they are working correctly on the bench is the only way to ensure they will keep their adjustment so you achieve the best results. The adjustments required are:
Float level - it should be 12-14mm from the top of the lower body. I could not find any combination of shims under the float valve that would lower the float level from 16-17mm. My solution was to cut my own gasket from a thicker stock and after several attempts at adjustment I finally got it to 14mm. The way these carbs are designed, you cannot bend the float tab without disastrous results.
Accelerator pump - it should squirt 0.5-0.9cc per pump (pump ten times and take the average). 72 carbs only squirt 0.3-0.6cc. Do not attempt measurement until you get the float level correct. Use a graduated cylinder for measurement ($5 at your local chemistry supply store).
Throttle plate - needs to have the correct gap. You will need a set of metric round feeler gauges in the 0.5 - 1.5mm range. This step is very important because it syncs the throttle position before any further synchronization is done (you will have to re-sync from here because the engine breathing will probably vary from one cylinder back to the other). 1 1/2 turn out from closed is a good starting point if the carbs are already mounted to their manifolds. You need to perform this step with the cross bar disconnected so it doesn't influence the setting.
Equally important is that the throttle shaft bushing in not worn out. If so, the leakage there could render these carbs untunable. Rimco is such a company that can re-bush these carbs.
Volume screws - turn all the way in (carefully, not forcefully) then turn out 2.5 turns to begin.
Idle screw - Bentley makes no recommendation so start 2-3 turns out
Idle mixture screw - again Bentley is speechless so mimic the other volume screws are go 2-3 turns out to begin with using the same care.
Bentley has some exploded photos of the carburetor. Check Bentley for the exact measurements because they vary from year to year. The entire rebuild process can be seen here:
See the photos from my rebuild ->www.ratwell.com/dotmac/Sites/Parts/PhotoAlbum80.html
The kits are made by three companies:
The first two are junk (see note below) but the Walker kit is good.
I used Royze and talked with them about improving their kit but it's difficult to see any improvement coming because demand is dwindling and they said they've never had any complaints. Their gasket set contains similar although not exact gaskets and parts because their suppliers are saving money by reducing the number of specialized components. Problems I had:
Note: if you can find a NOS Royze kit from the 70's or 80's they are the best often containing original Solex Needle Valves and some even have the Volume Control Screw (aka Mixture Screw). It's only the NEW ones that are problematic. Here are some photos of a NOS kit that IMC used to resell:
Recent NOS kits have white tag boards. Older ones are light green and really old ones are yellow. The older the kits, assuming the rubber seals are still good, the better.
It must be difficult on mechanics that fix these carbs having to deal with crapola replacement parts. No wonder they want to convert 72-74 buses to webers and Kadrons that have parts available still.
I like to use a German made synchrometer from STE. It's commonly called the snail because of it's shape.
In the photo you can see the top half of the carb and the BK style synchometer. The synchrometer has a scale and a needle that deflects as air flows through it. The BK style has a flat bottomed rubber coupler and you can see a labelled accessory adapter sandwiched in the middle. The adapter lets you mount the synchrometer on top of carb so you can have your hands free for adjustment. Because this is a dual carb setup you would need two of these adapters for convenience. Not only are they hard to find, but they are expensive. Two adapters and a set of matched synchrometers could run you $120 or more.
Luckily, because of the shape of the opening of the 34-PDSIT carbs the base of the coupler is stable and makes good suction with the carb without having to press down on the synchrometer. I would still hold it in place by hand while you take your reading because you'll have to move it to the other carb anyway.
The SK style synchrometer has a cone adapter on bottom. Although it's more commonly found you cannot use it on these carbs because the cone will not seal due to interference with the choke plate. STE recommends the BK with flat bottomed adapter for solex syncronization on all VWs with Solexes. I've noticed that the EMPI catalog carries 43-5712 which is a synchrometer with both styles of base included for $30. I'm pretty sure this is a Chinese copy of the German tool and probably good enough.
If you have the other style of syncing tool, the traditional Unisyn, it better be an original from the 60s.
Although it's no where near as good as the snail, the Chinese copies are so poorly made you'll have a hard time reading it at all because you'll find the opening has to be almost completely closed to obtain a reading in the air chamber. This has the effect of starving the carbs of air and stalling the engine. If you think stalling at idle is difficult, try to sync the carbs at 2-3000 rpm with the Unisyn.
The synchrometer is a pass thru style and can sync at any rpm. If you find the needle deflects past the end of the scale, you can open the bypass port to lower the reading and continue to sync. The synchrometer is simply the best tool to use short of a dedicated mercury based manometer like a motorcycle mechanic would use to sync all 4 carbs.
