Weber Tuning Rif Raf
 

i Still have not got my 1970 2.2L 911 w/ dual weber setup to idle, or to quit "spitting" at low RPM, also the engine races and won't come back down to idle range for like 20 seconds! I currently am borrowing a synchrometer but im lost at this point of wht to do next. haha. i've done enough "****ing" around. lol i need some help...

...Like a step by comprehensive step list of the complete tuning procedure.
All help appreciated.

Patrick
1970 911 T/S

P.s. My ignition timing is pretty darn good.

You sure the carbs have clean jets and such?

nope ... but i think they are clean b/c the car runs fairly well at higher RPM 3-4000. IM pretty sure it all tuning.

More advice appreciated

Patrick
1970 911 T/S

The experienced Porsche guys I know seem to look through their jets pretty frequently. And sometimes they look in there and say "Ah ha!"

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'83 SC

Its been a long time but tuning webers is not too hard, just takes some patience, once they are set up they will run without too much fuss. This assumes that the timing and ignition are all in order:

1.They GOTTA be clean!
2.Fuel pressure has to be correct 4-5lbs and the float level must be checked with a sight glass.
3.Disconnect the linkages from the carbs. You can't do this with them connected.
4.Turn the idle stop screws (both sides) in an even number until you have +-1200rpm.
5.Put the guage on the left rear throat, The round cone shaped gauge works better.
6.Adjust to about half way up. Check each throat, if it's different it needs adjustment.
7.Adjust the air bleed screw on the side until you get consistent measurements at each throat. Recheck each throat.
8.Back both sides down evenly to 900rpm.
9.Tighten each mixture screw in until the motor slows, then back out until it runs smooth, each engine is different, I used to back out a quarter turn more, try up to 1/2 turn in or out and see what runs smoothest. Once you have gone around once, go around again. Note that each throat will not be exactly the same number of turns in or out.
10.After mixture, recheck the sync. Then mixture again.
11.Now, and this is important, adjust the throttle linkage so it starts to lift the throttle plates equally on both sides, otherwise you will be opening one side too soon and the car will run poorly and spit.
12.Double check the throttle valve compensator, I think this is why your engine races. It is a sort of bell shaped thing adjacent to the base of the carb.
13.Loosen the locknut at the top and screw the adjusting screw in all the way.
14.Rev the motor to 3-3500rpm and release, it should drop to +/-2,000rpm.
15.If it is higher, lengthen the rod (this sounds like your problem)by screwing the ball end longer (It's best to have your girfriend or wife present to assist here...)
16.Adjust the screw on the top for 900rpm.
17.Burp the throttle, If it takes too long to return to idle screw the it in, if it hunts and tries to die back it out. If you can't get this adjustment I think you need a new compensator.

I hope this helps. Once you've mastered this you will get your Weber merit badge and impress your friends with your automotive prowess...




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93RSA

Thanx Richard! That's the ticket...I'll post tonite to tell how i did.

Patrick
911 T/s

Thanks here too! Very comprehensive and appreciated.

My problem is different though. My car will idle all day, but loses RPM's while cruising every few miles. The car sat for 4 yrs. I flushed all the fuel out, ran cleaner through the carbs, new filters and plugs, set the timing which is close. Put in a pressure regulater and (was told 4.5 psi max and 3.5 was ideal) that did not make much difference to the 8psi we were running at.

The car pops once and a while holding at mid rpms, and will backfire if you let off the throttle. The timing is much closer than it was.

It's like Its choking itself out I think. I didnt want to frigg with the carbs. I do not have probs with idle, and the car runs and revs quite smooth with no load on it, but cuts out while driving.

Any insights/suggestions on my situation would also be appreciated.

The car is a 2.2 T with SSI exchangers.

