QuestionHi. Long story short, I have a leaf blower that won't start. It's a Craftsman, Model # 358. 797931, Serial Number 96330N300281.
The fuel that was in it had been there for a while. I dumped out the old fuel and replaced it with fresh fuel, but no different.
I have not yet replaced the spark plug, and that is what I will do next.
I read some stuff in some other places about cleaning the carburetor. People also have some very helpful comments about checking for fire this and that, but it's very high level stuff.
Trouble is, I have no idea what they're talking about. I tried to take the thing apart tonight, but I couldn't get any further than getting the air filter cover off (at least I can report that the air filter is clean!). Honestly, I've never even replaced a spark plug before, so that'll be somewhat of an adventure, too.
Don't get me wrong, I'm not an idiot. I'm mechanically minded and have no doubt that--with a little guidance--I'll be fine figuring this thing out. I just have no experience with gasoline engines at all.
But I want to learn! This blower was given to me and if I can't make it work, I'm going to throw it away. In other words, I'm not going to pay anybody to service it. But it seems like the perfect piece of equipment for me to learn a little bit about small engines on.
What I was hoping you could do was point me in the direction of a service manual. Or a video of somebody working on this unit (or something similar). Or--if nothing else--more specific instructions on what I should be doing.
Thanks in advance. The internet is an awesome place because people like you are willing to help people like me learn new things!
AnswerMatt
I will try to give you some insight about the workings of the engine.
Ok Number 1 it may sound stupid but this is what we use to do troubleshooting of engines.
Check the Fuel tank to see if it is empty or shutoff valve is closed, or fuel line or fuel tank cap vent is clogged.
There is water in the fuel.
Carburetor is overchoked.
Carburetor is improperly adjusted or needs service.
Ignition system or its wiring is defective or ignition switch is off, or timing way off (e.g., broken flywheel key).
Deadman or other cutoff switch is open or defective.
Spark plug is fouled, improperly gapped, or damaged.
Engine compression is poor.
This is probably the most common problem you are likely to encounter. The cause is very often the same - lack of maintenance.
Note that the assumption here is that it cranks - the crankshaft and blade rotates in a normal manner but the engine never catches. Some larger (Briggs and Stratton) engines may have a low-oil cutoff switch which will stop the engine if the oil level is inadequate. However, this is not likely on a push mower.
In the case of a recoil starter, you are able to pull on the cord and the crankshaft with the blade rotates and it feels normal but the engine does not start. If it feels like nothing is engaging, then the starter mechanism or clutch may be broken. Of course, if the cord breaks, then the problem is obvious!
In the case of an electric starter, the engine cranks but never catches. If there is no response to the button or key, then the outlet may not be live, the battery may be weak or dead, or there could be a bad connection or bad starter motor. If the motor spins but doesn't engage the engine, the overrunning clutch or gear could be broken.
If you are unable to pull the cord (or the auxiliary starter on one with electric start), there may be a clump of grass stuck between the blade and the deck or there could be serious internal damage, especially if you just encountered an immovable object. See the section: Lawn mower will not start after the blade hit an obstruction. However, you didn't forget to engage the dead-man bar, did you? On most inexpensive mowers this safety interlock is needed to both enable the ignition system and release the blade brake.
Determining why it won't start
Think: FAST - Fuel, Air, Spark, Timing. Diagnosing a balky engine is not difficult but a step-by-step methodical procedure will make it a lot less traumatic. Despite all the warnings, serious problems rarely develop on their own. Most likely, there is a simple, easily remedied cause.
Fuel
Obviously, the engine won't run without gas!
Is there some in the fuel tank? If it is near the bottom, add enough so that there is no doubt about there being enough to reach the outlet pipe regardless of any slant on which the lawn mower is located.
Make sure any shutoff valve is open.
Check for a clogged fuel filter, if there is one. There may be a sediment catching screen at the bottom of the tank as well.
If your engine uses a primer bulb, does it feel like it is doing something? There is a distinctly different feel when it is actually squirting a little gas into the intake pipe. Check that the rubber hasn't deteriorated but if many pushes still doesn't do anything (and you're sure there is gas in the tank and the engine hasn't flooded from TOO MUCH gas), the carburetor and/or fuel line may need cleaning.
