Leaning Carter carbys on my '47 Buick

Hi there from down under in Australia.

Have twin Carters on a Edmunds twin carby manifold fitted to my 248" Buick.

How can I lean the high speed mixture/ At the moment I only have a rolling chassis and the body is still work in progress.

But it is running really rich in the shed.

Open to any suggestions except putting it back to a single carby!!

Thanks

Trackless

You might get a little relief by lowering the fuel level but really the proper solution is to install smaller main jets.

First make sure that the enrichment circuits are not stuck open. If I remember correctly, there is a 'power valve' that is opened when the manifold vacuum drops, ie: when the throttle is past roughly 1/2 open and the engine is under load.

If you look down the throat of the carbs while the engine is running, you will see raw fuel dumping out of the main venturi even at low rpms or idle. It may be only one carb, so take a look before opening up the carbs.

GLong

Before doing anything to the carburetor, check the ignition.

A perfect carburetor will run exceptionally rich on an engine with poor ignition.

Jon.

Thanks to all for the help.

The engine has just been fully reconditioned with new everything and a balance thrown in.

Running an electronic ignition as well and have the original dissy recoe'd as well.

Both carbys were reconditioned in the States by Daytona in Florida who did a great job on them.

The problem as I see it is that I have 2 carbs running of which either will supply the fuel requirements by itself.

I see a need to get smaller main jets for both, but do not have any ideas where to get them.

Welcome any more suggestions!!

Tks

Trackless

Carter used metering rod technology.

The carburetor has stepped, often tapered rod(s) that are inserted through the jet(s).

The effective metering area is the area of the jet less the area of the section of the rod then in the jet orifice. The section of the rod then in the jet is controlled by the vacuum piston spring.

Since you are dealing with multiple areas, which are a function of pi, changing jets MAY get you closer; but rarely is one able to solve a calibration problem by changing jets. Changing the rods are the recommended modification.

I DID read your post as to what has been done. I still would check the following in the order presented:

(1) Compression test

(2) CHECK FOR A VACUUM LEAK !!! A vacuum leak will cause the wrong section of the rod to be in the jet orifice, and will result in a rich condition.

(3) When you changed to the electronic ignition, did you also upgrade the electric charging system to an alternator??? If not, try putting the points and condensor distributor back in the engine.

(4) Try to find someone with a diagnostic oscilloscope, and check firing voltages at the plugs.

(5) Check the fuel pressure. You did not specify which Carters, but most from that period would not like more than 5 psi, some even less.

If all of the above TESTING turns up nothing, then time to look at the carburetors. Rods and jets are available, but they are not inexpensive, PLUS if you have to get them from the US, shipping costs to Australia are not cheap.

Jon.

Are you actually able to drive it? or is it idling, no load, in the shed?

If it is in the shed idling then you are not on the main jets/metering circuits - you are on the idle circuit.

You are right, either carb will satisfy the fuel/air needs of the engine and you jet that carb per Carters' specs for that engine - that will give you the proper fuel/air ratio. When you add the 2nd carb the air will be supplied thru 2 carbs and both carbs will meter the fuel for the amount of air that passes thru them at the proper ratio. Make sense?

I would imagine the engine rpm will be happiest about 600-650rpm, higher than what buick spec'd, because the air signal moving thru 2 carbs at 450ish rpm is so weak that it'll be uncomfortable to drive - when approaching a stop sign it could easliy stall. You don't want it idling any higher because it'll try to pull fuel thru the main wells and that will definatly be too rich.

[h=1]Carburetor Tuning: The Air/Fuel Equation[/h]

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Now that the ignition spark timing advance curves are optimized for the blend of reformulated and/or oxygenated gasoline your customers are using we will now show you how we use tools such as a 5-gas exhaust analyzer and wideband Lambda air/fuel (A/F) meter to tune the mixture.

The Air/Fuel Mixture

A lean fuel mixture can cause an engine to have a surge or miss at idle and part throttle stumble on acceleration, leading to engine overheating and lack of power. A rich fuel mixture can cause an engine to “load up” at idle, foul the spark plugs, and also lack power or run sluggish.

