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Troubleshooting Briggs and Stratton Carburetors

TROUBLESHOOTING CARBURETOR SYSTEMS

As much as we would all like to have a short, simple and effective way of troubleshooting engine problems, there is no short cut to accurate and effective troubleshooting. There is no magic bullet, no teaching method that will change the learning curve drastically. Effective and efficient troubleshooting results from a combination of a thorough understanding of theory, logical as well as accurate testing procedures and experience.

The purpose of this session is to provide the technician with the necessary information and procedures to effectively diagnose and repair carburetion system problems correctly and efficiently the first time. Listed below are the three areas this section of your notebook will cover in detail.

Most carburetor system problems fall into three distinct categories:

Starting Problems

Performance Problems

Leakage

There are times when a carburetor may exemplify all three of the problem categories at the same time. This can be confusing. The information, tests and procedures that follow can be used when a singular problem occurs or when multiple problems are evident. Understanding carburetor theory of operation, following the tests and procedures carefully as well as collecting accurate symptom information will provide you with all of the tools needed to effect an accurate and efficient repair.

If you wish to learn more about carburetor theory or troubleshooting, contact your regional Central Sales and Service Distributor or the Customer Education Department at Briggs & Stratton.

In order to isolate the various systems for troubleshooting (ignition, compression and carburetion) this text assumes that the recommended ignition system test (using a #19368 spark tester), and a cylinder leakdown test (using a #19413 leakdown tester) have been performed and the technician has no data indicating that these systems could be defective in any way.

TROUBLESHOOTING CARBURETION SYSTEMS

Starting Problems

CHOKE OPERATION

As simple as it sounds, there are a significant number of starting problems that could be solved by a proper choke adjustment process. The “Golden Rule” of the systematic troubleshooting process is “Do the easiest thing first.”

The customer complaint of “Hard starting” or “Hard to start cold, starts fine warm” will lead the seasoned technician immediately to a complete check of the choking system, cables and linkage. Too often the carburetor is removed, cleaned and re-installed only to find the problem was a cable out of adjustment.

PRIMERS

Fig. 1

Some engine models use a primer bulb system to force fuel up through the main nozzle and into the venturi. This adds additional fuel to start the engine.

Fig. 2

To verify that the primer is indeed working, remove the air cleaner assembly and look directly into the throat of the carburetor. A pulse of fuel should be seen when the primer is depressed. The amount of fuel coming out of the nozzle is directly related to how hard and often the primer bulb is depressed.

FUEL SUPPLY

Fig. 3

A simple check of fuel supply and delivery to the carburetor is accomplished by mechanically clamp- ing off the fuel line in order to stop fuel flow, then removing the line from the inlet fitting of the carburetor. See Figure 2.

With the fuel cap installed, release the clamp, and allow the fuel to run into a container approved for gasoline. The fuel flow should be as wide as the inside diameter of the line.

If the engine is equipped with a fuel pump, remove the spark plug high tension lead(s) (attach the leads to a good engine ground) and perform the same style test. The fuel should be delivered with distinct pulses at cranking speeds.

TROUBLESHOOTING CARBURETION SYSTEMS

Starting Problems (cont.)

Fig. 4

ANTI-AFTERFIRE SOLENOIDS

Begin troubleshooting the solenoid by checking for battery voltage at the spade connector (with the key switch in the “start” as well as the “run” position). The next check is to listen at the bowl of the carburetor for a distinct click when the ignition switch is turned to the on position. If a distinct click is not heard, remove the solenoid for further testing.

To test the solenoid itself, attach one pole of a 9 volt transistor radio battery to the spade terminal and the other pole to the solenoid case or black ground wire if so equipped. A sharp distinct movement of the plunger should be seen.

A poor solenoid ground connection or dirty/stale fuel are common causes of anti-afterfire solenoid problems.

STARTABILITY TESTS

There are two simple ways of confirming that the choke or primer system is delivering fuel to the combustion chamber. As stated previously under PRIMERS, remove the air cleaner and visually inspect the gasoline stream created in the throat of the carburetor when the primer bulb is depressed.