If you are using gas from last season, discard it and start fresh. While old gas will usually work in an engine in good condition, this is not always the case, especially with one that has seen better days. The more volatile fractions evaporate leaving behind higher flash point gas. Why add another unknown factor to the puzzle?
There may be water in the gas. If the carburetor has a drain plug, operate it to rid it of the bottom layer which would have the water. If there is no drain, repeated pulling on the starter cord should eventually clear any reasonable amount of water.
Once you have exhausted these obvious problems, determine if gas is reaching the cylinder as follows: Perform the normal starting sequence and then, assuming it shows no signs of wanting to start, immediately remove the spark plug. If fuel is reaching the cylinder, the spark plug should be damp with gas and there should be a very distinct odor of gas from the spark plug hole. If there is none, then there could still be a blockage in the fuel line or the carburetor may need cleaning.
A flooded engine, most likely due to extended unsuccessful attempts at starting or a defective carburetor (float valve stuck open or gas-logged float) will result in inability to start as well and a distinct odor of gas. You might find raw gas coming our of various orifices - air filter as well as exhaust. (Note that in severe cases, enough gas gets mixed in with the oil to significantly increase the level in the crankcase and reduce the effectiveness of the oil. This will require an oil change.
The optimal air:fuel ratio is around 14:1. This must be lower for a cold engine and thus a choke plate or other means to increase the richness of the mixture is usually provided. A choke plate restricts air intake forcing more gas to be sucked into the cylinder. A primer bulb effectively squirts gas into the intake pipe to augment the normal carburetor action. Some carburetors have no choke and no primer but incorporate a small gas reservoir which fills when the engine is off and provides some extra when starting.
To much air results in a mixture that is too lean, burns too quickly, and can result in engine damage over extended periods of operation.
Check that any choke is not stuck in the open position and not doing its job.
The carburetor may need adjustment or cleaning.
Too little air results in a mixture that is too rich - there will be loss of power and possibly black smoke from the exhaust. This could be due to several factors:
Check the air filter. For testing, it can usually be removed to see if the engine will start. However, do not run it for an extended period of time without a properly functioning air filter in place. Some are designed to be washed and reused while others must have their elements replaced.
Check that any choke is not stuck closed. Though needed to start cold, if the choke remains closed, the engine will not restart and will quickly stop (truly choke!) due to an overly rich mixture.
A defective carburetor may also cause the mixture to be too rich or too lean.
Spark
All common lawn mower engines require a precisely timed spark to ignite the air-fuel mixture in the cylinder. The existence of a spark can easily be tested as follows:
WARNING: make sure there is no gas in the vicinity when performing the following test!
Remove the spark plug wire and insert the blade tip of an appropriately sized and well insulated (plastic) screwdriver inside the boot or clip in place of the spark plug. While holding the *insulated* part of the screwdriver, position the metal part of the blade about 1/8th inch from the block or frame.
An alternative technique is to use an old, but good, spark plug whose gap has been increased to about 1/8 inch or one specially made for exactly this purpose. In this case, simply connect the spark plug wire to the test plug and hold its threaded part against the cylinder head or other part of the chassis (away from the gas tank!!).
Note: Just positioning the spark plug wire a short distance from the spark plug terminal is not recommended as the results of this test will then depend on the condition of the spark plug as well since the spark will have to jump two gaps.
Have a buddy crank the engine at normal starting speed so that you will be able to hold the screwdriver or test plug steady and be close enough to see any spark clearly. Shield the gap from the sun or bright light if necessary.
You should see a nice healthy spark jump the gap several times on each pull (actually, once per rotation of the crankshaft/blade on both 2 and 4 stroke engines). Note: 4 stroke engines ignite the air-fuel mixture on every other rotation of the crankshaft. The extra sparks fire harmlessly into the exhaust gasses and are wasted. Can you believe it?!
CAUTION: if you are not well enough insulated, *you* will jump several times per rotation of the crankshaft/blade if the ignition system is functioning properly! Hey, that *is* a valid test!
If this test confirms the spark, it is still possible that the spark plug is fouled or bad.
Lack of spark
If your mower is less than 15 years old, there is an excellent chance that if uses an electronic ignition system. These are very reliable as there are no points or condenser to go bad and no need for routine tune-ups.
