Mechanical Tune-Ups

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Tune-ups are just the thing to pep up any car, old or relatively new. In order to get the best performance, mileage, and life from your car, you need to tune it twice a year. Tuning involves adjustment of worn parts or systems. There are many books published on this topic. In this section and the two following, we present theories and recommendations we hope will enable you to perform better, more complete tune-ups that will help prolong the life of your car. Again, follow the manufacturer’s recommendations for performing the tune-ups themselves. Follow the recommendations given here for frequency of tune-ups and special tips. You’ll be glad you did.


An engine and a human being are alike at least in one respect; they both need fuel and air to function. In the case of our bodies, the fuel is the food we eat. In the car the fuel is either gasoline or diesel fuel. The carburetor, fuel pump, intake manifold, and valving all work together to provide the proper amounts of air and fuel to power the car.

The carburetor on your car is nothing more than a large air valve. When you de press the gas pedal you operate the carburetor by linkages or cables to allow more air to flow into the engine. The main job of the carburetor, then, is to introduce a proper air and fuel mixture into the engine cylinders under correct temperature and pressure.

Carburetor operation depends entirely upon differences in pressure within the carburetor. It works on the venturi principle (Fig. 1). Air flows through a tube, called a barrel, inside the carburetor. Most carburetors have one to four barrels, depending on engine size and the performance desired. As the air progresses downward toward the intake manifold, it encounters a necked-down or restricted area in the barrel. Since the diameter (B) in the figure is smaller than diameter (A), the air must speed up at the necked-down area in order to maintain the same amount of total air flow through the barrel: what goes in must come out.

Fig. 1. Venturi principle.

As the air speeds up at the necked-down region of the barrel, its pressure drops, creating a partial vacuum. If we attach a fuel tube to the barrel in this region with the other end of the tube in a bucket (or bowl) of fuel, the fuel will be siphoned out of the bucket because of the vacuum effect. If we further attach an air tube to the fuel tube, as shown, air will be drawn into the fuel and a combination air-fuel mixture will flow into the barrel, through the intake manifold, and into the engine cylinders. A throttle valve located at the bottom of the barrel is connected via linkages or cables to the gas pedal to control the amount of air-fuel mixture delivered to the engine. The more air-fuel mixture delivered, the faster the engine speeds.

Modern carburetors are much more complicated than shown in Fig. 1. They are made up of numerous passages, ports, jets, and pumps to control not only the total amount of air-fuel mixture, but also the proportion of air to fuel delivered to the engine. When a higher proportion of gasoline to air is needed, the ratio is called a rich mix. Rich mixes are used at idle, for starting, and for accelerating. When a lower proportion of gasoline to air is needed, the ratio is called a lean mix. Lean mixes are used at any part-throttle operation.

Carburetors consist of a number of systems that do various jobs. The float system keeps gasoline at a constant level in the fuel bowl. The main metering system consists of tubing and jets that deliver the air-fuel mix from the fuel bowl to the barrel in the proper quantities. Each barrel has its own main metering system. The idle system supplies sufficient air-fuel mixture for engine operation at idle when the throttle valve is closed. The power/pump system is designed to supply the especially rich air-fuel mixture for the times that engine power and speed are required—for example, when passing another car. The choke system supplies the very rich air-fuel mixture needed for cold engine operation, as at start-up. The anti-stall dashpot system, used on cars with automatic transmission, acts to prevent stalling if the throttle is closed suddenly.

Our purpose here is not to explain carburetor operation in detail, but rather, to establish that, although the basic theory behind carburetor operation is simple, the many different driving conditions the car encounters tend to make the design complex. Proper engine performance and life depend on a clean, efficient carburetion system. For this reason, the carburetor must be kept in top condition.

Some high-performance cars use a fuel injection system instead of a carburetor. The air-fuel mixture is injected directly into each cylinder instead of being mixed before entry into the intake manifold, as with a carburetor. The advantage of a fuel injection system is that ideal fuel distribution is possible at every injection. This reduces fuel consumption and air pollution. The only disadvantages of a fuel injection system are the high cost and the fact that it must always be serviced by a factory-trained technician. In fact, work is often performed in a “clean room,” which is not available to normal service stations. If your car has a fuel injection system, don’t attempt your own service unless you know what you’re doing. Have it checked out at least once a year for proper pressure levels, air-fuel mix, and performance. By the way, all diesel engines use fuel injection.