On a choke type carburetor, after verifying the choke plate is fully closed, inspecting the spark plug is the easiest way to confirm that fuel is being delivered. If the spark plug is not wet with gasoline after repeated attempts to start the engine, it indicates that fuel is not being delivered. An extremely wet plug may indicate that the choke could be stuck closed or the engine has been over-choked or over-primed.

Fig. 5

DEBRIS

Any debris lodged in the main jet of a carburetor can cause an engine to appear under-choked. In some cases (even on primer systems) the debris in the jet can completely stop any fuel from flowing into the throat of the carburetor.

The lesson here is that there are multiple factors that can affect startability. The faulty operation of the choke is common but not the singular answer for all “hard to start cold” situations.

Fig. 6

TROUBLESHOOTING CARBURETION SYSTEMS

PERFORMANCE PROBLEMS

Performance issues are the most common carburetion problems seen in the day to day operation of a repair facility. Performance problems come in many varieties. We will cover the most common symptoms and the recommended procedures for successfully troubleshooting the problem.

Engine won’t idle – hunts and surges/runs poorly at idle

Engine hunts and surges/runs poorly at top no-load speed

Engine hunts and surges/runs poorly under load

In many of these cases, the common problem is a lack of fuel or “lean” condition as opposed to an overly rich condition. Most hunting, surging and performance problems can be directly attributed to an obstruction of fuel flow.

In excess of 95% of all (carburetor) performance problems are caused by dirt or debris in the system.

Most performance problems are described as “hunting and surging” in various modes of operation. Hunting and surging at idle, top no-load or under load make up the majority of customer complaints.

A challenge in diagnosing performance problems in a carburetion system is deciding whether you have a governor problem or a carburetor problem. With this in mind, the first test for a technician is to differentiate between the governor and carburetor systems.

SEPARATING THE GOVERNOR SYSTEM FROM CARBURETION SYSTEM

Separating a possible governor problem from a carburetor problem can be different for each complaint or combination of complaints. This is where the use of carburetor theory becomes very important. To begin the troubleshooting process, make sure the governor system, linkage and spring(s) are in the proper positions and orientations without interference from sticks, grass or other debris. Some engines have multiple positions for the governor spring. Any deviation from the correct position will cause symptoms that are difficult to accurately diagnose and correct.

Once the governor system is inspected for obvious problems, the next step is to check the static governor adjustment. The procedure is in the repair manual for the engine series you are working on.

TROUBLESHOOTING CARBURETION SYSTEMS

Fig. 7

HUNTS and SURGES AT IDLE (ONLY)

An engine that hunts and surges at true idle (when the linkage is against the idle speed adjustment screw and after idle mixture adjustment) can only be caused by a fuel delivery problem or an air leak.

Because the throttle plate is held stationary in the carburetor bore, a rhythmical hunting & surging problem must be caused by an improper air/fuel mixture related to an air leak or an obstruction in the idle circuit.

The reasoning behind this statement is that at true idle, the governor has no effect on the engine at all. The governor spring produces no force at true idle.

ENGINE HUNTS and SURGES AT GOVERNED IDLE

Fig. 8

Perform an idle mixture adjustment (if so equipped).

To separate the governor system from the carbure- tor at governed idle speed, hold the throttle plate linkage against the idle speed adjustment screw and increase the idle speed to the governed idle specification. If the engine runs well without any hunting or surging, return the engine to the correct true idle speed.

Next, bring the engine up to the proper governed idle speed using the governed idle spring. If the hunting and surging returns, chances are good that you have a governed idle spring or linkage problem.

Fig. 9

ENGINE HUNTS & SURGES AT TOP NO-LOAD SPEED (ONLY)

To determine if this symptom is caused by a carburetor or governor problem, follow the same procedure described for hunting and surging at governed idle speed.

Once the idle mixture adjustment is performed (if so equipped) and the engine idles smoothly, increase the engine speed using the idle speed screw. You must hold the throttle plate linkage against the idle speed adjustment screw until the engine reaches its top no-load speed.