However, a number of other problems can result in lack of spark:
Make sure stop switch/stop wire is in appropriate position - confirm with a multimeter, check that flywheel is being spun by starter and that flywheel key is intact to assure proper timing, check condition of points/condenser and setting (if applicable), test magnet (on flywheel) for strength, check the gap between flywheel and magneto core. If these are all fine, test or replace the magneto.
In more detail:
Check for a faulty or misadjusted STOP switch. This may be activated by releasing the dead-man bar or by a throttle control lever (STOP, RUN, START). Inspect the cable, linkage, and wiring for damage or for something that may have come loose. Make sure you have the controls set properly to run!
Check that your starter is actually spinning the flywheel. If the flywheel is not rotating properly when you pull the cord or turn the electric start key, then there is a problem with the starter, not the ignition system. Or, the flywheel is not tight due to a sheared flywheel key or improperly torqued flywheel nut.
Check for a flywheel that is loose and not seating properly on the taper. This could result in no spark if the air gap between the flywheel magnet and magneto core is then incorrect. However, due to the close spacing, you would probably feel and hear serious scraping in this case.
Items (2) and (3) are likely if your just attempted to move a curb with your mower blade (or if someone inadequately tightened the flywheel nut during some previous maintenance).
Check for bad connections or defective wiring including faulty or water logged insulation. If you just gave the mower a shower, wait ample time for it to dry out. High humidity may result in more problems if the insulation is not in good condition as well.
Check for a weak (or missing) flywheel magnet. Both of these faults are extremely unlikely unless you have been hammering and whacking the crankshaft and flywheel in an effort to remove the flywheel. (This is not recommended - see the section: Flywheel removal.)
(a) Electronic ignition - There is likely a single potted module which includes the circuitry and ignition coil. If anything goes wrong with this module, replacement is the only option. Once the wiring and resistance of the secondary has been checked, there are really no addition tests that can be performed on an electronic ignition module without special equipment. A defective ignition module will have to be replaced.
(b) Breaker point ignition - Possibilities are bad, dirty, corroded, or loose points or points that are grossly out of adjustment, a bad condenser, or a bad magneto coil. See the section: Maintenance of point-type ignition systems.
See the section: Testing the magneto.
First, check that the dead-man bar is properly disengaging the stop switch when pulled and/or throttle control is properly disengaging the stop switch when in the start or run position.
For anything beyond this, disassembly will be needed to identify and replace any defective parts.
If the no-spark condition happened after the blade hit an obstruction, (1) or (2) are likely. See the section: Lawn mower will not start after the blade hit an obstruction.
Checking the spark plug
Use the proper socket to remove the spark plug and inspect it for damage and general appearance:
Light gray or brown and smooth - this is the normal appearance. The mixture is correct and there likely no major problems with the engine.
Excessive black carbon - the mixture may be too rich or the spark plug may be the wrong type for your engine.
Damage to the electrodes - the mixture may be too lean, timing may be set incorrectly, or the spark plug may be the wrong type for your engine.
The best thing to do at this point is just replace it with a new spark plug and worry about the old one later. Actually, nearly every small engine maintenance book will recommend changing the spark plug every season anyhow.
Testing the magneto
The magneto, like the ignition coil on an automobile, contains two windings:
A primary with a few turns of heavy wire.
A high voltage secondary with thousands of turns of super fine wire.
In an automobile, the battery supplies the primary current; in a magneto, the magnet on the flywheel moving past the core at high speed acts as a generator and induces current in the primary.
As the magnets spin past the pole pieces of the magneto core, the points are closed and current builds up in the low voltage winding (and flux builds up in the core). At or slightly before Top Dead Center (TDC), the current (and flux) should be maximum and at this instant the points open. The flux then collapses (and the condenser (capacitor) across the points acts as a snubber allowing the current to bypass the open points and preventing arcing at the point contacts). This rapid decrease in flux results in coupling of the stored energy to the turn high voltage winding and results in up to 10,000 V or more at the spark plug.
(For EE types, this is somewhat similar in basic operation to the flyback converter in a switchmode power supply except that the moving magnet supplies the input power instead of the rectified AC line and the points act as the switch instead of a power transistor.)
