Joined: October 18, 2002
Location: All Across The Country
|Posted: Sat May 20, 2006 9:28 am Post subject: Adjusting Dual Carbs. . . for rustybus and all
|[url=https://www.thesamba.com/vw/forum/viewtopic.php?t=173175&start=0&postdays=0&postorder=asc&highlight=]Adjusting Dual Carbs. . . for rustybus and all[/url]
1972-1974 Dual Solex PDSIT Carburetor Adjustment Procedure
Preliminaries: This set-up is a matrix. Each adjustment affects the others. Follow the sequence. Diagrams have some continuity as far as numbers and colors, but be alert to artistic license.
1.) Adjust valves cold - .006" all intakes, .006" exhaust, .008" sodium-filled exhaust.
2.) Adjust points - .016" for a dwell of 44*-50*
3.) Test vacuum retard. Idle should speed up when you pull hose F diagram one. If it does not speed up, check for leaky hose, leaky diaphram, or no vacuum signal from left carburetor throttle body. You will want to replace the hose or vacuum unit if they leak, if no signal from carburetor, sit tight.
4.) Set timing only below 1,000 rpm (to prevent centrifugal advance from skewing results)
5* ATDC (1972 and all auto), 10* ATDC (1973-4), 5* BTDC (no retard signal). If your idle refuses to drop low enough because everything is bulloxed on your engine, temporarily block the central air intake with your finger, it's the brass tube under the I elbow on the left carburetor, colored in pink on Diagram One.
5.) Inspect all hoses and connections per Diagram One for cracks, leaks, or omissions. 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.
Consult Diagram One
6.) Perform compression test on warm engine. Add cylinders 3 & 4, and 1 & 2, together and divide by two. Note which side of engine has the higher average reading than the other, later we will allow any minor variation in carburetor airflow to go to the weaker side. Clean and gap plugs to .028" and re-install with a light film of oil on threads.
This concludes preliminaries.
Terminology/Locations: Consult Diagrams.
I removed my EGR and air pump , but have the plumbing and filter illustrated. The top-shots have no depth perception, so things like the G 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.
Initial Adjustments: Take off left and right air horns. We want the central idling tube "I" to remain on the brass intake tube so we can properly calibrate the central idling circuit mixture. Likewise for G, the preheater vacuum hoses must remain attached to the right air horn even when it is off the right carburetor.
Start the engine with two or three pumps of the accelerator then start with no 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. If one choke opens faster than the other, it can give you a shaky lopy fast idle. Search for the index marks on the choke housings and just make a note of where they are. They may not be lined up. After four or five minutes, the choke plates should be almost vertical and the idle should be pretty quick, tap cross shaft to bring it down. It may not want to go down, or it may stall or it may get shaky and slower, or it may get smoother if it was shaky, it all depends on how synchronized your chokes are and how synchronized your fast idle links are. Please note, as long as the left fast idle link is holding the left throttle plate open, your vacuum retard orifice is going to be blocked and your timing will be advanced! As soon as the engine is warm and off the fast idle, the retard comes in bang, your chokes must remain open long enough for your engine to transition to the retarded idle. Otherwise, it is stall city. Warm engine? Shut it off.
1.) Set the throttle stop screws (we are not going to do the Bentley rip the carburetor off and stick feeler gauge in throttle body 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 F off the left carb. Check for vacuum signal coming from the little bass nipple. If you have it, good. If you were missing the vacuum signal during timing adjustment check, then you get to reset timing to proper value now. I'll wait here.