Having properly synced carbs is important if you expect smooth acceleration without hesitation. Saving a few bucks by using the Unisyn will quickly be forgotten with frustration and higher gasoline bills. I don't believe this is an issue of skill. The Chinese Unisyns are simply junk.
Adjustment is a circular process if you skip steps or have a broken part. You should be able to follow these steps to the end and have perfectly sync'd carbs. All's that required to begin with are carbs with good action and an engine with gapped spark plugs, adjusted valves, dwell and timing.
Before attempting to adjust, warm up the engine for 10 minutes. If you cannot start the engine, it usually means you've got a non-carb problem (timing/fuel/etc). The carbs can be way out of sync and still start the engine.
Before you perform any adjustments, be sure the idle speed screw isn't turned all the way in (see step 7). Set it somewhere in the middle of it's adjustment.
1) You have to remove the dashpot.
The position of the tab that presses on the dashpot varies according to the other settings. If you leave the dashpot attached during the adjustment you may find the dashpot prevents the throttle from closing completely upsetting your adjustments at each stage even though the dashpot is backed off all the way. Undo the Adjustment nut and remove the dashpot from its mounting.
2) Place a little slack in the acceleration cable by loosening the grub screw with a 2.5mm allen key, pulling out the cable slightly and tightening it again. If you remove the spring first, you will find adjustment much easier. When finished, reattach the spring.
You should be able to push on the barrel a little without moving the cross bar. If not, put more slack into the cable: you don't want the cable keeping the throttle open during adjustment.
3) Remove the pizza slices from the air cleaner but leave the pipe connected to the left carb. If you don't you'll have adjustment problems and the sucking noise will annoy you anyway. Now you are ready to sync.
Check that the bolts holding the carbs onto the manifolds are tight. In fact I recommend that you use a good blob of green Loctite on those bolts after the tuneup. If you use blue Loctite you may pull a head stud later so be prepared for that. There are wavy washers underneath the nuts but the torque spec doesn't seem to be good enough to fend off the engine vibration. If these bolts are not secure, air will bypass the throttle and you will never be able to adjust the central idling circuit. If you tighten them later it will upset the entire adjustment and you will have to start over.
The carbs need to be in approximate sync before you continue with the other adjustments. This helps to minimize the other adjustments. Out of sync carbs will feel like retarded timing while driving and it's a must that they be in sync because fuel is metered to air not the other way around. There is a noticeable difference is throttle response and feel when the carbs are "dialed in".
Syncing is simply a matter of reading the syncrometer dial on one carb, moving it to the other carbs and comparing the reading. Some people use a matched pair of syncrometers to speed up adjustment. I've found that the syncrometer with the BK base and no adapter makes a good seal and moves easily from one carb to the other.
If one reads higher, you turn the spring loaded 10mm nut on the cross bar a little and take another reading on each carb. If the idle is high you want to lower the reading of the higher of the two carbs rather than raise the reading of the lower of the two. If the idle is too low, take the reverse action. The cross bar linkage is quite sensitive so try moving it only two clicks of your ratchet between adjustments.
After you sync at idle, you must re-sync at 2-3000 rpm. If you only sync at idle the linkage may not be in sync at higher rpms and smoothness/performance will suffer since you are only at idle for brief moments. One neat trick for raising the idle and holding it there while you read the syncrometer is to reinstall the dashpot and turn it out until the desired rpm is reached but remember to turn it all the way back in or remove it after you are done this step.
The 72 and 73-74 rod linkage is slightly different. See Bentley for the 73-74 procedure.
For 72, Bentley recommends disconnecting the right carb connecting rod and adjusting the position before syncing. I found that a high idle condition was caused by the left connecting rod being too long. Bentley makes no mention of adjusting this rod but not knowing what the specified length should be I decided to adjust it to cure my high idle.
What you need to do is leave both the left and right rods connected for adjustment. Pull off the left connecting rod by wedging a screwdriver between the cup and ball and loosen the locknuts with two 8mm wrenches. Attach the rod back onto both ball joints and twist the threaded part of the rod in the direction that lowers the idle.
At some point the idle will begin to rise and the reason for this is that the linkage is starting to open the other carb. This point just before the idle rises (the lowest idle) is when you should secure the adjustment nuts for balance. If you were to skip this step one throttle would open before the other.
LUBE: you want to put a little grease in each ball joint and the cross bar needs to be greased as well so it doesn't stick.