The secondary will always be accessible for testing but the primary of an electronic ignition may be not be due to the electronic components:
Secondary: 3 K ohms (maybe a little higher but not open). Much lower would indicate a shorted winding.
Primary (if non-electronic and accessible): very low - guessing less than an ohm.
Wires can break due to corrosion or vibration. This would result in an open winding - infinite resistance. Shorts can develop between adjacent windings or to the core. This may be detectable as reduced resistance but without knowing exactly what it should be, there is no way of knowing if a slight discrepancy represents a problem or just slight variations in design or manufacturing.
A more complete test would involve checking the 'Q' or doing what is called a 'ring' test and even more for an electronic ignition. This requires special equipment. Therefore, it is best to swap in a known good unit. They are not that expensive.
Timing
For power to be developed, the ignition of the compressed air/fuel mixture must take place at exactly the correct instant - just before the piston reaches Top Dead Center (TDC) on the compression stroke. With automotive engines, there are mechanisms to advance the spark at higher revs but simple lawn mower engines do not have this complication, at least.
Timing is set on older mowers with point type ignition systems by adjusting the point gap and generally only changes due to wear. However, these changes are gradual and unless the points come loose for some reason, will not likely suddenly prevent the mower from starting. On newer electronic ignition systems, there is basically no adjustment as the position of the electronic ignition coil/module fully determines ignition timing and this is fixed.
However, timing can be grossly messed up if the flywheel key gets sheared and the flywheel then rotates a fraction of a turn on its mount on the crankshaft. The result may be a mower that does not start, backfires or runs erratically, lacks power, won't run and/or start when hot, etc. This is very likely to happen should the blade strike a rigid object causing the mower to stop instantly. In this case one or both of the blade lock key and flywheel key have sheared to (hopefully) protect the very expensive internal parts from damage.
There are likely not going to be any timing marks for that old timing light you have sitting gathering dust somewhere. The only test really is to inspect the flywheel keyway to determine if damage has occurred.
See the section: Lawn mower will not start after the blade hit an obstruction as this is the most likely cause of a sheared flywheel key.
Lawn mower will not start after the blade hit an obstruction
The following description applies to most small rotary lawn mowers with direct driven blades. The vast majority of these use either Tecumseh (as found a variety of Sears/Craftsman equipment) or Briggs & Stratton engines. However, similar comments apply to others as well including Lawnboy two stroke engines and the more modern Honda and other overhead valve type of engines.
The assumption is that the engine started and ran normally prior to the incident. Now, no matter how many times you yank the starter rope or run the electric starter, it will not start at all, bucks, kicks back, backfires, or fails to develop enough power to keep going on its own.
If the blade struck a solid boulder while the engine was set on 'high', more severe damage is possible as even with soft metal keys locking the blade and flywheel to the crankshaft, the inertia of the rotating blade is acting sideways against the crankshaft in addition to suddenly stopping its rotation. This can result in a bent crankshaft. The end of the crankshaft with the blade adapter could be bent without affecting the bearings or internal parts. This would need to be tested for as well. Not that such an occurrence is that much better - the crankshaft would still have to be replaced but at least the bearings in the crankcase will not be damaged.
If the starter will not turn the crankshaft (assuming you remembered in your haste to engage the safety bar) - it is seized or will only rotate part of a revolution before hitting against something solid inside - then you probably have serious internal damage that will require a complete strip down and replacement of some (expensive) parts. If it turns but much more tightly than you recall (assuming you do have the safety bar engaged!) then the crankshaft may be bent - again very expensive. Repair may not be worth it.
However, in most cases, what has happened is that either or both of the blade lock key and/or flywheel key have sheared to protect the crankshaft from serious (and terminal) damage.
If the blade lock key broke, the blade will no longer turn rigidly with the crankshaft and provide the inertia required by many small engines with undersized flywheels. In this case, the engine may try to start but die out with a few "putt-putts" or even kick back on the starter cord. (As a side note, attempting to use a lawn mower engine as a replacement on a piece of equipment that doesn't have something to substitute for the blade's inertia may not work for this reason.)
If the flywheel key broke, the ignition timing will likely be totally wrong and the result may be no ignition, backfiring, kickback, or weak or total loss of power.