I have a nearly indentical problem with my 2.2 Zeniths. I cleaned the idle jets, but no resolve with the popping or racing. I have to use the hand throttle to get the car back to idle. I'm starting a Techron treatment this week. I have seen some inquire about the Marelli distributor's centrifigal advance being a factor in the engine racing and not returning to idle. I used to have the instructions to tune the Zeniths, but I have lost them. I am needing to know about the 2 adjustments near the middle of the carbs around the accelerator pump distribution block. One is like a throttle compensator. I'm sure this is causing my engine racing, but I can't remember how to adjust them. Anybody?????

It is possible that you may have some contanimants in the gas tank, which could be causing a restriction when you are calling for more fuel.

I had a similar problem with a 55 chevy that sat for about a year. Varnish deposits from the fuel sitting in the tank collected in the screen for the line going to the fuel pump. When the engine was at idle or not moving, everything was fine. The car would even run but every now and then it would quit, like it was being starved for gas.

You may want to check it out.

Steve

I had that situation too, well still do. There are still rust particles in the tank and I have to change the fuel filter now and then.

Fear no more I have the answer.

Get paper towel or rag and stick it under the connection of the line from tank to fuel pump. Make sure car is off. Undo line and let the few drips of fuel fall into the paper towel and then throw that out. Unscrew gas cap and set aside. Now scrunch your head down into engine bay (this is tricky but possible, you will see) and blow the SHHIT outta that line back to the tank until you can hear bubbles gurgling in the tank. Problem solved! HOWEVER, this is a temporary solution. The intervals in which you must do it gradually decrease until it is almost a daily practice. Mine has gotten so bad that I now have no choice but to yank the tank and get it boiled. I will posting that question a little bit later!

Good luck, and you are describing EXACTLY what I have had in the past so I hope this fixes it.

Joe 68L

...I forgot to add, the block is purportedly the screen at the outlet of the tank not your filters. That was what was killing me, filters looked o.k. but still wouldn't work!

[This message has been edited by joehahn (edited 08-21-2001).]

For a rusty tank take a large magnet and place it on the bottom of the tank. I use old speaker magnets. Steve

I couldnt take out the line that goes from the tank to the fuel filter. I did though remove the botoom line (I guess the overflow) line and drained all the gas out of the tank, after all , it is below the pick up line.

I wanted to remove the pick up line , but it was harder to get at with the tank installed on the car as it is higher up on the tank.

I have chevron techron in the tank now. I really do not want to mess with these carbs and am hoping it cure itself.

Am I dreaming?
and thanks for all of the info!




[This message has been edited by Reg (edited 08-21-2001).]

i have done the blowing back through fuel line many times myself, i even went as far as having a seperate 2 foot lenth of hose tucked away in the engine compartment that i could quickly attatch to hard fuel line coming into the engine compartment. the idea of the maggnet works real well also. eventually you will end up doing these things less frequently, but you will probably never get it all, atleast i havent and ive been doing it for about 3 years now, on and off, good luck

On the sides of the zeniths there is a large pair of screws, the Aux. Mixture control and the flow rate adjustment (the big one in the middle) these two settings will affect the driveablility of the car. And need to be set from side to side much like individual throats on the webers. On both types check your accelerator pump delivery shot, they need to be consistent or you will get some running fat while some are lean during accel which=popping. I have seen backfires and over rich conditions actually burn (melt) the bottom of the emulsion tubes. The car popped and stumbled, only repair was to replace the tubes. Throttle shaft wear can wreak havoc with these carbs as well, try some carb cleaner at the shafts while the car is running to see if you have a leak.

On the tank you could drain it with the drain at the bottom. run compressed air through it to dry it out. Blow through the main back into the tank. Once you are convinced it is dry and NO vapors, stick a shop vac with a plastic tube taped to the end into the tank. with it running blow into every hole you can find to stir the debris while moving the tube around the tank. If you pull the sensor out it gives you a big hole to work through and you don't have to worry about damage. Then get a big, clear fuel filter to act as a pre filter to your main filter.

Anyway just my .02

Happy wrenching...

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93RSA

I have a few questions as to J Richard's list above.

2.Fuel pressure has to be correct 4-5lbs and the float level must be checked with a sight glass.