Now let’s see how we can avoid problems and help lengthen the life of the carburetion system.


Air is free. Gasoline costs money. Sometimes, we might forget that air (actually the oxygen in the air) is just as important to the operation of the engine as gasoline. The modem gasoline engine uses about 15 parts of air to 1 part of gasoline by weight. Gasoline weighs approximately 600 times as much as air at sea level. That means 1 pound of air will occupy 600 times as much space as 1 pound of gasoline. Therefore, we need to supply 600 x 15—or 9,000—cubic feet of air for every cubic foot of gasoline to run our cars. That’s a lot of air to handle.

Air is supplied to the engine through an intake tube on the air cleaner assembly that is mounted on top of the carburetor. It must pass through a filter or air cleaner before it travels down through the carburetor barrel and into the engine. Atmospheric air contains dust and, especially in industrialized areas, may carry particles of dirt, smoke, and coal— which are abrasive. In heavy traffic, the air will be contaminated with car exhaust, which can contain droplets of unburned or partially burned gasoline or oil. The air cleaner must filter all these pollutants and abrasives out of the air before they reach the carburetor.

There are two types of air filters in common passenger car use: the dry corrugated paper filter and the oil bath filter. In either filter type, the filtering medium must not be torn, clogged, or overly dirty. Inspect the filters every other month.

With the dry paper filter, hold the filter up to a bright light. You should be able to see light through the corrugations. If not, try to tap the filter or blow air from inside out to clean it, otherwise replace it. For optimum performance and cleanliness, replace the air filter once per year regardless, because many times small holes that allow dirt to pass into the engine go unnoticed even though the filter looks clean.

Oil bath filters need to be cleaned every six months or sooner, as conditions warrant. Clean with the specified solvent and reassemble according to manufacturer’s instructions.

With either type filter, be sure the elements are properly seated atop the carburetor to prevent unfiltered air leakage in the carburetor. Remember: dirt is the biggest enemy of your car engine.


Petroleum is the second most plentiful liquid on earth. Only water is more plentiful. Crude petroleum oil is a mix of many different kinds of hydrocarbons. Gasoline, diesel fuel, jet fuel, lubricating oil, kerosene, grease, plastics, and many other products originate from crude petroleum oil. All gasolines are hydrocarbons. The complete combustion of any hydrocarbon with pure oxygen yields water, carbon dioxide, and energy. Because air contains other gases besides oxygen, many other combustion products are realized. Pollution control equipment on modern cars is designed to neutralize or render harmless otherwise environmentally harmful combustion products.

The chemistry of gasoline is a complex subject. Gasoline formulations include additives to act as varnish removers and cleaners, additives to improve burn-ability, additives to increase octane number and, for cold weather driving, additives to rid the gasoline of water vapor. As a car owner, there are some additives you can add to your fuel tank. They include cleaning solvents and water vapor absorbers (made basically of alcohol).

All major name-brand gasolines are suitable for use in a modern car engine. The only areas in which you need to make a choice are in octane numbers. Scientifically, the octane number of gasoline is the percentage by volume of iso-octane (2, 2, 4 trimethylpentane) in a mixture of iso-octane and normal heptane. Iso-octane is the substance needed to prevent knocking. The more iso-octane a particular gasoline has, the higher the octane number. Normally, the higher the octane number of a particular gasoline, the less it will knock, and the more power, better performance, and better mileage you will get. If a particular fuel performs better than pure iso-octane, its octane number can be greater than 100. Aviation fuels are typically rated at 130 and above.

At the fuel pump you might notice a label stating that octane numbers are arrived at using the (R & M)/2 method. The R stands for the Research Octane Number (RON), the M stands for the Motor Octane Number (MON). The RON is a measure of antiknock performance under mild operating conditions at low to medium engine speeds. The MON is a measure of antiknock performance under severe conditions during power acceleration at high speeds. Averaging these two numbers gives an indication of overall performance of the particular gasoline. Table 8-1 lists current antiknock requirements for gasolines.

Table 8-1. Antiknock Requirements for Gasoline.

Octane Number

(RON + MON)/2


Less than 87




95 - 97.5

For cars with low antiknock needs.