To diagnose, proceed as follows:
First, pull off the spark plug wire and tie it securely away from the spark plug terminal (several inches minimum) or remove the spark plug entirely so that there is no chance of the engine accidentally starting. Even though it will not start now no matter what you do, the underlying problem could actually be a flooded carburetor or something else which may correct itself while you are working. Never take chances.
Drain the gas or remove the fuel tank. This will prevent gasoline from spilling out the gas cap vent hole or flooding the engine through the carburetor since you will need to tip the mower to get underneath.
Set the mower on its side (carburetor side up).
CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere.
The mower can usually be set on its side for a few minutes without harm but if these occur - you will have to work with it tipped less than 45 degrees or so - propped on wood blocks. Or, use this as a good excuse to perform an oil change and drain the oil (even if the engine is cold, most of the oil will drain out - it will just take a little longer). Just don't forget to refill the crankcase with fresh oil once you have completed your work!
Using an old rag and/or proper work gloves, grasp the blade and attempt to rotate the blade and crankshaft.
CAUTION, despite your lack of maintenance, the blade may be sharp!).
The blade and crankshaft should rotate together. If there is slippage, the key has broken and will require replacement of just the key or the entire blade adapter plate depending on design. If it appears to be intact, then you can assume the flywheel key has broken. The blade key may be broken as well but it is not likely the reason for your failure to start. You should remove the blade to determine this for sure before restoring the mower to service in any case. See the section: Non-violent blade removal.
You can possibly avoid removing the flywheel for inspection of the key by unscrewing the sparkplug, rotating the crankshaft so the piston is at TDC, and noting the location of the magnet on the flywheel relative to the magneto coil pole pieces. The magnet should be pretty close to the magneto in that position. If this is not the case or just to be sure, the flywheel will have to come off to inspect and possibly replace the key.
To get at the flywheel key itself, some disassembly is required.
Flywheel removal
Remove the shroud (blower cover) if you have not done so already. This is usually fastened with 4 screws and hopefully does not involve any head bolts - if so, you will need to tighten them to the proper torque using a torque wrench once you have remedied the problem. You may need to remove the fuel tank (if you have not done this already) and other trim pieces as well.
You should now see the top of the flywheel. In most cases, a large nut fastens the flywheel to the crankshaft. (However, in some designs, part of the starter mechanism is actually used and this is supposed to require a special wrench to remove. However, using a piece of wood as a buffer and tapping the ears in a counterclockwise direction will work also. Refer to your engine manual for details.) Use the proper socket to loosen this nut (counterclockwise). It may be necessary to brace the flywheel securely to gain enough leverage. Make sure this is done against something that can stand the force. Once loose, remove it by hand and then remove any washers or other parts that are under it. Make a note of how these were positioned including which side is up on some cupped washers.
You should now see the keyway. The slots on the crankshaft and flywheel should be aligned. There are two common types of keys:
A rectangular or D shaped piece of soft metal that locks the flywheel and shaft. You should be able to see if the two identically sized slots are still aligned.
A piece of soft metal with an L-shaped cross section. The slot on the crankshaft is narrower than the slot on the flywheel and is slightly offset (thus, the L). Again, it should be obvious if the two slots are still aligned.
You may even find that the flywheel is relatively loose on the crankshaft if rotating the blade while holding the flywheel stationary is possible. Either the blade key or the flywheel key or both are broken in this case.
You will have to remove the flywheel to replace the key if it is broken or damaged.
If the flywheel is loose at this point, then the following will not be needed as it can be lifted off.
There are several approaches to flywheel removal:
The best way by far is to use a special puller designed for your particular engine. Briggs & Stratton and Tecumseh flywheels usually have 2 or 3 holes placed around the center of the flywheel which are used with special puller blocks. These have self tapping bolts which you thread into the holes and then tighten down nuts to pop the flywheel off of the crankshaft. I have made my own blocks for this purpose from scrap steel. If you have a drill press, it is not difficult. Alternatively, you can purchase these from the engine manufacturer. The use of a puller really does reduce the use of 4 letter expletives and virtually eliminates the chance of damage to the flywheel or crankshaft by the alternative techniques.
