QuestionI have a B&S 17.5 HP riding mower: model 31C707 type: 0303E1
It had been running fine, but yesterday, it sputtered just before I turned it off and then BANG!!! (as loud as a pistol).
I changed oil and fuel but think some dust/dirt went down into the carb. while I was changing air filter.
How do I properly remove carb and clean?
THANKS!
AnswerTravis
The first thing to do when rebuilding a carb is to shut the fuel off then remove the air filters. I would suggest that you take pictures with a digital camera. The reason to take pictures is so you can put it back the same way it was.Now if you have a photo graphic memory don't worry.Make sure you lay out all of the old pieces and when yo buy a rebuild match up what you took off the carb,maintenance kits can be used for other carb's so you may not use all the pieces.Most carburetors on Craftsman mowers are variations on a common float design. Newer mowers tend to have no adjustments and no choke - which greatly simplifies cleaning and adjustment. With respect to adjustment, there is none - it either works or it doesn't. If it doesn't, your cleaning was not thorough enough, some parts need replacement, or the problem is not in the carburetor. Reassembling the linkages EXACTLY as your found them is critical to the happiness of your engine. * Under the bowl is a large hex head bolt. On the non-adjustable carburetor, this is closed on the bottom. It is also not a simple bolt but includes the precision main fuel metering hole which will need to be cleaned thoroughly. Use a proper socket to unscrew this bolt (counterclockwise). Drain any residual gas from the bowl. CAUTION: I've heard of people breaking the bolt by either attempting to loosen it the wrong way or overtightening upon reassembly to stop leaks.
* There is a fiber washer under the bolt. There may also be a fiber washer on this nut inside the float bowl. Don't lose these or get them mixed up.
Turn the carburetor upside-down.
* Carefully remove the bowl and O-ring. Inspect these for damage. (Note: there is probably a dimple in the bottom of the bowl in the lower level side. This is normal and probably there to keep the float off of the bottom where gunk and varnish collect because you didn't drain the gas.)
* The float will now be visible. Rotate it to the fully up position. The inlet needle will come up with the hinged part of the float. It is held in place by a wire clip but will now be free. The inlet needle is actually a four sided metal rod with a polished conical tip. Remove the needle and clip.
* Use a pair of needle nose pliers to pull out the hinge pin which will free the float. Careful - the float is made of relatively thin brass and is susceptible to damage.
Check the throttle plate for free movement - there should be absolutely no hint of binding or tightness. If there is, then this will need to be disassembled as well and cleaned:
* Use an open-end wrench to loosen the intake pipe and then remove the nuts and bolts. The manifold will probably come free with the gasket intact. Don't lose the metal strip to which the governor spring attaches. Check for dirt and other debris and set aside.
* Use a 1/8" straight blade screwdriver to remove the screw in the center of the throttle plate. Note the position of the hole in the plate and the orientation of the plate. (The hole should be toward the engine side with the carburetor body upright. Mark it before removal if in doubt.)
* Tap the carburetor if needed to remove the throttle plate.
* Pull the throttle shaft out of the carburetor body. Take care not to lose the spring with the felt and/or metal washer. Note their positions.
Cleaning the carburetor parts
Use carburetor cleaner and lint free cloths or paper towels to remove all built up brown or green gunk, varnish, and other contamination from the metallic parts. Pay particular attention to the machined passages and metering holes.
WARNING: Carburetor cleaner is both flammable and the fumes are toxic. Do all cleaning away from open flames and outdoors if possible. Wear eye protection. The stuff will also eat plastics including some plastic eyeglass lenses.
Take care where non-metallic parts are still in place as extended contact with harsh solvents may degrade their properties (inlet seat and primer bulb, if present). Low pressure compressed air may be used to blow out passages but only use this on the fuel line from inside the carburetor body - else you may end up with the inlet seat clear across the driveway never to be found again.
DO NOT use wires or metal instruments to clear any of the passages as their size is critical.