If the A/F mixture that is delivered to the engine is excessively rich for too long the engine could leave leftover fuel from the combustion process, washing the oil off the cylinder walls. Without the oil to act as an anti-wear agent, the pistons and rings will make metal-to-metal contact with the cylinder walls. Also, if enough fuel gets past the rings and into the crankcase the oil can become diluted and lose much of its lubricating properties and accelerate engine wear.

Theoretically, the ideal stoichiometric A/F mixture (the chemically ideal mixture of air and fuel that is required to provide a complete burn) for a properly tuned engine running on pure gasoline is 14.7:1; that is, 14.7 lbs. of air to 1 lb. of fuel. However, because of operating losses in the induction system due to intake runner and cylinder wall wetting, plus the fact that fuel may not fully vaporize in the combustion chamber, a 14.7:1 A/F mixture is often too lean for actual operating needs. A more realistic light-load, cruise A/F mixture for a stock carbureted engine running on reformulated unleaded gasoline is in the 14.1:1 range.

The A/F mixture always varies from cylinder to cylinder, therefore we tend to tune the average A/F mixture slightly on the rich side to avoid engine misfire in the leanest cylinder. It is possible to target an A/F mixture leaner than 14.7:1 for maximum fuel economy but this can lead to driveabilty problems if any one cylinder is leaner than the others. The power mixture we target for maximum horsepower is in the 12.2:1 – 13.5:1 A/F range, depending on the engine package and its combustion chamber design.

The original equipment carburetor(s) that came on a muscle or classic vehicle’s engine was tuned for the leaded gasoline of the day, so in most cases the engine will tend to run lean with the reformulated and/or oxygenated unleaded gasoline of today. The gasoline of today also has lower volatility than the leaded gasoline of days past, which will cause most carbureted engines to need a slightly richer A/F mixture at idle and light load part throttle driving conditions to have the same drivability as it had with the leaded gasoline of the ’60s and ’70s.

Back in the 1950s and early 1960s, the car manufactures tended to calibrate their carburetors on the rich side of the ideal A/F mixture needs of the engine with the leaded gasoline of the day. Then starting in the late 1960s, the carburetors were calibrated more toward the lean side of the ideal A/F mixture needs of the engine so the vehicle could pass the exhaust emission standards that were just coming into existence.

The modern reformulated conventional and oxygenated gasoline of today will cause the A/F mixture to shift leaner when compared to the leaded gasoline of the 1960s and 1970s. This means if the A/F mixture was lean with leaded gasoline it will be even leaner with today’s gasoline blends.

The high performance and replacement carburetors sold today are sold with an A/F mixture curve designed for a generic engine; therefore they must be tuned for both the specific engine and the blend of gasoline they will be used with. These aftermarket carburetors should be designed with an A/F mixture that is rich enough for a wide variety of engine packages with different exhaust systems, but this is not always true. Some of the aftermarket carburetors we see need a lot of tuning work to get the A/F mixture correct for the engine’s demands with the reformulated unleaded gasoline of today.

Air/Fuel Mixture Tuning Guidelines

Back in the days of leaded gasoline an experienced tuner would adjust the A/F mixture the engine was getting from its carburetor by reading the color the fuel left on the insulator of the spark plug in the exhaust port and in the first 6 inches of the exhaust header. The reformulated unleaded gasoline we have today has made reading spark plugs almost impossible because it leaves little or no color on the spark plug insulator.

However, modern technology has made available at an affordable price both portable 5-gas exhaust gas analyzers and wideband Lambda (“oxygen”) sensor based digital A/F meters that can be used to accurately “read” the A/F mixture in an engine by analyzing the content of the engine’s exhaust gases. These modern tools can allow you to observe what A/F mixture the engine is getting from the fuel system while driving the car in real world conditions at any rpm and load condition.