Keep in mind that without any appreciable load, the circuit providing the fuel is the idle/transitional circuit.

TROUBLESHOOTING CARBURETION SYSTEMS

ENGINE HUNTS and SURGES AT TOP NO-LOAD SPEED (ONLY) CONT.

If the engine continues to hunt and surge, chances are good that the carburetor is causing your problem. The reasoning behind this is that with the idle speed screw controlling the throttle plate position (therefore the engine speed) the governor has no real effect on the performance of the engine.

If the engine runs well while controlled by the idle speed screw but continues to hunt and surge while being controlled by the governor spring, the governor system would then be suspect. Check the governor linkage for resistance or binding then replace both the governed idle and the main governor springs.

ENGINE HUNTS and SURGES UNDER LOAD (ONLY)

The chance of a governor system causing a hunting and surging condition under load is very remote. The reason for this is that when the carburetor delivers fuel under load, it is feeding fuel almost exclusively through the main nozzle. Most loads are constant enough to maintain the rpm of the engine. Once the engine is doing work, the governor system has little additional effect on the performance of the engine.

Fig. 10

HARMONIC HUNTING AND SURGING

It is possible that at some speed settings, the governor spring could begin to vibrate excessively. This can result in very unstable governing. To test for this rather uncommon but possible problem, run the engine at the speed that it hunts and surges the greatest. Observe the motion of the spring. If the spring seems to be oscillating or vibrating excessively, take a pencil and place the eraser end lightly against the vibrating spring. This should dampen the vibration. If the hunting and surging disappears, replace the governor spring with the exact part number dictated by the model, type and code numbers of the engine.

TROUBLESHOOTING CARBURETION SYSTEMS

COMBINATION SYMPTOMS

There are times when an engine will exhibit multiple symptoms. This is where your troubleshooting skills will be tested time and time again. It is important to remember basic carburetor theory while performing your tests.

Multiple symptoms tend to follow a pattern in which common combinations are found.

ENGINE HUNTS and SURGES AT IDLE and TOP NO-LOAD; RUNS WELL UNDER LOAD

An engine that exhibits these symptoms and runs well under load is a classic example of where understanding carburetor theory will help in the diagnosis.

The circuit that is feeding the fuel to the engine during idle and top no-load is essentially the same, the low speed circuit. If the engine runs well and produces acceptable power under load, then the main jet, main fuel supply, and main air bleed can be presumed good. The problem must reside in the idle/transitional circuit.

Carefully check the pilot jet (if equipped) or the transitional holes for debris.

Fig. 11

ENGINE RUNS POORLY UNDER LOAD

Fig. 12

Other than compression component problems (which we have tested and eliminated) an “under load” performance problem is almost exclusively caused by debris in the main jet/nozzle or air bleeds.

To assist in determining which of these components may be at fault, examine the exhaust system while the engine is under load. If black smoke is evident when the engine becomes symptomatic, chances are there is an excess of fuel. That would indicate that there is an incorrect float level setting, a clogged main air bleed or debris is lodged between the needle and seat.

If the exhaust exhibits no smoke, and has no black residue on the muffler deflector, chances are that it is the main air bleed or main jet that is obstructed.

TROUBLESHOOTING CARBURETION SYSTEMS

PERFORMANCE PROBLEMS – SPECIAL HELP

Fig. 13

CLOGGED/DAMAGED MUFFLERS

A clogged/damaged muffler or spark arrestor can cause poor performance as well as an apparent carburetor leak. The classic symptoms of a muffler problem are:

□Lack of power, difficulty in achieving rpm

□Distinct change in exhaust sound level or quality

□Gasoline soaked air filter

□Noticeable large plume of gasoline vapor around the carburetor (with the engine running)

To check the muffler for problems, replace the muffler and re-test. (Don’t forget that a loose exhaust valve seat can closely mimic a clogged muffler).

USING THE CHOKE AS A DIAGNOSTIC TOOL

Using the choke as a diagnostic tool is one of the quickest and easiest methods of determining whether you have a lean condition causing your performance problem.