The small hole in the hollow bolt on the bottom is most critical. Make sure it is cleaned down to the shiny brass and that this hole is unblocked and fully open:If you are absolutely sure there is no hole in the bolt (some models may forgo this), check further up on the central tube - there may be a tiny hole about 1/2" from the bottom. There has to be a hole somewhere for the gas to be sucked up through the carb!
I first use carburetor cleaner inside and out with cotton swabs to remove all traces of gunk from the inside. Use as many as needed till no more discoloration shows up. Then, use the broken end of a wooden toothpick or popsicle stick to clear the .5 mm diameter hole in the side. In severe cases, this hole may not even be immediately visible due to the varnish and gunk buildup.
If this hole is narrowed or clogged, the engine may start but then die in a few seconds. Gas enters the reservoir in the nut slowly or is forced in by priming but the normal suction cannot replenish it quickly enough.
Fine steel wool may be used on the float hinge pin if it is rough or there is evidence of rust but do not use anything abrasive on any of the other parts. Persistence with carburetor cleaner and cloths or paper towels should prove sufficient.
Inspect the inlet needle and seat. The needle should have sharp uniform edges and no visible damage to the conical tip. Any damage half way down the conical part - where it actually contacts the seat - will result in leakage and flooding. The seat can be removed if damaged by pulling it out with a hooked wire - careful - you do not want to scratch the body! If removed, do not reuse but install a replacement. The new seat goes in groove side first (lubricate with a drop of oil) and can be pressed home with a blunt rod.
If the throttle plate was disassembled, clean these parts with carburetor cleaner. Use a cotton swab to get into the bearing surfaces in the carburetor body.
DO NOT attempt to disassemble the carburetor beyond this point - the pressed in main fuel nozzle is precisely fitted and is not removable. The welch plug (pressed in disk) should not be removed unless you suspect contamination in the primer chamber (if any).
Carburetor rebuild kits are available and are economical where almost any parts need replacement.
If soaking the carburetor in cleaner:
When you remove each part from the dip tank rinse it with warm/hot water (as hot as your hands can reasonably stand). The parts that have passages - force water through the passage. This does two things: (1) rinses the internal passage of the cleaner and any old junk (2) gives you a VISUAL check that water is coming out the other end of the passage. The visual check is the key here - you could use compressed air to rinse the passages but you don't see the exit stream. On a clean passage the exit stream will be nice and solid indicating no particles hanging up in the passage.
Now as for the hot water - this is to help dry the parts off - evaporation. Sometimes once I've rinsed the parts off I'll use compressed air to further dry the passages - or at least manually blowing through them.
Critical considerations for proper operation
* The inlet needle and seat must be in good condition or else the carburetor will flood due to leakage or result in erratic operation due to uneven gas flow. If there is any evidence of damage, these parts will need replacement. The 4 edges of the needle should not be worn (the sharpness would change about 2/3 of the way from the pointed end). If the edges are noticeable rounded, replace the needle. There may be varnish deposits on the needle, rubber seat, and the metal casing in which it is installed. These must be totally removed using carburetor cleaner and soft (wooden or plastic) tools.
* The float height adjustment should be fairly accurate. With the float and inlet needle reinstalled (and the seat replaced if it was removed), invert the carburetor - the float should sit just about horizontal. For more precision, a .210" (#4) drill bit should just fit between the body and the non-hinge end of the float.
* The machined passages must be free and clear and not damaged - never use wires to clean them. Use compressed air, carburetor cleaner, wooden sticks, etc. However, do make sure that they are fully open. There are no blind passages in these carburetors so a strong light should permit you to see that they are unblocked (the following are typical - your model may differ slightly):
Air bleed, inlet side angled down toward main jet.
Passage to primer chamber, inlet side.
Slot towards center at edge of welch plug (may not be present).
Pair of main fuel passages in central cylinder in main body.
Main metering hole in bowl bolt.
The hole in the bowl bolt is the main metering orifice and it is critical to the proper operation of the carburetor. This area also tends to collect a lot of crud. It will yield to repeated use of carburetor cleaner, cotton swabs (Q-tips), and wooden sticks. Continue cleaning until you are down to shiny brass. Just don't become impatient and use any wires or sharp tools to speed the process!