The ideal A/F ratio for maximum power or fuel economy may be best calculated at the factory with the engine on a dynamometer, but the readings that are available from a 5-gas exhaust gas analyzer allow you to tune the A/F mixture for what your engine needs in real world driving conditions. The readings from an infrared exhaust gas analyzer will indicate A/F ratio, engine misfire, engine combustion efficiency and excessive combustion chamber heat (detonation) by looking at the following exhaust gases:

CO (Carbon Monoxide): The reading from an infrared gas analyzer that we use to determine the air to fuel ratio when the A/F mixture is on the rich side of stoichiometric. (Note: CO is partially burned fuel.)

The other readings that exhaust analyzers provide are:

HC (Hydrocarbons): The amount of unburned fuel in the exhaust (a indicator of an engine misfire).

CO2 (Carbon Dioxide): A gas that is the product of complete combustion (the best A/F mixture gives you the highest CO2 reading). The ideal ignition-timing advance will also create the highest CO2 reading

O2 (Oxygen): A high O2 reading indicates a lean mixture; an exhaust leak or the engine has a “hot” cam. Note: if O2 content is above 2 to 3 percent, air dilution of the exhaust gases being measured is indicated and the accuracy of the all of the gas readings may be negatively affected.

NOx (Oxides of Nitrogen): A gas created by excessive combustion chamber heat. This gas can be used as a precursor to detonation.

The readings you can get from a 5-gas exhaust gas analyzer can help an experienced tuner calculate what A/F mixture and how much ignition spark timing advance the engine needs to perform at its best.

A wideband sensor lambda sensor based A/F meter calculates the A/F mixture by “reading” the unburned combustible content of the exhaust gases (note: a lot of people call the lambda sensor an oxygen sensor but Bosch calls it a lambda sensor). The wideband lambda sensor measures the amount of oxygen that must be added to or subtracted from the exhaust gas to form a stoichiometric gas mix in its reference chamber, the A/F meter then calculates the A/F mixture of the exhaust gas from that value.

The readings you get from a wideband lambda sensor based A/F meter can be quite accurate, but false readings can be created by an exhaust leak, engine misfire, or an engine with a high performance camshaft at lower engine speeds. These false readings are caused by the Lambda sensor misreading the unused oxygen and/or unburned combustibles that are in the exhaust gas mixture

Tuning with a 5-Gas Analyzer and Wideband Lambda Meter

The use of a portable 5-gas exhaust gas analyzer and/or a wideband sensor based A/F meter can allow a tuner to observe the A/F mixture the engine is getting from its fuel system at any engine operating condition.

A starting point for A/F mixtures for most mild performance engines is:

• Idle: 1.0% to 3.0 % CO or a 14.1-13.4:1;

• Cruise rpm: 1.0% CO or a 14.1:1 with a mild performance engine; or 1.0% – 3.0% CO or a 14.1 – 13.4:1 with high performance cam; and

• Power mixture and acceleration: 6.0% CO or a 12.5:1 for a “normal” engine or high performance engine with improved combustion chamber design such as a Pro Stock or a NASCAR engine; in some cases you may be able to use a slightly leaner power mixture of 4% CO or a 13.0:1.

When we are tuning fuel systems, we use both infrared exhaust gas analyzer and the wideband Lambda sensor methods. This way we can take advantage of the strengths of both tuning methods. The infrared exhaust gas analyzer, while slower in reaction time than a wideband sensor based A/F meter can actually best determine A/F mixture needs. The misfire rate can be observed with the HC (hydrocarbon) reading.

Efficiency can be observed by the CO2 reading (carbon dioxide) reading, and the NOx reading (oxides of nitrogen) can also be used as a precursor to detonation. A wideband Lambda sensor-based A/F meter systems available from companies such as Innovate Motorsports or FAST have almost no delay, while a 5-gas exhaust gas analyzer has a 6 to 10 second delay.

If the engine you are tuning has an air-gap style intake manifold and/or high performance camshaft you may need to tune the idle and cruise mixtures richer than a stock engine with the same gasoline. The added performance from an air-gap intake manifold and the increased valve overlap from a high performance camshaft can often come at the price of lower fuel vaporization at lower rpm operating conditions.

