Lawn Mower Parts

Tips on Making a Great Looking Lawn, Lawn Mower Tips, Mulching, Landscaping, and Tips for your Garden, also Finding Great Deals on Mower Parts for Lawn Mowers.

Author and Copyright

Author: Samuel M. Goldwasser

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page. Copyright © 1994-2009 All Rights Reserved Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying

Most mowers and other small pieces of gas powered yard equipment use a self retracting recoil type starter. You pull on a handle attached to a cord wound around a one-way clutch affair. Pulling on it rotates the engine's crankshaft and the clutch allows the engine to run without pulling you back into the mower! If yours has an electric starter, then you don't need to tug on anything - plug it in and push a button or turn a key. Of course, finding an outlet at the far end of a large yard may prove to be a challenge. With larger equipment like riding mowers, power is usually provided by an on-board rechargeable battery. In either case, there will be some kind of backup recoil or rope starter should the electric start be unusable or inconvenient.

For the following, we will assume you pull a self-retracting starter rope. With an electric starter, replace the words: 'pull...times' with 'crank for several seconds'.

Move the mower or other equipment to the place where it will be used - no sense in dragging a chugging lawn mower through the neighborhood. Position it on a solid level surface. Make sure there are no loose stones, twigs, branches, logs, etc. underneath to get sucked up and thrown about once you succeed in getting the engine started (if you ever do).

When attempting to pull on the starter cord, it may be helpful to put one foot on the mower deck to brace it. Whether this is needed will depend on the design of your mower and in what direction the cord exits from the starter.

You or the starter motor supplies the power to get it started. However, at the low speed of starting, special modifications may be required to the fuel system for the engine to catch. These may take one of the following forms:

Primer - a rubber bulb or pushbutton squirts a little extra gas into the intake pipe. Your engine manual will detail the procedure and number of 'pushes' required under various conditions - typically between 1 and 12. Too few and nothing will happen. Too many and you will flood the engine (excess gasoline will prevent it from starting). See the next section.
A typical starting procedure for an engine with a primer might be:
- Set the throttle control (if any) to the START or HIGH position.
- Push the primer 5 times.
- Pull the starter cord once or twice (if it doesn't start on the first one).
- If it still doesn't start, prime 2 more times.
- Pull the starter cord again.
- Set the throttle control to the desired speed once it starts.
Automatic primer - a chamber in the carburetor which fills with the engine stopped and provides an additional squirt of gas when starting.
Automatic choke - temperature and engine vacuum control the fuel-air mixture.

A typical starting procedure for an engine with an automatic primer or automatic choke might be:
- Set the throttle control to the START or HIGH position.
- Pull the starter cord once or twice (if it doesn't start on the first one).
- Set the throttle control to the desired speed.
Choke plate - a control on the carburetor which partially closes off the air intake and forces additional suction to increase the amount of fuel drawn into the cylinder. Normally, the choke is closed when starting cold and gradually opened in the few seconds after the engine starts. It is left fully open once the engine is running and may not need to be closed when starting warm.
A typical starting procedure for an engine with a choke might be:
- Close the choke (usually on carburetor).
- Set the throttle control (if any) to the START or HIGH position.
- Pull the starter cord once or twice (if it doesn't start on the first one).
- If it still doesn't start, open the choke half way.
- Pull the starter several more times until it starts (hopefully).
- Gradually open the choke to keep the engine running smoothly.
- Set the throttle control to the desired speed.

If the appropriate procedure is not successful, the engine may be flooded. You can give it 15 minutes or so for the gas to evaporate and try again or, if there is an IDLE or LOW speed position, open any choke and pull the rope several times in this position which should clear out the excess gas. Then repeat the recommended starting procedure.

Blade sharpening

The blade in a rotary lawn mower doesn't need to be sharp as a carving knife or scalpel but serious dents and nicks will result in a less than perfect lawn (to say the least!). In addition, a seriously unbalanced blade can result in excessive vibration and eventually, possible internal damage as well.

First, drain the gas or remove the gas tank. If you will be filing steel, you get sparks. Sparks are not the greatest thing to have around gasoline vapor. Enough said. Disconnect the spark plug wire and tie it safely away from the spark plug or remove the spark plug entirely. Turn the mower on its side.