For most 1971 and later cars.

For most 1970 and prior cars designed to operate on regular gasoline.

For some cars designed to run on premium gasolines.

For most 1970 and prior cars with high compression ratio engines, and for later model cars with high compression ratio engines.

Note: The octane number for use in areas where altitude is greater than 2,000 feet may be reduced 0.5 number for each succeeding 500 feet, but is not to exceed a total of three octane numbers.

Check with the owner’s manual, dealer, or manufacturer in regard to the minimum octane number gasoline for use in your car. Ask about using higher octane number gasolines and any associated changes in engine timing or carburetor adjustment. If permitted by the manufacturer, always use a gasoline of the highest octane number. In other words, always use the hi-test gasoline. Your engine will run smoother, you’ll feel more power at the pedal, and your car will get better gas mileage. In addition, many of the hi-test gasolines have additional cleaners incorporated into their chemistry, so in time your car will run cleaner also.

Caution: Never use a leaded gasoline in an engine designed to run on unleaded gasoline or vice versa. Stick with the factory recommended type only. And stay away from gasolines formulated with alcohol; their use may void car warranties.

Other than using the best gasoline available, proper filtering is really the only concern in regard to fuel. Inspect the gasoline filters on the car twice a year. Replace them if they are dirty or clogged. Consider replacement once a year regardless, because the replacement cost is so low.


The ignition is affected by the mechanical timing adjustment. Adjust the timing to specification twice per year. Timing out of adjustment leads to poor performance and wasted fuel. Ignition will be covered more fully in section 9.


Some problems with the air-fuel delivery system include ping, knock, and rough idle. Ping. Ping is actually a light knock that results from post-ignition. Post-ignition occurs when some unburned fuel ignites spontaneously after the spark plug has fired the major portion of fuel. This creates two pressure fronts that collide like a thunderstorm inside the cylinder, creating very high pressures and temperatures. Normally, a slight ping while accelerating or driving up a hill is not cause for concern. However, it does indicate that something is wrong and, if left uncorrected, could cause problems.

Ping can result from using gasoline with too low an octane rating, incorrect ignition timing, overheating engine, and incorrect spark plugs, to name just a few. Correct ping as soon as practical.

Knock. Knock is a more serious form of engine ping. It basically is abnormal combustion, occurring before the spark plug fires or at pre-ignition. It can become so severe that it causes piston rings to break and can even burn holes through piston heads. It is usually caused by hot carbon deposits inside the cylinder or on the piston, high valve temperatures, or incorrect spark plugs. Knock must be corrected immediately. It can lead to diminished engine life and/or complete engine failure.


Don’t overlook bolting when performing a tune-up. Loose bolting can lead to performance- robbing air leaks, unnecessary vibration of engine parts, and in some cases, loss of engine parts simply because they fall off the car.

Before you start your actual tune-up, take a trip around the engine compartment and check for any loose bolting. Tighten engine bolting to specification twice a year. In particular, check for loose carburetor to intake manifold bolting, air cleaner assembly to carburetor bolting, intake and exhaust manifold to engine bolting, head to block bolting, oil cover bolting, battery case bolting, and bolting that secures any and all auxiliary equipment to the engine proper or fender or fire wall.

Do not overtighten any bolting. Tighten only to specification and in the factory- specified sequence, if any. Be especially careful not to overtighten the air cleaner assembly to the carburetor. This bolting can easily be stripped or pulled out of the carburetor body.


Compression and vacuum are the two keys to proper engine breathing. Compression and vacuum adjusted to factory-designed levels allow the engine to develop full power and efficiency. Let’s discuss compression first.

All modern gasoline-powered passenger car engines base their operation on the four- stroke cycle. These strokes, in order of occurrence, are: intake, compression, power, and exhaust.

The intake stroke starts as the piston moves down through the cylinder from the uppermost (top) position in the cylinder. As the piston moves downward, it creates a partial vacuum, which helps to draw the air-fuel mixture into the cylinder through the intake valve.

The compression stroke begins as the piston reverses direction at the end of the intake stroke to start moving upward within the cylinder. With both the intake and exhaust valves closed, the air-fuel mixture has no place to go, so it becomes compressed. As the piston continues its upward movement, it compresses the air-fuel mixture more and more, raising its pressure to a limit as designed at the factory.