After making all possible carburetor adjustments, try SLOWLY closing the choke plate while the engine is performing poorly. If performance improves, the engine is experiencing a lean condition. If performance gets worse, you may have a rich running condition.

The primer can also be used as a diagnostic tool but with slightly less accuracy as it is more difficult to control the amount of fuel each pulse supplies.

Fig. 14

USING A “KNOWN GOOD” TEST CARBURETOR

Another common practice (that is practical in some instances) is to attempt to confirm a carburetor problem by installing a known good carburetor on the engine.

Taking into consideration shop labor rates, the time to remove and replace the component as well as the cost of the carburetor may make the practice of troubleshooting smaller carburetors impractical.

TROUBLESHOOTING CARBURETION SYSTEMS

LEAKAGE

One of the most annoying problems in the field is that of carburetor leakage. Many years of research have gone into effective remedies for this problem and true factory defects are very uncommon. It has been determined through testing, that a large portion of the carburetors returned to Briggs & Stratton for warranty consideration have been found to be structurally acceptable. Most carburetors that are returned from the field have been contaminated with dirt, debris or water.

The overwhelming majority of carburetor leaking problems are caused by DIRT OR DEBRIS.

Fig. 16

Carburetor leakage problems typically fall into three categories:

Leakage while not running (known as static leakage)

  • Leakage while running
  • Leakage during/after coast down

When attempting to troubleshoot carburetor leakage problems, it is especially important that you not automatically assume a component is good or overlook any possible cause. Although an overwhelming majority of the leakage is caused by dirt or debris, there is a small percentage of carburetors whose component parts may cause a symptomatic leak. Because of the relatively high return rate for leakage repairs reported from the field, extra care in proving the integrity of the carburetor is very important to increase the rate of first time, successful repairs.

TROUBLESHOOTING CARBURETION SYSTEMS

FLOAT STYLE CARBURETOR LEAKAGE WHILE STATIC

To help technicians more accurately diagnose the true cause of a leaking carburetor, the Customer Education Department has put together some procedures and specifications for all of our float style carburetors.

Briggs & Stratton Carburetor Specifications/Procedures (All Types)

Needle/Seat minimum sealing pressure: 2 psi (14kPa) (wet with fuel) for minimum of 5 minutes Static test 2 qt. (1.9-2L) fuel tank: 12”-18” (30-45cm) above the inlet fitting

The tools suggested by Briggs & Stratton to perform the following troubleshooting tests are:

Walbro 4 cycle “thumb pump” part # 57-21, McCulloch carburetor pump part # 215432 or a similar equivalent.

Before testing a float style carburetor with air pressure, be sure that the float assembly contains no liquid. Any retained liquid causes increased weight and reduced buoyancy.

LEAKAGE WHILE STATIC

Fig. 17

To tell where a static leak is occurring, remove the carburetor from the engine and remove the float bowl. Invert the carburetor with float and needle installed. Wet the needle and seat area with a small amount of gasoline. Use the “thumb type” pump (with clear non-rubber hose) to pressurize the inlet needle to a minimum of 8 psi (55 kPa). The inlet needle should pop off and re-seat at 2 psi (14 kPa) or greater. If the needle does not seal, clean or replace and test again. If the needle seals and holds pressure at 2 psi (14 kPa) or greater for at least 5 minutes, consider it acceptable. If the carburetor still will not hold 2 psi (14 kPa) for 5 minutes or continues to exhibit a leak when the engine is in service, additional testing is indicated.

POROSITY/EXTERNAL LEAK TEST

To test a carburetor for porosity or external leakage, invert the carburetor and submerge in a water bath. Hold the float with a finger to assure a seal between the needle and the seat. Pressurize the inlet needle to at least 8 psi (55 kPa). If there is porosity in the carburetor, bubbles will be present, erupting from the casting between the brass seat insert and the metal casting or from the fuel inlet fitting.