The richer A/F mixture can help cover up the driveability problems when the fuel is not completely vaporized. The heat the intake manifold gets from the exhaust gas crossover in a conventional intake manifold helps the engine vaporize the fuel as it travels from the carburetor into the cylinders combustion chamber.

A/F Mixture Delivery Circuits

A carburetor has an accelerator pump, idle, main jets, and in most cases a power system that is designed to supply the correct A/F mixture for the demands. The accelerator pump system adds fuel as the throttle valves are opened. Tuning the accelerator pump squirter volume and duration is mainly done by trial and error to obtain the best throttle response, but a 12.5:1 A/F mixture is a good place to start.

An idle system will have an idle jet/restriction that must be changed to supply the desired fuel mixture for idle and off idle engine demands. If the engine you are working on is equipped with a power valve (no metering rods), the main jet size is what determines the A/F mixture that will be delivered to the engine at light-load/cruise speeds.

The power valve restriction (under the power valve) determines what A/F mixture the carburetor will supply when the power valve is open; under high power demands a 6.5? power valve will be open, supplying richer A/F mixture any time the vacuum is below its 6.5? opening point.

Power valves have a reputation for being a weak link in certain designs, but the carburetor can be retrofitted with backfire protection, which will improve reliability. A carburetor that uses metering rods in the primary jets will use the metering rods to change the A/F ratio for both the power and cruise mixture demands of the engine; the larger the metering rod diameter the leaner the A/F mixture.

After the basic engine condition and tune-up (fuel pressure, timing curve, etc) is confirmed to be correct, as well as checking to be sure there are no vacuum leaks, the next step is to determine what the A/F mixture is at idle through 3,000 rpm. If the cruise mixture is off, first change the jets to get the A/F mixture correct at 2,500-3,000 rpm cruise range. Then check and set the idle mixture. If the A/F mixture is too lean at idle or part throttle and the idle mixture screws do not provide enough adjustment, the correction may involve enlarging the idle jet.

If the mixture is still lean at 1,000 through 1,800 rpm after enlarging the idle jet, the idle channel restriction (if used) may have to be enlarged slightly to allow more fuel to be delivered at part throttle. It is important to note that any changes other than basic adjustments and jet changes should be done by a “carburetor expert” to avoid damaging a vintage carburetor. If the carburetor is damaged a replacement numbers matching carburetor could be quite expensive.

A modular design carburetor, such as a Holley, with a metering block does not use an idle channel restrictor. When we want to richen the part throttle we often must slightly enlarge the idle well in the metering block. When the A/F mixture is too lean at part throttle the engine may miss or stumble on light acceleration and at 5 – 25 mph light throttle cruise conditions.This lean off idle problem has become more prominent as the ethanol content in today’s gasoline is increased and as the gasoline formulation is changed.

If the A/F mixture is too rich at idle and/or part throttle, the idle jet or part throttle idle restriction may be too big. You may need to be replace it with a smaller one. Once you have the idle, part throttle and cruise A/F mixture curves correct, the next step is a road test.

A road test using a portable infrared exhaust gas analyzer and/or a wideband oxygen sensor will allow you to check the cruise speed A/F mixture, followed by a check of the power A/F mixture under load. This type of test allows you to see what the A/F mixture is under real world driving conditions. During this road test you will be able to read and then correct the A/F mixture.

If you see an A/F mixture reading that goes too lean at high engine loads, the first thing to do before you change jet size is to check the fuel pressure. The fuel pressure must stay above 5 psi at wide-open throttle; if not, the carburetor will starve for fuel.

The most common accelerator pump-related complaint we hear is a hesitation on quick acceleration. This hesitation is most often caused by the changes in the gasoline’s volatility and changes in carburetor manufacturing. The accelerator pump duration spring used on most replacement carburetors is not as strong as the spring that was on these same carburetor designs used in the 1960s.

We use an accelerator pump upgrade kit on most Holley modular style carburetors that consists of a stronger duration spring, a 0.031? squirter and a “pink cam” (Ole’s p/n 1330), this makes the accelerator squirt more active.