Ok, checking I have a brass screwed fitting that has a level guage on it. I assume this is what I use to check the float level. Where should it be on this guage? It has two sets of double lines.. is it correct to assume it needs to be within these limits?

If this is the proper way to check the level, how does one adjust?

3.Disconnect the linkages from the carbs. You can't do this with them connected.

I am just wondering why they need to be disconnected, and is there a better way of doing it?

6.Adjust to about half way up. Check each throat, if it's different it needs adjustment.

I have the cone type guage that pelican sells in this catalog. Half way up ? My scale reads 0 to 30 pounds, but half way on the scale forgetting the numbers is about 5 pounds, whereas 15 pounds would be 3/4's up the scale. Just a clarification here please.

10.After mixture, recheck the sync. Then mixture again

You mean after I do the adjustments , place my cone airflow guage on the stacks and see how close they are to being at the same level? If they guage is the same on all 6 throats , the carbs are set , aside from the even linkage?

Is it just the air mix screws I should be fooling with?


I hope to have it running good on the weekend, go for a first smooth drive is teh goal! Thanks so much http://www.pelicanparts.com/ultimate/smile.gif

Reg

2. The float level should be between the lines, depending on what year and model of webers you have the float adjusment varies from a small screw to a bendable tab. It's hard to say unless you look at the carb.

3. The throttle linkage must be disconnected (only at the ball joint, don't take the whole thing apart!) so that it is not holding one bank of throttle plates open due to it being tied to the other side. You want to adjust and balance the carbs and then hook up the linkage so they are both working from the same idle adjustment.

6. All the synchrometer is doing is giving you a relative flow rate, with the cone type you just put it on and read it, with the unysin type you have to adjust the guage to get the ball to read initially.

10. Yep, you're just going over it one more time. usually it requires two passes at least to get it all right. The thing is that adjusting the mixture on one throat can cause it to use more or less air due to a more or less efficient combustion cycle. So the first adjustment gets you in the ballpark, the fine tuning is the next pass.

Go back up and follow the process in my original post. After the air screws then mixture, then air screws, then mixture, then Idle compensator. (and then I usually went back and checked the air screws and mix again!) Everytime you change something in the system you affect everything else, you are working your way to a common state with each pass. And you can't skip a step or fuss with stuff out of sequence or you'll just be wasting your time over a hot motor.

Before you start you may want to just check a few things:

1.Is the timing right on? Including dynamic at 6000rpm? it really needs to be checked at 6000, or your advance could be sticking and messing you up. Pull the cap off and check the advance mech is operating and free. Suck on the advance hose and make sure the plate moves and you have no leak. If you have a marelli distributor, dump it and get a bosch.

2. Try the Sync on each of the throats before you disconnect the linkage. Are any way out? or all fairly consistent?

3. Rev the motor and see which throat is backfiring on release. Is it just one in particular? a burned valve can cause a backfire.

4. How was the compression? any cylinder low?
Valves adjusted?

5. On your orig post you said that the car idles but looses RPM while crusing. Is this an abrupt loss of power and stumble/backfire or a gradual reduction in power? I would be looking in two different places depending on whats going on. Sounds dumb but if it is slowly loosing power you may have a fuel tank venting problem or the wrong gas cap.

Anyway I hope some of this is helpful, It's really hard to do this without hearing the thing. I would spend the cash and get a weber tuning book there are several out there which will give more specific info.

Good luck, have patience, it will take some...


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93RSA

5. On your orig post you said that the car idles but looses RPM while crusing. Is this an abrupt loss of power and stumble/backfire or a gradual reduction in power? I would be looking in two different places depending on whats going on. Sounds dumb but if it is slowly loosing power you may have a fuel tank venting problem or the wrong gas cap.


It just starts to gasp really... you floor it as it is slowly loosing rpm's at highway speed. sputters , backfires some , pops.
It idles all day well, but seems a slight miss , very slight to me while at idle. It does not hesitate when you open up the throttle with your hand at the carbs.