Caution: Excess finger pressure will cause damage to the needle and/or seat surface. The viton portion of the needle and seat combination should be replaced after performing this test.

TROUBLESHOOTING CARBURETION SYSTEMS

FLOAT STYLE CARBURETOR LEAKAGE WHILE STATIC

Fig. 19

TEST RESULTS

If the pressure test reveals no bubbles from any component parts, check the float carefully by immersing it in warm water and watching for bubbles erupting from a seam. If all components test good, it is safe to assume that the original leakage problem was cased by dirt or debris.

To effect a repair that will retain a high level of quality, it is important to follow the next steps without deviation.

SUCCESSFUL FIRST TIME REPAIR TIPS

To maintain a high level of success repairing leaking carburetors caused by dirt or debris, the first place to start is the fuel supply. The fuel tank should be drained completely and inspected for debris, dirt or scaling (if the tank material is metal). Clean or replace as indicated.

After confirming the integrity of the tank, remove all fuel lines and filters and replace with new. It is highly recommended that fuel filter part #493629 be used in all but a few rare instances.

The only instance where the #493629 filter should not be used is when the fuel supply is gravity feed and the bottom of the fuel tank is less than 1”

(2.5 cm) above the inlet fitting of the carburetor.

GO THE DISTANCE

Fig. 20

Fig. 21

It is important to go the distance and remove every “removable” internal part and check for dirt or debris. Many well intentioned repairs have returned to the shop due to dirt hiding in some small, difficult to expose area of the carburetor.

If the inlet seat in the carburetor body is removable, do so, especially in the case of a chronic or repeated leak. Check in the repair manual for the proper procedures. Dirt and debris can hide behind the seat and, if disturbed, may cause leakage at any future time. If any dirt or debris is found, all (removable/replaceable) jets, air bleeds and pilot jets should be carefully checked, cleaned or replaced.

INLET SEAT CAVITY

TROUBLESHOOTING CARBURETION SYSTEMS

FLOAT STYLE CARBURETOR LEAKAGE WHILE STATIC

An engine that exhibits leakage after it sits for a period of time can be caused by a number of problems. The most common of these is dirt. Any imperfection on a needle or seat surface, such as a small particle of dirt, can cause an engine to perform well in all other scenarios but still leak after sitting a period of time. In addition to dirt, an improperly vented fuel cap can allow pressure to build in the tank which could overcome the float assembly and result in a sporadic leak.

All fuel filters are rated by the size of the particle they will allow to pass. If the filter size is too large, debris capable of plugging the jets will enter. If the filter size is too small, the fuel flow will be reduced and may cause an engine to run lean.

When an engine is not in use, the fuel in the bowl is exposed to the atmosphere via the bowl vent. Fine particles of dirt or debris that normally would be suspended in the fuel and pass completely through the carburetor, settle out and end up at the lowest point between the fuel inlet fitting and the inlet needle. As the fuel slowly evaporates, the float responds by lifting the inlet needle very slightly off its seat. The dirt present moves in and lodges itself between the needle and seat. Now a slow but steady flow continues into the bowl, eventually showing up as a static leak.

STATIC LEAK TEST

One method used successfully in the field to static leak test a carburetor is done with a simple 2 qt. (1.9-2L) free standing tank. Fill the tank with gasoline and place it between 12” and 18” (30-45 cm) above the test bench measured from the bottom of the fuel tank. Attach the fuel line to the carburetor to be tested and open the in-line fuel shut off. See Figure 22. Place a piece of paper under the carburetor and let it stand in a large catch basin or drain pan. The carburetor should not exhibit any leakage if left to stand overnight.

Fig. 22

DON’T CAUSE YOUR OWN LEAK

One last service tip may help relieve some problems seen in the field. When a fuel filter is removed, a small amount of the inner rubber hose is rolled up as the barbed end of the filter is pulled out. This small amount of rubber can find its way into the carburetor and cause obstructions as well as leaks. The debris is very small and difficult to see with the naked eye. Whenever you remove a fuel filter, replace the fuel line. Especially if the carburetor has exhibited a chronic leak.

