QuestionI have replaced coil, points, condensor, still no fire at the plugs this engine is on a miller welder 1978 year model.Also plug wires
AnswerKent
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. See the section: Checking the spark plug.
If there is no spark, then there is a problem with your ignition system.
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