If you sit the car for say 5 minutes, it starts right up and rev's freely. If you start the car too soon after this all hapens you play with the throttle and its rough for a bit , then breaks free and away you go.

Sad thing is a couple times it revved really nice, and pulled my neck back while adding a big smile to my face! I want that again!

My other friend suggested to t off the fuel line with a pressure meter in the cockpit and monitor the pressure at speed when the car dies out. We have it regulated at 3.5 psi, and can only regulated it to 4 psi, without going to 8 psi without this regulator installed.

BTW , the car ran the same way at 8 psi, but of course like now, it is not running correctly!

The car is a 70 t , 2.2 with SSI's and the 40 webers..not sure if anything other than stock inside the engine, but am assuming so.


Thanks again!

[This message has been edited by Reg (edited 08-31-2001).]

[This message has been edited by Reg (edited 08-31-2001).]

I had the same kind of problems.

I had fuel filters in my car that i did not know of, full of rust. Almost no gas came out at the carb end. Replaced the fuel lines +filters and has been working since (gotta change filter pretty often though

I did por-15 tank sealer kit (Wayne, you guys should carry this, 50 bucks!) This is truly idiot proof. Once I put tank back in and whacked the side of carbs with screwdriver handle, car ran fine (and holding).

yes, I have a new fuel filter installed, and am placing additional in line clear plastic filters right before each carb.

Lots of variables... ahhhhhh!

I'd like to add some diagnosis to the suggestions above.

If the carb is popping at idle, it could be a vacuum leak somewhere. No one mentioned leaking throttle shafts as this is a common area of wear (vacuum hoses too). The throttle shaft bore machined in the carb. housing wears and allows air to bypass normal air intake. You can verify this by pressing on the shaft lever in one direction or another and see if the idle changes. You can also squirt some carb. cleaner around the suspected areas and see if the idle changes. If so, you have a leak there. Not inexpensive to repair. Eurometrix used to get around $400/pr. to rebush the carb. housings.

PMO solves this on their Weber versions by using a ball bearing to support the throttle shafts. Neat.

If the engine idles fine, but falters at cruising/high speed, check the fuel pump volume. Pressure could be fine but insufficient volume means the engine uses fuel faster than the pump can supply it.

Hope this helps,
Sherwood Lee
http://members.rennlist.org/911pcars

Though I mentioned the throttle shafts on my second post, I guess I didn't make it clear...

However, If the car runs solid and then looses power gradually then I'd go with Sherwood, it is probably a fuel delivery problem or a sticking vacume advance.

You could go through the test for the fuel pump volume, you need to find out what mfgr. you have but it should be be somewhere between 30-45 oz. per minute.

Also try taking the fuel cap off and driving the car to see if there is any difference.

If you put in clear fuel filters are you getting any particles?



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93RSA

the new clear Fram g3 filter has shown some discoloration. I will take a closer look tomorrow to see how much for particles there is inside. I am adding an additional two filters just before each carb. Thanks for the amounts of fuel requirements.

I keep wondering now about dropping the tank. I did not take out the line out to the engine when i drained it. I removed the return line as it was lower on the tank. I thought I'd get all the fuel out that way, plus the pick up line was higher and harder to get at with the tank not removed from the car. I wonder if there is a screen there, that is full of sediment now.

I guess the fuel oz. per minute should tell me that though, right? Or will that too make a difference under load while driving?

Will knowing the flow rate with the pump on into a measuring cup elimintate the car starving under load at speed problem, if it falls under your numbers given above?

[This message has been edited by Reg (edited 08-31-2001).]

There is a screen on the tank pick up, if you have a lot of buildup it could be clogging, the fuel flow test should tell you, you might want to run a gallon or so through to see if it loads up and slows down.

Do take the time to take the gas cap off and drive it just to make sure the vent is clear and you aren't vapor locking with a blocked fuel tank vent.

Good luck, keep at it.