2.) Synchronize air flow: 1972 has it easy, your adjustment is on the left side, a little spring-loaded screw that splits the orientation of left and right levers. 1973 and 4 have to loosen the upper and lower locknuts at both ball joint ends on the right throttle link before starting engine. Our air horns are still off, get your Uni-Syn approach plan figured out before you start the engine, it is going to be noisy.Open the Uni-Syn a good amount. Now get your tach-dwell meter set to rpm and hook it to coil #1 (green wire to distributor) and the other to a ground ( I use the coil mounting 10mm bolt), start the engine, and grab a little vise-grip out of your tool box.You will clamp it only on the accelerator cable end to trap the cross shaft " U" where the cable comes through until you get 3,000 rpm. Move the vise-grip up the accelerator cable end until it is snug against the cross shaft "U" and lock the vise-grips 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. 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! Now switch over to the right carb. Turn the right throttle link (or the screw on the left cross shaft if you're '72) to raise the right throttle lever if your Unisyn red slug is reading low, or lower the right lever if the slug dances up high. This will change the engine speed. 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. Hold the right throttle link steady. Read the new left airflow. Now go back to the right and move the throttle link to make it the same as the new left reading. Did the engine speed change? Go back to the left. When they are both the same, remove vise-grips. Shut off engine. Now your ears are ringing. Take a break.
Goofy Nerd Time: For Perfectionists
When they are both the same, recall which side of the engine had the lower compression readings. If it was the right side, (you '72s can just barely add a smidge of airflow to the weak side) hold the throttle link steady and turn the upper ball joint CW until it is about to pop out, lock the locknut on the upper ball joint, then rotate the lower ball joint CW to it's edge and lock its locknut. Check your Uni-Syn readings again. The right reading should be just a tick higher than the left, probably only noticeable at the margins of its little dance up and down. If the left side was the low compression side, tighten the lower ball joint socket locknut, let the entire link rotate to preload both ball joint sockets before you lock them. Check your Uni-Syn readings. The left side should be barely higher than the right, probably noticeable only at the upper margin of its little dance in the sight glass. Remove vise-grips. Shut off engine. Now your ears are ringing. Take a break.
3.) Synchronize idle airflow: This will be on an idling engine, so you won't be going deaf. 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. Close down the Uni-Syn pretty far. Start engine, reference left carb to that preferred 1/3 mark on the Uni-Syn. Make a quick reading, go to the right carb. For example, if right carb is reading higher, back out throttle stop screw 1/8 (45*) of a turn, 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 fast idle links: 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. Loosen the upper nuts, then resecure finger tight. Push left choke plate gently towards closed. It will happen across a resistance. Push right choke plate gently towards closed. It will happen across a resistance as well. If they don't find that resistance in the same spot in their respective locations, check your indexes. Loosen the three perimeter screws and rotate choke housing until indexes line up with mark on carburetor. Use discretion. If your car has always had a difficult first cold start, move the choke that doesn't find resistance soon enough in the direction of the arrow on diagram, Left will be clockwise towards longer/richer, Right will be counter-clockwise towards longer/richer. If your choke is always going on a fast idle after a short visit inside the store, move the choke that finds resistance too quickly away from 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. Push on cross shaft to unload the fast idle cams and the resistance in the choke plates will be gone. 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 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, so don't worry about it right now with your warm engine. Reference Uni-Syn to mark on left carb, check right carb. Now you need to make a judgment call. 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. 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. To decrease the airflow, turn the lower nut back 1/2 turn, so the fast idle link is allowed to move upwards a bit. Then resecure upper nut finger tight. Compare airflow. (if the fast idle cam drops, 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, 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 close to 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. This whole deal takes five minutes when you're familiar with it. Really.