The power stroke begins as the piston nears the top of the cylinder at completion of the compression stroke. Just before the piston reaches its uppermost travel in the cylinder, the spark plug fires, igniting the compressed air-fuel mixture. The subsequent explosion expands the burning fuel mixture and drives the piston downward, and because it is attached to the crankshaft, helps turn the engine and move the car.

The exhaust stroke finishes the cycle as the piston again reverses direction to start another upward trip through the cylinder. During this time the exhaust valve opens, allowing the piston to push the burned gases out of the cylinder and into the exhaust system.

The importance of the compression stroke on engine power levels and efficiency is paramount. For the engine to meet its full power capability, the degree of compression measured in pounds per square inch (psi) must meet design specifications. It’s also important that compression readings for all cylinders be compared with the maximum variation between cylinders as specified by the factory. Checking compression is the only way to make sure that the pistons, piston rings, valves, and cylinder head gasket are properly sealing. You cannot perform a proper tune-up until compression problems are solved. Here are a few things to consider.

Let’s establish the specified compression pressure for your imaginary V-8 engine at 130 to 160 psi with a maximum variation between cylinders of 25 psi, for example. The first set of readings shown would indicate that the engine is breathing well.


# 1—140 psi # 5—140 psi

# 2—135 psi # 6—140 psi

# 3—135 psi # 7—145 psi

# 4—140 psi # 8—135 psi

All the cylinders are within both specified factory limits.

The next set of readings indicate a problem with cylinder #3. It is not below specified pressure, but it is below the maximum variation of pressure as compared to cylinder #4. (160 psi - 130 psi = 30 psi).


# 1—160 psi # 5—160 psi

# 2—155 psi # 6—160 psi

# 3—130 psi # 7—155 psi

# 4—160 psi # 8—155 psi

A third example shows that a couple of cylinders, #2 and #6, are below the minimum specified pressure—indicating a worn engine.


# 1—135 psi # 5—130 psi

# 2—120 psi # 6—125 psi

# 3—135 psi # 7—130 psi

# 4—130 psi # 8—130 psi

Low compression pressure in cylinders that are adjacent to one another may indicate piston ring leakage and/or valve leakage. Low-pressure readings in adjacent cylinders can be a sign of cylinder head gasket leakage. Higher-than-specified readings are harmful to the engine, also. They indicate carbon build-up in the cylinders. Any of these problems must be corrected as soon as possible.

As pointed out earlier, changing the engine oil, the oil filter, the air filter, gas filters, and maintaining proper bolting torque on engine parts will help to keep abrasive dirt out of the cylinder area. Dirt can cause cylinder, piston ring, and valve wear that eventually lead to low compression pressure readings. To further guard against cylinder compression pressure problems, consider using any of the quality cylinder head cleaners or top oils available on the market. They remove potentially harmful carbon and varnish deposits. They usually work by either adding them to the gas tank or directly through the carburetor. Follow directions on the can. Perform this preventive maintenance procedure once a year.

The other way the engine breathes is through intake manifold vacuum. Intake manifold vacuum is the decrease in air pressure created by the pistons moving downward within the cylinders. Intake manifold vacuum helps to operate the heater controls, power brakes, distributor advance mechanism, antipollution controls, automatic transmission modulator and, in some vehicles, the windshield wipers. The amount of vacuum depends upon engine speed and load. It is also affected by the condition of the pistons, piston rings, valves, exhaust system, and by proper engine bolt torque. While vacuum readings that are slightly out of factory specification do not need to be cause for alarm, they should be corrected because poor vacuum will affect engine performance. Table 8-2 explains how to interpret vacuum gauge readings.

Table 8-2. Interpreting Vacuum Gauge Readings.



Vacuum pointer drops

Pointer floats up and down

Low vacuum reading

Unsteady readings

Slowing dropping readings

Sticking valves due to gum or varnish deposits, carbon buildup, weak valve springs, bent valve stems, sticking valve lifters.

Incorrect air-fuel mixture.

Leaks, compression problems, late ignition, valve timing out of adjustment.

Leaks, carburetor adjustment needed, incorrect distributor point spacing, valve adjustment needed.

Restricted exhaust system—probably plugged muffler or catalytic converter.