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93RSA

I just posted this to another post (from my new book)

Project 29: Carburetor Adjustment, Balance, and Tuning

Time: 1 hr
Tools: Carb sync tool
Talent: 2
Tab: $25
Tinware: -
Tip: Adjust the carbs, and then drive the car around for awhile and recheck all your adjustments, have plenty of patience
Performance Gain: Better engine performance
Comp Modification: Check the engine timing

Well adjusted carburetors are key to good engine performance. If your carbs are unbalanced, then different cylinders in your engine will receive different amounts of fuel and air, and the all important air/fuel mixture ratio will be off. This will result in less horsepower, a poor idle, and higher emissions. It’s a wise idea to check the balance of the carburetors each time that you perform a major tune-up on your engine. The most popular system for use on the 911 are the Weber triple-throat carburetors. The remainder of this project will discuss the procedure used for balancing a 911 engine equipped with Weber carburetors.
If your carburetors are poorly balanced, then you will have a multitude of problems with your engine. The engine will have a tendency to spit and backfire through the exhaust. Unbalanced carbs may also give the engine an uneven idle, or make the car very difficult to keep at idle without stalling. It’s important to make sure that your engine is in good overall health (good compression), and that your idle jets are clean as well. Dirt in the idle jets, poorly adjusted valves or a burnt exhaust valve can also result in the same symptoms that are associated with unbalanced carburetors.
The first step in adjusting your carburetors is to make sure that the rest of your car is tuned and performing properly. Make sure that your valves are properly adjusted (Project 18), and the timing and dwell are set properly (Project 23). Make sure that your spark plug wires are in good condition, your plugs are new, the carburetor floats are properly adjusted (Project 28), and the idle jets are clean. Let the car warm up to its normal operating temperature before starting to adjust the carburetors. Once the car is warm, begin by adjusting the idle to about 1000 to 1100 RPM. You want to set the idle to be a little bit higher so that the car will not die out when you are making adjustments. Adjust the engine to this value by carefully turning the idle adjustment screws located on the ends of the carburetors.
Now, remove the air cleaner from the top of the carburetors. The stock air cleaner is simply clipped onto the top of the engine, and can easily be removed. If you have an aftermarket filter assembly, you can remove these by simply unbolting the air cleaner housing on each carburetor. Now, disconnect one of the two small drop links on the throttle linkages so that there is no connection between either of the two carburetors.
Once you are sure that the timing is set correctly, and the idle speed is set to your working range of 1000-1100 RPM, it’s time to synchronize each of the carburetor throats. A special synchronization tool (sometimes called a Unisyn or synchrometer) is used to measure the air flow being sucked into the manifold by each carburetor throat. The goal of the synchronization task is to make sure that the amount of air flow into each throat is the same for all six cylinders.
Start with cylinder number one, located on the rear left side of the engine. Place the synchronizer over the top of the carburetor stack and adjust the tall glass stack to a vertical position. Turn the adjusting disc on the synchronizer until the small indicator bubble inside the glass rises about half-way up the height of the synchronizer. This basically calibrates the synchronizer tool for cylinder one.
Now, place the Unisyn on one of the throats of the right bank of cylinders. Adjust the idle adjustment screw located at the end of the carburetor until the ball in the Unisyn is at about the same height as it was on cylinder one. The idle speed should either rise or drop off a bit from the level that you had it set at.
Starting with cylinder number one, adjust the mixture adjustment screw at the base of the carburetor. On each Weber carburetor, there are three mixture adjustment screws that have a spring wrapped around the inside to prevent them from turning. Turn this screw in until the idle begins to drop. Then back off the screw about an 1/8th of a turn or until the idle comes back to its previous level. The goal is to turn in all the mixture adjustment screws right to the point where they begin to affect the idle. Repeat this procedure for each cylinder on each side of the car. If your mixture adjustment screws are turned in too far, then you will be starving your engine of air. This will result in a popping-type backfire out of the exhaust. When all the mixture adjustment screws have been set, then readjust the idle adjustment screws located on each end of the carburetor until the idle is adjusted back to 1000-1100 RPM. Use the Unisyn to make sure that both banks of carburetors are drawing the same amount of air while the car is at 1000-1100 RPM.
Next, take the Unisyn and measure the level of air flow that each cylinder is drawing through the carburetor. Make a note of the level of the highest cylinder – you will adjust all the other carburetors to level of this cylinder. There are three air correction screws that are located at the base of the carburetors and have a small nut that locks them down to prevent them from rotating. Adjust the air correction screws so that the level of each cylinder as shown by the Unisyn is equal to the reading on the highest-level cylinder. Adjusting the carburetors in this fashion will give the car a strong idle and make it run very even. If for some reason, you cannot adjust all the cylinders to the reading on the highest cylinder, then adjust that one down slightly to the point where all the cylinders can be synchronized together. After adjusting all the air correction screws, reset the idle back to the 1000-1100 RPM range, using the Unisyn to verify that both sides are evenly balanced.
Once all the cylinders are set and synchronized, it’s time to adjust your linkage. The accelerator linkage must open each carburetor by the same amount at the same time. In other words, both the left and right carburetor linkage must be synchronized with each other as the throttle is depressed.
First, carefully inspect your throttle linkage. All of the links and ball joints should be tight and have almost no slop or backlash. If there is significant wear on these joints, then they should be replaced, as you will find it very difficult to adjust the linkage and balance the carburetors.