CAUTION: Immediately check for oil leaks at the oil filler pipe or elsewhere. If there are any, you will need to work on the mower just propped up by 45 degrees or so. 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 but it will take a little longer). Just don't forget to refill the crankcase with fresh oil once you have completed work on the blade!

Check that the blade isn't bent. Locate a reference point on one side and note the height of the blade tip at that location. Rotate the blade 180 degrees and check the height of the opposite blade tip. There should be no significant difference - say no more than 1/8" or so. If it is greater, the blade is bent or the crankshaft is bent. Either will require further investigation as running the mower under such conditions will probably result in excessive vibration and can be dangerous.

Assuming this is fine, inspect the blade:

Slight nicks and dents can be cleaned up with a file while the blade is still installed on the mower. Unless you have run into a curb, this is probably all that is needed on an occasional basis. Removing this small amount of metal will also not unbalance the blade enough to worry about. Refer to the section: Non-violent blade removal if it needs to come off the mower.

If the damage is severe, consider replacing the blade entirely - they are not that expensive (usually under $10). Otherwise, you can use a file, a bench grinding wheel, or a grinding wheel mounted in an electric drill (there are special attachments for this specific application).

Since the rotating blade also contributes to the proper air flow, you do not want to upset the shape. Grind in such a way that the original blade angle is preserved. It doesn't need (or want) to be razor sharp. A 1/64" edge is fine. Anything finer will quickly be dulled by little bits of stone and dirt in any case. Safety is not the main concern here - if any part of your anatomy contacts the whirling blade, you WILL be in trouble no matter how dull or sharp the blade might be!

Attempt to remove approximately equal amounts of metal from both ends and in roughly similar areas if possible. If there are a few large nicks, it isn't necessary to remove them completely - your lawn (and neighbors) will never know the difference.

Check the balance by positioning the blade at the center hole location on a pencil or other rod - you don't need a fancy blade balancer but can use one if you like. If it tips one way or the other, remove more material from the heavy side a little at a time.

Replace the blade along with all its mounting hardware. Make sure you get all parts in the same relationship as they had originally. The blade must have its sharpened edges pointing downward. Don't forget the install the key if it is separate and DO NOT substitute a hard steel key for the soft metal one that should be used. See the section: Why soft metal keys must be used. If the locking key or blade adapter key appear damaged in any way, replace it.

Lawn mower will not start

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! See the section: Recoil (pull) starter repair for more information.
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.

Lawn mower too loud

Small engines - especially those on cheaper mowers - are usually loud, no question about it. However, if yours sounds like it is about to explode or take off, there may be something actually wrong.

Obviously, if it is backfiring every other stroke, you have a problem with the ignition timing, mixture, valves, etc. What this section deals with is just the normal noise assuming the engine runs properly - and how to reduce it. Else, you need to perform the proper maintenance first.

Make sure everything is tight - it may be the engine rattling against the deck or something simple like that.
Your muffler could be worn out - probably rusted out. Inspect it and if in doubt, just replace. They only cost $3 or so.
Some lawn mowers discharge under the deck. This should reduce the noise level but the proper (probably spark arresting) muffler must be used or else you risk igniting dry grass or whatever as you mow!
It may be possible to purchase an after-market quieter muffler - check with the suppliers listed in the section: Mower and engine parts sources.

Note that for a 2 stroke engine, the muffler is particularly critical for proper operation and substitution may be more difficult.

Types of rotary mowers

In addition to options with respect to gasoline or electric power, rotary mowers can be classified based on such things as: walk-behind vs. ride-on, to bag or not to bag, and rear or side discharge or mulcher:

Walk-behind mowers may be either of the push type - you provide the power to move the mower; the engine or motor spins the blade - or self propelled where the engine (usually only for gasoline types) drives the front wheels via a fixed or multiple speed transmission. Typical engine power is from 3 to 6 HP with the trend nowadays toward the higher end of this range especially for self propelled models.
Side discharge mowers eject the grass clippings from the right side (usually) to the rear. These are probably the most common type in use today due to their low cost. Some perfectly serviceable new mowers of this type sell for $100 or less. Optional grass catcher bags permit some of these to be converted to side-baggers if desired but the bags tend to be much smaller and thus less convenient than those for rear bagging mowers to prevent them from becoming side-heavy when the bag fills up.
Rear baggers eject the grass clippings into a bag mounted behind the mower. Bags tend to fill quite quickly - especially if you keep putting off your mowing assignment - and need to be emptied or changed frequently. The weight of the grass clippings in the bag also contributes to the mass of the mower - which you have to push if it is not self propelled.
WARNING: Rear baggers should never be operated without a bag unless the opening is fully blocked or the proper deflector is installed. Grass cutting performance will then be similar to that of a mulching or (side) discharge type mower respectively but since this is a compromise, the resulting appearance of the cut lawn may suffer.
Mulching mowers do not discharge the clippings but continue to chop them up under-deck until they are small enough to cease being sucked up by the airflow. The best will result in almost no detectable evidence of grass clippings though the worst may leave big clumps behind. The appearance of the lawn from a mulcher may be as good or very nearly as good as that from a bagger but this is not assured. Some mowers - both discharge and bagging types - can be converted to mulching mowers by using a special cover to block the discharge port and possibly a different mulching blade.
Side discharge and bagging mowers can often be converted to mulching with the use of a mulching kit which includes a means of blocking the discharge port and possibly a special mulching blade. However, performance of one of these may not be as good as that of a mulching mower since the airflow requirements differ and these are largely determined by the design of the deck.
Riding mowers are basically larger versions of the self propelled bagging mower with a seat for the operator, steering wheel, and more controls for cutting rate and forward speeds. The engine is typically in the 8 to 10 HP range and is mounted in the rear beneath or behind the seat.
Lawn tractors may be even more sophisticated than riding mowers with larger blades and additional options for non-lawn mowing tasks. The engine is higher power - 12 to 18 HP or more - and is mounted up front under a hood as in a real tractor.

WARNING: Most lawn mowing accidents result from reckless or careless use of riding mowers and lawn tractors, not walk-behind mowers. These are not as stable as an automobile especially when their large bags are full of heavy, wet, grass clippings and may tip over on inclines that would not be a problem with someone in control of a walk-behind lawn mower. They are best and safest for use on large flat open lots. A walk-behind mower - perhaps even a manual reel mower - should be used for sloping or irregular areas and for mowing or trimming around obstructions like trees, shrubbery, landscaping timbers, posts, fences, planters, boulders, and so forth.

About engine oil

For the common 4 stroke engines such as Briggs & Stratton or Tecumseh, you don't need to buy specially high priced engine oil. The type likely used in your automobile will work fine and will be much less expensive. Although small engine manufacturers may recommend SAE 30 oil, they usually also state that 10W-30 or 10W-40 are acceptable substitutes - and these are what your car probably uses. New oil should have at least the SG rating though if you have some SF in your garage, that will be fine also (for your lawn mower, not your car!). However, there are situations like equipment that must run in extremely cold weather (probably not a problem with grass mowing!) where specific alternative oil recommendations should be followed.

Rotary mowers

These may be gasoline or electric (AC line or battery) powered. A spinning blade or blades whirls at the selected cutting height and lops off the tops of you grass by side impact. This is most definitely brutal treatment of your poor defenseless grass! The length of the cutting blade determines how much of a swath is cut on each pass - typically 18 to 24 inches for a walk-behind mower; up to 36 inches or more for a riding mower. Mowers with large swath widths may use two or more smaller blades instead.

The uniformity, consistency, and just general appearance of a lawn mowed with a rotary lawn mower is not quite up to the standards of that of a professional reel mower. You will never get the perfect manicured look though some models may come close. However, the simplicity, lower cost, and need for less and more easily performed maintenance will generally overcome the desire for perfection unless your lawn is featured regularly in "Better Homes and Gardens" magazine.

Gasoline powered rotary lawn mowers are by far the most common type used by homeowners and many professional landscapers as well. Most walk-behind mowers use a manual recoil (pull) starter though electric start is available on more sophisticated (and more expensive) models and generally standard on riding mowers and lawn tractors.

The main disadvantages of a gasoline powered mower are the need to deal with the handling and storage of gasoline and routine engine maintenance at the end of the mowing season to assure easy starting next season and to prolong engine life. However, most of this is pretty straightforward. See the section: General preventive maintenance. Small gasoline engines also do contribute to air pollution but new mowers must meet more stringent EPA requirements as of September, 1996.

Electric rotary mowers are also available in both plug-in and cordless (battery powered) versions. However, since gas mowers are generally more powerful and not limited by the length of a power cord or charge of a battery, they continue to dominate the market. Electric mowers are, of course, non-polluting but the electricity had to be generated somehow.