To keep manifold vacuum up to par, repair any of the problems indicated in Table 8-2 as soon as possible. Check for leaks between the intake manifold and engine block, the intake manifold and carburetor mounting, and at hoses and other vacuum lines throughout the engine. Other than fixing leaks, keeping the cylinders, pistons, and valves clean by using the cleaners or top oils discussed under engine compression pressure is the only preventive maintenance you can perform for the metal parts of the vacuum system. For rubber vacuum hoses, consider replacing them every 25,000 miles or two years, or when they develop leaks through cracking.


Car designers need to provide a means to supply the carburetor with a sufficient quantity of gasoline at the correct pressure to meet a variety of driving conditions. In very old cars, this was typically accomplished by mounting the gas tank somewhere above the carburetor, which allowed the gasoline to simply spill into the carburetor. Modern high- performance engines need to have the gasoline properly pressurized, therefore, the older gravity method of fuel supply would not work.

Fuel pumps move the gasoline from the fuel tank under pressure and supply it to the carburetor in precisely the correct amount at the needed time. Fuel pumps are either mechanical and run directly off a cam inside the engine, or are electrical and run from electrical power supplied by the car’s electrical system. One advantage of an electric fuel pump is that it can be located away from the hot engine, thus reducing the chance of vapor lock in the pump. In either case, the fuel pump acts to keep the carburetor fuel bowl filled with gasoline.

Fuel pumps are notoriously long-lasting, usually requiring little service. However, when their performance drops off or they begin to fail, they should be repaired or re placed immediately. For example, a ruptured diaphragm in a mechanical fuel pump will result in a leak between the engine crankcase and intake manifold. This will cause engine oil to be drawn into the cylinders, fouling the carburetor, spark plugs, valves, and cylinders. Antipollution equipment can also be fouled via this failure.

Modern fuel pumps normally cannot be repaired; they must be replaced. There is no service you can perform on most of them. Some pumps, especially on older cars, can be repaired or rebuilt. Rebuilding kits are available for this. If your car has a mechanical fuel pump, consider replacing or rebuilding every 100,000 miles. Electric fuel pumps can be run until they develop problems. Fuel pumps for diesel-powered cars are susceptible to water damage. Use a dry gas formulated to be added to diesel fuel to absorb water in the fuel tank. For best results, add every 12,000 miles.

Test fuel pump operation for specified pressure flow and vacuum every year or 12,000 miles. An avid practitioner of car care will also consider having the gas tank cleaned and flushed every 2 years or 25,000 miles to keep abrasive dust and dirt away from the fuel pump.


There are three types of valves on modern cars you should be aware of: engine valves, exhaust gas recirculation valves, and positive crankcase ventilation valves.

Engine Valves

Every cylinder in the car has two valves. The intake valve allows the air-fuel mixture to flow into the cylinder at the proper time during the intake stroke. The exhaust valve allows burned fuel to exit the cylinder at the proper time during the exhaust stroke.

During the compression and power strokes, the valves must remain tightly sealed. To do this, valves are precision-ground along their edges to seal against the cylinder valve seat, which is also precision-ground. Carbon buildup, or varnish and other foreign particles such as abrasive dirt, will foul or scratch these smoothly ground surfaces and impair valve sealing—and, therefore, engine performance. Valve grinding is necessary from time to time to keep these surfaces in top condition. Have the valves ground every 100,000 miles, assuming no problems develop before then.

Valves need to be kept in perfect alignment to facilitate good sealing as the valve closes on the valve seat. Valve guides provide this alignment. The valve stem moves up and down through the valve guide in a precision fit that does not allow any appreciable side play (Fig. 2). If the valve guides are the replaceable type, install new ones or have them reamed every time the valves are ground.

Fig. 2. Valve assembly.

The valve spring is the other major component of the engine valve assembly. It provides the correct force against which valves open and close. A weak spring will cause improper valve seating, resulting in poor engine performance and possible damage to the valve. A spring that is too strong or distorted could result in excessive camshaft lobe wear. Test the springs every 100,000 miles at valve regrinding time.