When we are working on a engine with an Edelbrock Performer or Thunder series carburetor we use an improved accelerator pump (Ole’s p/n 1010). This accelerator pump has a stronger duration spring that allows the pump to be more active and thus help cure most of the accelerator pump related hesitation we see with these carburetors.

Selecting the Correct Carburetor

The big four suppliers of 4 barrel carburetors today are Edelbrock, Holley, Quick Fuel and Barry Grant, each of these carburetor designs have strong and weak spots. The carburetor that we would recommend is based on how the customer will drive their car and the engine package that is in the car.

The Carter-designed Edelbrock Thunder and Performer are reliable low maintenance carburetors with great electric chokes but if the driver likes to drive fast around corners they may not bethe best carburetor to select for that application. The off-idle system design in these Carter-designed carburetors can lead to a lean off-idle stumble problem when the engine has a “hot cam” or an air-gap style intake manifold. Enlarging the idle channel restrictor on the 500 thru 650 cfm units will often cure this lean off-idle stumble problem but we have not had the same success solving this lean off-idle stumble problem on the 750 and 800 cfm carburetors of this design.

Modular carburetors manufactured and sold by companies such as Holley, Barry Grant and Quick Fuel are very good carburetors to select when the driver likes to drive fast around corners or when you are tuning for maximum power. Quick Fuel also sells billet metering blocks with changeable idle jets, power channel restrictors and emulsion well restrictions for the Holley style modular carburetors, which allow you to custom tune the fuel curve.

When the customer wants a high performance modular carburetor with an electric choke we often recommend a Holley brand carburetor because their chokes have a choke pull-off built in. When we are tuning a high performance engine with a “hot cam” (over 240 degrees of duration @ .050?) or any engine with an air-gap style intake manifold, we often recommend a race-designed modular carburetor with a four corner idle system.

Tuning Results

You can build your customer the perfect engine but unless it is tuned for the correct formulation of gasoline the engine will not perform like it should. The first person that most customers blame if the engine does not perform like they think it should is the person that built the engine. The best way to ensure your customer is satisfied with the dollars they spent to rebuild their engine is to either offer a tuning service or give them a tuning guide.

Once the engine has been tuned so that it has the correct ignition spark timing and the correct A/F mixture for every operating condition the engine will perform like it should. Properly tuned engines will also have fewer alleged warranty claims since a properly tuned engine will not suffer from problems such as the piston rings being washed down from overly rich A/F mixtures, piston damage from detonation or engine tuning related driveability problems.

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The above article is the best I have seen in simple terms of tuning a carb. We use a five gas machine and a chassis dyne. Also on antique cars we re calibrate the distributor for timing and a faster roll in curve. It's amazing how many poorly running antique cars there are on the road. I enjoy letting club members drive my 32 Pierce to compare it's performance to theirs. More than half the time they send us their carb and distributor. Even better results can be had but fuel mileage goes out the window, and low end performance drops off very quickly. Carbking spends a lot of time here giving good advice. Often times the people who seek it are not capable of understanding it and repairing the car themselves. Now my two cents. Most people spend thousands of hours restoring a car. Why wouldn't you spend 5 to 10 hours final tuning it? Ed.

Another problem I commonly see here and on other web sites is the inability of people to correctly or accurately diagnose the problem with the car. Carbking often points it out in his posts. I bet 99 percent of the time people skip over it and go straight to the carb. Fact is a highly skilled test and tune technician is a rare thing to find today on the older and antique systems. I could make a great living testing and tuning cars that are fresh out of the restoration shop. At a recent Concours we attended there was a car valued in the millions that would not run right, the restorer was "out of options" and couldn't get anywhere. Long story short, we determined missing parts and incorrect parts in the carb was the issue. Simple fix, it was just an issue of a shop not willing to farm out work to someone who was more capable of a correct repair. After a restoration bill approaching seven figures, the owner was NOT happy. Sometimes the most expensive repair is the best value. Ed.

Thanks to all who provided such a lot of info to work on.

I am dividing my time between the engine and completing the body but I'll post my findings.