Never use a bolt or threaded object to plug a fuel line during repair. The threads will cut the rubber hose

and aggravate the debris problem. Fig. 23

TROUBLESHOOTING CARBURETION SYSTEMS

FLOAT STYLE CARBURETOR LEAKAGE WHILE RUNNING

There are some situations that could cause a symptom of leakage while operating. These are rare and can be challenging to troubleshoot accurately. Most cases of leakage during operation can be attributed to the same problems that cause static leakage. There are however, additional possibilities when it comes to this scenario. It should be noted that there are two types of leakage while running.

The first type of leakage is external. This could be caused by deteriorated tank and strap assemblies and loose, torn or missing external gaskets (adjustment nut packings or bowl nut washer etc.). The second type of leakage is internal. A symptom of this would be an engine that displays a leak through the brass “drain” insert in the throat of the carburetor or “wets” the air cleaner element with fuel. This type of leakage can be caused by any number of problems:

  • Loose or damaged main nozzle (2 piece flo-jet)
  • Clogged air filter element (externally vented carburetors only)
  • Excessive vibration
  • Incorrect fuel inlet seat size
  • Improper float level
  • Clogged muffler
  • Porosity in the carburetor casting
  • Improper valve clearance
  • Intake valve seat loose

TROUBLESHOOTING CARBURETION SYSTEMS

SPECIFIC CARBURETOR TROUBLESHOOTING – REPAIR TIPS

Fig. 24

TWO PIECE FLO-JET LEAKAGE

The two piece flo-jet carburetor has a unique design that can cause some misunderstandings in the field. This carburetor has a main nozzle/jet that bisects the carburetor body. Near the base or large end of the main nozzle/jet tube is a chamfered area that provides a seal between the emulsion cavity and the fuel bowl of the carburetor. If there is any defect, dirt or corrosion on the surface of the sealing area (either the brass tube or carburetor casting) the carburetor could exhibit a leak. The symptoms include: unexplained and inconsistent leakage while at rest, leakage during long coast down periods, poor overall performance.

BENCH TEST

You can quickly and easily bench test this condition. Install the nozzle/jet assembly into the lower carburetor body. Thread a 1/4” bolt into the base at the speed control bracket boss to use as a “handle”. Grip the bolt head in a vise and position the body so that it is level. See Figure 25.

Add fuel to the bowl until it is three quarters full. If the tapered sealing area is intact, there will be no fuel leakage into the carburetor throat.

Fig. 25

TWO PIECE FLO-JET LEAKAGE (cont.)

Fig. 26

If the sealing area integrity is in doubt, take the old nozzle and remove the threads with a bench grinder. Place a small amount of lapping compound on the nozzle at the taper and insert it into the body of the carburetor. With a jet screwdriver, gently lap the nozzle just as you would when you lap a valve in. Remove the old nozzle and clean the carburetor body completely.

Using a new nozzle/jet, install the Teflon washer from a # 391413 carb repair kit over the tube and seat it on the sealing surface of the new nozzle and install. The Teflon washer acts like a gasket providing a better sealing surface for both the nozzle/jet and the body of the carburetor.

ADDITIONAL SERVICE TIPS

  • The proper and thorough cleaning of a float style carburetor includes the removal of all parts including the welch plugs (on larger carburetors).
  • The use of commercial carburetor cleaners is limited to a total soaking time of 30 minutes.
  • Always use good quality carburetor spray solvents with long nozzles. These help immensely in reaching internal orifices and passageways.
  • Always wear eye protection when cleaning a carburetor. The spray from the aerosol can easily be deflected and get into your eyes.
  • The removal, cleaning and inspection of the inlet seat area is critical for optimum performance especially in chronic leakage cases.
  • Always install new fuel lines and fuel filter (#493629 recommended) when repairing a unit with a carburetion system leak.
  • Always instruct the customer to use the fuel shut off valve when transporting the equipment. If the unit does not have a shut off valve, install one.
  • Always clean your hands and work surface before starting any carburetor repair.
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