Valves are operated by rocker arms, actuated by push rods and lifters on non- overhead cam engines. As the camshaft rotates, it operates the lifter which, in turn, moves the pushrod. The pushrod acts to pivot the rocker arm, pushing the valve down. For proper valve closure, a slight clearance—called valve clearance—must exist between the closed valve and rocker arm. This clearance is necessary to allow for expansion of warm engine valve parts. Adjust this clearance to specification every 12,000 miles. Overhead camshaft engines might or might not require periodic valve clearance adjustment. Check with the shop manual, and if adjustment is required, perform at the same interval as for non-overhead cam engines.

Exhaust Gas Recirculation (EGR) Valve

The EGR valve is a part of the EGR System. The EGR system is an antipollution system. It diverts some of the engine exhaust gases into the cylinder, which acts to lower high combustion temperatures that form nitrogen oxide pollutants. The part of the system that diverts this flow is the EGR valve. Because cold engines produce no nitrogen oxide, the EGR valve operates only in warm and hot engines. The EGR valve is normally operated by engine vacuum and mounted on the intake manifold near the carburetor (Fig. 3).

Fig. 3. EGR valve.

Pinging, rough idle, and excessive production of nitrogen oxide as revealed by an emission test are telltale signs of problems with the EGR valve. Check vacuum hoses periodically for leakage or blockage and proper attachment to the EGR valve. A sticking EGR valve can sometimes free up if carbon deposits are cleaned from the valve stem and port area. Clean the EGR valve twice a year. If cleaning doesn’t free the valve, it must be replaced.

Positive Crankcase Ventilation (PCV) Valve

The job of the PCV valve is to return combustion gases to the carburetor that have leaked or blown past the piston rings and collected in the crankcase. Years ago these gases were merely ventilated to the atmosphere. Now, with strict air pollution control requirements, we can’t get away with simple venting to outside air (Fig. 4).

Fig. 4. PCV valve.

On modem cars, a vacuum hose runs from the rocker arm cover to the base of the carburetor. Engine vacuum draws combustion vapors from the crankcase, into the rocker arm area, and out into the intake manifold, where they are mixed with fresh air-fuel mixture for burning. The PCV valve regulates this flow of vapor.

Over time, the PCV valve will clog from an accumulation of sludge carried with the crankcase vapors. When the PCV clogs it will no longer be able to pull pollutants from the crankcase, and acids, sludge, and engine oil will build up and foul the engine.

Check the PCV system every 5,000 miles. Perform tests according to factory recommendations. Completely clean the system once a year or every 12,000 miles. Make sure all hoses are in good shape. If the PCV valve on your car is the cleanable type, clean it every 12,000 miles with the recommended solvent—otherwise replace it every 12,000 miles.


• Perform a mechanical tune-up twice a year.

• Keep carburetor linkages clean and lubed.

• If your car has fuel injection, have the system checked yearly.

• Inspect air lifters every other month. Replace the dry paper type once a year. Clean the oil bath types twice a year.

• Use the highest octane number gasoline you can get.

• Inspect fuel filters twice a year. Replace yearly.

• Tighten to specification all engine bolting twice a year.

• Use cylinder cleaners or top oil once a year.

• Replace rubber vacuum hoses at 25,000 miles, or every two years or when cracked or hardened.

• Test fuel pump operation every 12,000 miles or yearly. Consider replacing or rebuilding mechanical fuel pumps every 100,000 miles, or sooner if trouble develops.

• Dress valves every 100,000 miles. Install new or ream old valve guides at this time. Also, test valve springs at 100,000 miles.

• Adjust valve clearance on those cars that can be adjusted every 12,000 miles.

• Clean the EGR valve every 6 months.

• Check the PCV system every 5,000 miles. Clean the system every 12,000 miles. Replace PCV valves that can’t be cleaned every 12,000 miles.


• Keep a supply of mechanical tune-up parts available with your other auto supplies for both routine and emergency service.

• Don’t try to service fuel injection systems yourself. They require almost clean room service. Even small amounts of dirt can impair their performance.

• Buy the better grades of air and fuel filters. They are more efficient and protect your engine better.

• Keep gasoline, oil, and solvents off rubber hosing.

• Learn to wipe down the engine and related mechanical equipment every few months. This little task will help to get dirt out of places where it can do harm.

• When servicing the carburetor air filter, keep the throat of the carburetor covered with a clean, lint-free rag, a piece of plastic, or aluminum foil to keep dirt out of the carburetor.

• Vacuum the carburetor air filter holder twice a year.

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