Jon, yep you are not wrong re the cost of freight --- most of my rebuild has come over from the states. Try a set of 4 springs and 2 front shockies!!

Thanks to all

Trackless

This is not new. 50 years ago Smokey Yunick wrote an article in which he said careful tuning of one of his stock car engines gave an extra 30 or 40 HP, AFTER a complete rebuild. Yet this was a step most people skip. The same kind of tuning could be done on any stock engine or hop up.

We're kinda drifting here, but I'll point out that when 'tuning' an engine I'll start at the valves and continue thru the ignition and finally the carburetor. Jon points out that most carb problems are in the ignition and while people get a chuckle out of it he is giving accurate advice. The carburetor is at the end of the induction/ignition system and it can't perform anybetter than the worst/weakest of the other systems. What will happen is that an engine will have a slight ignition problem and the carburetor will be adjusted to compensate for that problem, the result is 2 different problems that need correcting and you throw in a poorly adjusted exhaust valve and you have just described 80% of the cars puttin around today.

My comment about looking down the throat of the carbs is based on several experiences when a carb 'went south' on me. And also based on my experience of scratching my head, checking points, timing etc.. When I could have discovered the problem with a quick look down the throat of the carb.

My '33 Pierce Arrow 836, with a Stromberg EE3 carb did what I described in my first post. From one stoplight to the next it went from running fine, idling smooth, to belching black smoke and almost stalling when idled down.. it ran 'ok' when accelerating, once the plugs cleared.

In the parking lot of the meet/tour hotel i saw raw fuel leaking from the throttle shaft. A quick look down the throat of the carb showed raw fuel coming out of the main venturies even with the throttle closed. Fuel was pooling on the closed throttle plates.

What was found was a very small speck of brass or copper.. probably from some new fuel lines and fittings had made it to the power valve, and was causing the valve needle to not seat closed. Once the contaminant was removed, the valve functioned correctly.

I could have chased my tail all night, but the repair was complete in about 40 minutes.

So, a quick look can tell a lot, or nothing.. but how much time does it take to take a look ?? And how much expertise does it take ?

GLong

Carburetor problems were MUCH LESS prevelant with the older updraft carburetors than with the newer downdraft carburetors!

Does this mean the updraft carburetors were better than downdraft carburetors??? :confused:

Of course not! What it means is the updraft carburetor was more trouble to remove, thus "mechanics" that will immediately go to the easiest option would try other avenues before trying to remove the updraft carburetor!

Approximately 1952, Carter Carburetor Company in St. Louis asked that any vehicles with "carburetor issues" be sent to the Carter plant for study. 1000 vehicles of differing makes, and with differing makes of carburetors diagnosed with "carburetor issues" by "professional mechanics" (car dealerships, auto repair shops, etc.) were brought to Carter.

I don't remember the EXACT numbers (I do remember the percentages):

Approximately 20 vehicles had major compression issues (2 percent)

Approximately 930 vehicles had ignition malfunctions (93 percent)

Approximately 50 vehicles actually had carburetor problems (5 percent)

Somewhere around here I have the actual article, but don't remember where. For those of you that might wish to search, it was on one of the pink/orange Carter General Bulletins that were in the back of the large maroon Carter Master Parts Books.

A few observations:

An engine with defective (low) vacuum (or a vacuum leak) will result in the carburetor power valve staying open.....rich condition.

A "fully restored, painted" engine, MAY have so much paint as to result in a poor "ground" ("earth") condition, resulting in poor voltage...rich condition.

I personally bring out the worst in any electronics. I take electronic ignitions off (when possible) and replace them with points and condensor distributors. And have eliminated MANY issues. But if one is in love with electronic ignitions, and feel the need to "downgrade" your ignition system by installing one, you will have better results by adding an alternator at the same time. Electronics require a stable voltage, which is not something one will receive from a generator at low RPM's.

Finally, compression gauges, vacuum gauges, etc. are very reasonable in price. One does NOT have to guess when it comes to compression, vacuum, or ignition.

Jon.