During the carb rebuild, Bentley gives a spec for the maximum initial opening of the throttle plate as measured with a feeler gauge. This position is fixed by two stop screws on the carb body. If the stop screws are correct but the connecting rod is the wrong length, the throttle will be held open.
If the throttle gap is too big (it might even be spec) then the central idling circuit will have a reduced range of adjustment. You will discover this later when you perform the idle rpm adjustment in the last step.
Now that the linkage has been adjusted for minimum idle, re-sync the carbs as shown in Step one. This should only require mild adjustment at this point. Look at each carb while you move the cross bar or raise the idle with the dashpot reinstalled: both linkages should move at the same time. If they do not then you have not adjusted the left connecting rod correctly so go back to Step 2.
I find this the hardest step. Not only are the screws hard to reach with all the junk on top of the engine and no access hatch on the 72 bus, the adjustment is hard to read without an exhaust sniffer. Bentley says to set the mixture to 3-5% CO. Taking the average as 4% this translates to an AFR of 13:1. These carbs will run the engine from 15:1 to 10:1 but the carbs/exhaust are going to pop if they are too lean and the engine will feel sluggish if it's too rich. Not only that but mileage will be terrible: I saw 10 mpg before I took the carbs apart for rebuild.
The starting position is 2.5 turns out for each screw from the bottom. This is good enough to go for a test drive. Be careful when you bottom out the screws. They are easily dinged by the harder aluminum metal of the throttle body and retaining the cone shape is important for smooth fuel delivery. Bentley says to set the idle speed from 500 to 700 rpm before continuing.
I would turn out each screw by 1/2 a turn until you get your desired result. Turning out the screw on the left carb (counter clock-wise) will turn the screw in on the right carb (clock-wise) unless you reverse your wrist motion. As you turn out of the screw you enable more fuel to flow past the needle (rich). As you turn in the screw you lean out the mixture. Rev the engine and read the intake and exhaust. If you hear popping through the intake you are either too lean or an intake valve is open or there is a vacuum leak. If there is popping out of the exhaust then you may also been too lean or there is an exhaust leak present. This is why I find this step hardest: ruling out all the possible causes.
Once you get the mixture right, Bentley recommends syncing the mixture between the carbs by disconnecting the leads from the pilot cutoffs (not the central idling cutoff) one at a time. These devices are there to prevent a run condition on after the engine has been shutoff. Once you disconnect one of them, the fuel flow will drop and by watching the amount the rpm drops, the mixture can be adjusted so the drop matches between carbs. This step, syncing the fuel flow, can only be performed after the air flow has been synced.
You may find that the engine runs too rough for you to record a good rpm reading when the idle drops because you disconnected the cutoff. If so, use the dashpot to raise the idle to 100-200 rpm to help you.
The closer the sync between air and fuel the smoother the acceleration. We are almost finished and ready to reset the idle speed back to the Bentley value of 800-900rpm in the next few steps.
Now you can screw back in the dashpot. Bentley gives a spec of 0.04mm as the distance between the plunger and tab when fully pressed in. Not only is this awkward to measure but not very useful because the dashpot wears and the carb linkage is complex.
The way I adjust it is by ear. Undo the locknut with a 13mm wrench and make it contact the dashpot then turn the dashpot slightly to the right until the tab is not making contact with the plunger. If for some reason you can't back off the dashpot all the way, remove the lock nuts and secure it to the bolt from the other side of the dashpot mount but do not tighten.
Now pull on the cross bar to rev the engine and let it go from the open position. Listen to the rpms drop and watch your tachometer and verify that the rpms drop back to the idle rpm instantly. This is your starting point.
Now turn the dashpot to the left until the rpm begins to rise. At this point back off the dashpot by 1 full turn to the right. Now pull on the cross bar linkage again, let go, and listen to the rpms. If the rpms drop as you let go of the cross bar and then rise suddenly the dashpot plunger is too close. Back off the dashpot another turn and repeat the rpm test. At some point the return to idle will be a smooth transition. It may feel too long or too short to you. When you are happy with the setting, secure the locknut.
Lose the temptation to throw away the dashpot. Apart from being a simple emissions device to deal with overrun, it has the added benefit that it helps keep rpms up while you shift gears. With the long shifter in the bus, this really smooths out the driving experience despite the nature of the manual transmission.
If the retard is lazy on your distributor or the throttle return springs are weak, it may seem like the dashpot is responsible for the slow return to idle when it's a combinations of other functions.
You can now remove the slack from the cable. If the cable is too tight you'll have a high idle, if it's too slack the initial travel of the pedal will feel dead to you and you'll never reach top speed in each gear.