To adjust the linkage, you will need to change the length of the linkage arms by rotating the ball joints at the ends of the arms. The linkage has two drop links, whose length controls when the throttle bodies are opened and closed. If the one carburetor opens earlier than the other carburetor, then you need to decrease the length of the throttle linkage drop link until they both open at the same time. The drop links are threaded with both a left and right-handed thread (similar to the tie rods), so you can get a very fine adjustment simply by rotating the drop link in either direction. Tighten up the retaining nuts on each end of the drop link when you have finished adjusting the linkage.
In addition to checking both the idle mixture and the linkage, you should also check the adjustment of the accelerator pumps. These pumps are designed to inject extra fuel into the carburetor throats under acceleration. There are two of them, and they are located in the center of each carburetor. They directly affect the performance of the 911 up to about 30 mph. This pump jet squirts out a stream of fuel when the throttle is opened from being completely closed. It may be easier to perform this test if you remove the velocity stacks on top of the carburetors.
To check the adjustment of the accelerator pumps, place a small vial under the pump jet located inside the carburetor throat. You will have to fashion a small wire to hold the vial, as space is pretty tight inside the carburetor throat. Pump the throttle completely from closed to open twice, and measure the amount of fuel in the vial. For cold weather, the factory recommends 0.55-0.65 cc of fuel. For warmer weather, it’s recommended to have 0.40-0.50 cc of fuel. Adjust the length of the small linkage attached to the accelerator pump on the carb to increase or decrease this amount.
Early Weber carburetors don’t have adjustable accelerator pumps. A poorly adjusted accelerator pump will tend to make the car hesitate under acceleration. This symptom is also experienced when the diaphragm on the carburetor is cracked or leaking. It’s very often the case on 911s that have been sitting for many years for the rubber diaphragm to break. Replacement of the diaphragm during a routine carburetor rebuild will solve this problem (see Project 28).
When finished, go drive the car and check the performance of the engine. Often it is the case where several attempts will be required to properly tune and balance the carbs. When you have completely the entire procedure, it’s a wise idea to go back and check all the measurements one more time. If you find that you are having difficulty balancing or tuning the carburetors, it may be that they need to be rebuilt. Vacuum leaks around seals and worn throttle bodies will make the carburetors almost impossible to properly tune. If a carburetor throat doesn’t seem to respond to any changes in the idle mixture screw, then this is a clear indication that the idle jets may be clogged, or your carburetor may need rebuilding. See Project 28 for more details on rebuilding the Weber brand of carburetors.
Another problem common to carburetors is the clogging of the idle jets. These clogged jets will cause the engine to run rough. The solution is to simply pull them out of the top of the carburetors and clean them out with a little bit of carburetor cleaner and compressed air. You should make sure that your jets are clean before attempting to tune and balance the carburetors. Refer to Project 28 for more details. Approximately 60% of the time when you are driving, the idle jets will be what is primarily supplying fuel to the car. Dirt in the idle jets will make the car run sluggish, as if you have lost power in a cylinder.
Another common problem with Weber carburetors is that they sometimes spit out the top of the velocity stacks. This is usually caused by a design defect in the early specifications of carburetors used on early 911s. The spitting is caused by the idle jets being too small. The cure for this problem is to install larger idle jets. In the old days, you could check the jetting of your carburetors by taking a close look at the spark plugs. Unfortunately, you can’t read spark plugs as well with today’s unleaded gas formulations. In the older days, a black sooty plug meant that your car was running rich. A nice brown plug meant that the mixture was set properly. With today’s unleaded fuel combined with lots of additives, the plugs will often look black and sooty, even if the engine is running well.
Clean fuel is especially vital to good working carburetors. It’s a smart idea to add an extra fuel filter right before each carburetor just to remove an excess dirt that may clog the idle jets. Unlike high-pressure fuel injection systems, carburetors don’t get cleaned by the pressurized fuel flowing in and out of the small passages. It’s also very important to make sure that the air filter is clean when you reinstall it.
It’s important to note that Weber carburetors have no choke or starting circuits. You must use your foot on the accelerator pedal to pump the accelerator pumps in the carburetors when first warming up the engine. When starting the car warm, take the pedal and floor it, then turn the key. Don’t pump the accelerator pedal when the car is warm, as you will probably flood the engine. Weber carburetors have a tendency to boil over the fuel when the car is warm, and dump it into the combustion chamber, flooding the engine. Holding down the pedal is the correct procedure because chances are that there is already a lot of fuel in the combustion chamber. When the car kicks over, simply release your foot from the pedal and bring the car down to idle.
If after going through the entire adjustment procedure, the engine still doesn’t perform properly, you may have some problems that are caused by other factors. If your engine is not in good health and has a compression leak, then you may get backfiring through the carburetors. If the small passages within the carburetors are clogged, then they might need to be rebuilt. If turning either the idle mixture screws or the air correction screws doesn’t affect the idle of the engine, then you may have to rebuild the carburetors.
There are many different opinions on how to best tune carburetors, and the procedure described here is simply one of many. Just about every book I’ve read has a slightly different procedure for tuning the carburetors. In the end, practice and increased familiarity with how the tuning of the entire system affects engine performance will enable you to eventually tune the carbs ‘by feel.’