Any primer should be air-tight for it to function properly and for the engine to run properly. Any leaks will result the primer being partially or totally ineffective. In addition, the engine will run rich and contaminants may enter the carburetor. Check for damaged rubber parts or hoses that have fallen off.
If the primer bulb is on the carburetor, there is a 'welch plug' (a metal disk pressed into a mating cavity) sealing the primer chamber. On the side toward the center, there is a tiny rectangular hole that must be open - it often gets clogged and may not even be readily apparent. Do not attempt to remove the welch plug unless you seriously suspect something is inside. If pressing the primer bulb results in a blast of air out of the hole, it's probably fine. Where the primer is separate from the carburetor, there is usually no welch plug.
The float must be air (and gas) tight. Shake it - if there is any gas inside, the float will need replacement. (It's possible there may be metal particles or other debris sealed inside at the time of manufacture - this will cause no harm.) Put the float under water - there should be absolutely no evidence of bubbles and leakage. Pinholes sometimes develop in the thin brass and while these can be soldered, this practice is not recommended.
The large O-ring must seal properly. If it leaks, the engine will run rich and contaminants may enter the carburetor bowl. Replacement is usually recommended whenever the carburetor is disassembled. However, if it is in perfect condition, you can try to reuse making sure that the mating surfaces are clean and smooth. Use some engine oil on the O-ring to assure a tight seal.
Note: When tightening the float bowel, take care that the O-ring doesn't get pinched or kinked resulting in an inadequate seal or no seal. A narrow stick may be useful to help it along. If the carburetor hasn't been removed from the engine, a mirror will be needed to inspect it.
Reassembly
Once all parts have been cleaned and inspected - replaced where needed, proceed as follows:If you removed the throttle assembly:
Reinstall the throttle shaft along with its spring and felt and/or metal washer. Hook the spring onto the ridge on the carburetor body. Make sure it moves freely. DO NOT lubricate.
Attach the throttle plate to the shaft with the original screw. Make sure the plate is correct side out and that the hole is positioned on the right facing the upright carburetor from the throttle plate side. As you tighten the screw, slightly rotate the throttle shaft to allow the plate to seat properly - jiggle it a bit at the same time. When properly installed, the plate itself limits the return movement of the throttle. It should be fully closed at this point.
Confirm that the throttle plate moves freely between a fully closed and fully open position - there should be no hint of binding or stiffness.
* Reattach the air inlet pipe with gasket using the two sets of nuts and bolts. Don't forget the metal strip for the governor spring if your carburetor uses this. Tighten securely - 4 to 6 ft-lbs if you use a torque wrench.
Now for the main event:
* Install a new seat if you removed the old one. The new seat goes in groove side first (lubricate with a drop of oil) and can be pressed home with a blunt rod.
* Install the float using the hinge pin.
* Insert the inlet needle hooking the retaining clip on the tab near the float hinge. Check for free movement of the float.
* With the carburetor body inverted, check the float height adjustment. It should seat almost horizontally. For a more precise test, use a 0.210" (#4) drill bit as a gauge across the outer ring of the carburetor body - the float should just touch this. Bend the tab on the float to adjust. (Note: unless you replaced some parts, this setting will probably be fine.)
You can test for proper operation using low pressure compressed air (i.e., by blowing into the fuel hose), or water or gas. Water is safest but you must make sure to dry everything thoroughly before final assembly. To do this, temporarily reassemble the bowl with the hex head bolt. With the carburetor upright, dribble water into the fuel hose until it accepts no more - perhaps an ounce or two. There should be no leakage - the level of water in the hose should not change at all once it stops. If there is any leakage, there is still a problem with the inlet needle or seat - or the float is gas-logged.
* Install the large O-ring around the carburetor body. Use a small amount of engine oil to aid in assuring a good seal.