LUBE: Before you adjust the cable, grease the barrel so it doesn't stick on you while driving. The cross linkage also needs to be greased at the ends so it rotates smoothly. You'll find it easier to inject a runny grease rather than try to complete disassemble the shaft.
Adjustment is straight forward. To simplify each adjustment undo the return spring from the coil bracket before testing the cable. You don't need to press the pedal each time, simply push on the barrel and see if there is a small amount of slack before the cross bar moves. Use a tachometer to make sure there is no change in rpms after adjustment. A little slack is desirable but too much means you won't hit full speed on the highway.
NOTE: the purpose of the return spring on the coil is simply to keep the accelerator cable taught. It does not close the throttle plates. That job is left to a spring on each carb connected to the engine tin. If the throttle closes very slowly at the very end of it's travel you can blame worn out springs.
Now we are finally ready to adjust the idle speed. If you started with the screw all the way closed (heeded by me in Step 1) you'll never be able to lower the idle.
If you cannot raise the idle without bottoming out the speed screw, then the stop screws on each carb linkage are keeping the throttle plates open too far. Turn out each screw by half a turn until the idle lowers.
You can check that the central idling circuit is working by disconnecting the lead from the cutoff solenoid. If the rpms drop then the circuit is having an effect.
Tweaking the idle mixture is tricky with an exhaust analyzer...more on this when I revisit the article. Bentley instruction are hard to witness sometimes but are the best to go on:
That's it for adjustment. Go driving and check the idle speed again.
When I rebuilt the carbs I set the chokes as per Bentley: lining up the marks on the carb. I will probably need to change this setting from winter to summer usage. More to follow but it's not a critical step compared to the others.
So far, excellent. Easy starts, no run on and smooth acceleration. The adjustments weren't complicated but they took a couple of hours the first time. I made sure I drove after each adjustment to witness the effect. If you lack a vacuum advance then the carbs are going to feel sluggish. Same result if your timing is off or your carbs are not synced properly. There is a big difference when everything is tuned exactly right when you use dual carbs.
Although not strictly a carb issue, check the vacuum advance cutoff. Bentley describes this part at the end of the Engine & Clutch chapter. Basically this solenoid disables the vacuum advance on all 72, all 73 and 74 Automatic models only. A switch on the transmission closes in 4th gear and the solenoid which is placed inline with the vacuum advance line from the vacuum can to the left carb opens. Bentley also mentions a temperature switch that enables the advance in the lower three gears at temperatures below 54F but I couldn't locate this switch on the 72 bus (perhaps it's only on later models).
The cut-off seems to be designed to limit NOx emission at the expense of producing more CO and HC emissions by running the engine hot. Every engine needs vacuum advance under partial throttle conditions so disabling it seems like craziness that will overheat and wear out your engine sooner. Good thing VW had newer models for sale! VWs and Porsches in the early 70s were force fitted with these kind of emission control devices that resulted in reduce engine life expectancy so it's wise to remove them.
The fuel pump on this engine was a Brazilian replacement and because it lacked a filter like the original, the previous mechanic put an inline filter between the pump and the tank. Other than that, it's a stock setup. If you have 1 or more filters it will take longer for the fuel to saturate each filter and get to the carb. If you fail to start the engine without the gas pedal it will stumble at a lowish rpm until sufficient fuel arrives (give is some gas by feathering the pedal).
When I first tried to start the bus I poured 1 gallon of fuel into the tank. This wasn't enough to flow down to the pump. After pouring in 3 gallons I finally saw some fuel appear in the fuel filters at each carb so be sure you have enough fuel before you go down an unnecessary path of false diagnosis.
As complicated as these carbs are with all their gadgetry, you still have to exert manual control over the starting mechanism (there is no choke lever to pull on the dash as it's built into the accelerator linkage):
If you cannot start the engine normally you should check the rest of the engine before blaming the carbs.
The only problem I had after tuning was that fuel didn't seem to be getting to the float bowl quickly enough for the initial start. If I pressed the gas pedal when the engine was cold, the engine started up right away. When the engine was hot and I attempted a start, it was also instant. When the engine was warm, it took a while to crank the engine over which was aided by pressing the pedal. Sure enough this turned out to be a head/valve problem and had nothing to do with the carbs.
01/22/05 - Created
04/14/05 - Added more photos
05/31/05 - Added starting tips
06/18/05 - Added carb kit info from Jim
09/07/11 - Fixed broken photos, added translate button, updated footer
07/15/19 - Google update: new adsense code, removed defunt translate button