5.) Set carburetor mixtures: With pocket screwdriver (Tune Up Day Diagram), seat the left and right carburetor mixture screws gently. Back out 3 turns. Count with the pocket clip. Start engine, pull off retard hose (F Diagram One) from the vacuum can end and plug it with a screw or something, and kill the central idling circuit by pulling wire to Central Cut-Off . Your engine should run without too much complaint at about 6-800 rpm. Your tach-dwell meter is still with us, no? Find your Mixture Reference Hose ( G Diagram One Big Red Arrow) on the underside of right air horn it is the hose that has the vacuum, not the one that goes forward to the preheater diaphragm, and pull it off the thermostatic valve. It 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. Now off. Does idle go up when you take your finger off? Rich. Does idle go down when you take your finger off? Lean. Does idle remain the same when you take your finger off? Amazing. Since we put your mixture screws at 3 turns out, you should have found idle going up. You are now going to turn each screw 1/4 turn in( in is 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. Now we shall fine-tune. With the Central Cut-Off still off, and the retard hose off and plugged with a screw or something, you pull the wire off the right cut-off, read rpms, put wire back on, then pull the wire off the left cut-off, read rpms, put wire back on. Who dropped more, left or right? Follow closely. You are measuring the side opposite of the pulled wire. If you have an idle of 650, pulled the right cut-off, and the idle dropped 150 rpms to 500, your measurement is left 500. You then pull the left cut-off and the idle dropped only 100 rpms to 550, that is right 550. 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 your cut-offs and measure rpm drop again. Maybe now, pulling the right cut-off drops the idle 150 to 500 ( as it should because the output of the left side hasn't changed) but pulling the left cut-off drops the idle 125 to 525. OK, we're getting there, but now let's say your mixture test shows a big drop in idle speed when we finger off. Well, raise the left carb a bit, 1/4 richer let's try. Now we pull right cut-off and the idle drops 125 rpm to left 525. We pull left for a 125 drop to 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 go drop both mixture screws another 1/8 (45*) turn in CW because we really do want it running lean on the dual carbs only, the central will save the day. Put retard hose back on (idle will drop precipitously), Central Cut-Off wire back on, and "only push here" the throttles to kick on the central idling circuit (idle restores nicely).
6.) Set Central Idling Speed/Mixture: Central Air Screw 5 (diagram three) must be turned out enough to hear air sucking through the central idling air intake. You can pull the elbow I (Diagram One) to hear it extra loud. Set your idle speed to 1,050 rpm or so. Pull your mixture reference hose, finger off, and turn the Central Mixture Screw 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 set Central Air Screw idle speed to 1,000 rpm. 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.
Concluding Overview: 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.
Caveats: 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. That means it suffers from hot engine idle deterioration, and it suffers from transient cold engine lean outs. 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. If you leave your engine alone after this 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)
The factory provided an cold enrichment device to the central idling circuit when cold and it also attached a "hot-engine idle valve" to the throttle bodies on auto-trans buses that provides additional idle air to the dual carbs when extra hot. These are both simple bimetals that have a little brass plug 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 hot idle compensator is purely mechanical.
If you are mandated to have functional systems, please keep them in good repair.
Your evaporative charcoal canister and hoses and that stupid tee should be intact and functional. This system serves you by providing some nice fuel fumes when starting off after a sit. Better that the engine enjoy them than the cabin full of all of your friends and the dogs.
The crankcase breather hose should be inspected for kinks, 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 pre-heater diaphram can be checked by applying vacuum to the "other" hose at thermostatic valve G on the underside of air horn. It should close off the fresh air under vacuum. The thermostatic valve G on air horn should sing quietly when warm. Your mixture reference hose's day job is to provide vacuum to the preheater when cold. Your air filter intake fresh air supply should be hosed in from just above the battery. It is a big diameter pleated black paper/aluminum hose. I recommend a functional thermostat and flaps, these are engineered to work with your chokes, enrichment device, fast idle time, and EGR temperature switch, make it all happen as the factory designed it.
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. If there are any leaks in the EGR plumbing or valves, oxygen gets in there and burns your exhaust valves. I disabled my EGR in 1980, plugged the inlet to the balance pipe right at that day-glo asterik on the diagram, sealed the EGR valves at the vacuum nipples in case the diaphrams should ever leak, and strangely enough, my NOX emissions are well-within specs without it. 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. Make sure your EGR plumbing is air-tight from the muffler take-off to the balance pipe.
The Air-Injection system is an accessibility obstacle course and I deleted it as well, plugged the heads where the injection pipes threaded in andblocked 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 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 has to work if you're keeping it. The control hose opens the air pump outlet to the central idling circuit intake manifolds every time there is high vacuum, i.e. no load. If the connections ( pink stripes at anti-backfire valve on Diagram One) leak, you are introducing air that will burn up your engine under load. Pull the hose off the air pump-to-check valve 2 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. I am ponder/assimilating Adobe PageMaker as we speak, I will experience information-overload torture as well as you will here. It's nice to share the load. . .
If you have questions or comments, please email me at [email protected]
Last edited by Amskeptic on Sat May 20, 2006 10:33 am; edited 1 time in total