Picture 1: The Unisyn synchronizer is a very useful tool for balancing your carburetors. Make sure that it completely covers the top of your carburetor or velocity stack when taking measurements. Once you have the synchronizer set for a particular cylinder, don’t change the diaphragm setting until you are finished checking all the other cylinders. Also, avoid looking directly down the stacks of the carburetors while the car is running. If a backfire occurs it can come up the stack and hit you in the face.
Picture 2: It’s also very important to make sure that your linkage is properly adjusted. The small droplinks (shown by the yellow arrow) that are connected to the throttle and control the butterfly valves need to be appropriately adjusted. Make sure that they are opening the valves at the same time. The white arrow shows the idle adjustment screws that are located on the end of each carburetor.
Picture 3: This photo clearly indicates the adjustment points for the Weber carburetor. The white arrows indicate the idle adjustment screws. The green arrows point to the idle mixture adjustment screws. The yellow arrows indicate the air correction screws, and the red arrow points to the accelerator pump adjustment rod. The location of the idle jets which must be kept clean are indicated by the blue arrows.

Index Words:

Carburetor, Synchronize
Linkage, throttle
Synchronizer
Unisyn
Velocity Stacks, Carburetors
Idle Mixture, Carburetors
Accelerator pump, carburetors
Idle Adjustment, Carburators
Mixture Adjustment, carburetors

Great info!

Now to work I go! I hope to post good news later in the day.