* Place the bowl over this assembly making sure that it does not pinch the O-ring. Orient it so that the deep part is almost opposite the float hinge (it should actually point directly away from the engine when the carburetor is mounted.)
* Install the hex head bowl bolt and fiber washer. Tighten securely (one Tecumseh service manual lists 50 inch-pounds, but others have no recommended torque for this bolt). CAUTION: I've heard of people breaking the bolt by either attempting to loosen it the wrong way or overtightening upon reassembly to stop leaks. If gas leaks out in the area of the bolt head, the fiber washer may be missing or damaged. For testing at least, a non-hardening gasoline resistant sealer like Form-A-Gasket B(tm) can be used.
Carburetor installation
With the carburetor positioned in its approximate location on the engine:
* Reinstall the throttle and governor linkages Where there is no speed adjustment or idle position, the direct governor linkage goes in the hole closest to the engine and the spring hooks onto a fixed vertical metal strip with only one hole at one end and the lower hole in the governor lever at the other. Thus, in operation, the spring attempts to keep the throttle open and the governor pulls on the throttle to close it. Increased spring tension results in higher speed. Don't get these backwards when you go to reinstall the carburetor on the engine!!! See the section: Throttle/speed control linkages on Craftsman/Tecumseh engines.
* Reattach the primer tube, if any.
* Reattach the stop switch wire, if any.
* Install the carburetor onto the engine with a new gasket if needed. Tighten securely (6 to 8 ft-lbs).
* Double-check that the throttle linkage and governor spring are in the proper holes and nothing is binding - you should be able to move the throttle back and forth without any sticking or tightness. It should return to the full counterclockwise position instantly as a result of the governor spring tension.
* Do not replace the air filter at this time.
* Reinstall any throttle selector or cable. Confirm that it operates properly - usually STOP, LOW, HIGH. STOP should engage the stop switch. LOW should leave the governor spring tension where it was. HIGH should increase the governor spring tension slightly. If there is an IDLE position, the throttle plate should be almost closed.
* Reinstall any trim pieces that were removed.
* Reinstall the fuel tank and fuel hose with clamp, if any. Open the fuel valve, if any.
Throttle/speed control linkages on Craftsman/Tecumseh engines
The following is for one model! NOT ALL ENGINES ARE SET UP THE SAME! It is best to consult your engine manual. Getting it wrong is not something you really want to do! :(
The user speed control (if any) pulls on a spring which is attached to the 2nd hole from the top on the governor lever. For engines with no speed control, there is a fixed plate or tang to which the spring is attached. Some amount of adjustment is possible by bending this plate.
The carburetor throttle plate has several holes in it. The one that is probably used is that closest to the little cutout and the tip of the throttle plate return spring. You can probably confirm this by looking for which hole has the paint worn off! Initial tests
Add a small amount of gas to the fuel tank - perhaps half a glass or so. Just enough to assure that it will reach the carburetor even if the mower is slightly tilted or jostled.
Inspect around the fuel hose and carburetor body for fuel leaks. If gas starts dripping from the air inlet or anywhere else, there is still a problem with the inlet needle and seat. Disassembly will be required.
Only a few seconds are needed for the gas to fill the carburetor bowl.
Assuming there are no leaks, install the air filter and reattach the spark plug wire or reinstall the spark plug. Attempt the normal starting procedure - prime if recommended.
The engine should start on the first pull! Immediately move the throttle selector to LOW if you have this option. Confirm immediately that it stabilizes at a reasonable speed - stop it quickly if it sounds like the mower is preparing for takeoff - your governor connections are incorrect or binding. If it runs at a fast speed with the speed selector set at LOW, the governor spring is probably in the wrong hole. Check it.
Listen and feel for any significant unevenness, surging, or other unusual behavior. Stop the mower, wait a few seconds, and restart. It should restart with a single pull without priming.
Mow for a few minutes. Stop the engine and confirm that it restarts without priming. Listen and feel for any indication of lack of power or other unusual behavior.
Go take a dinner break. Then confirm that the engine will now start - priming may be needed since it will now be cold.
Good Luck
