QuestionHi,
I have a B&S pro classic 3500 generator.
type: EP272-Z
code 84101072CA213
3.0 KVA
I have checked the oil, cleaned the spark plug. I cannot see any obvious reason why it would not start but for the life of me, I cant get the dang thing to fire. could you offer any advice?
cheers for now,
Ivan (pulling hair out, stamping a lot and swearing in fluent anglo saxon!)
AnswerIvan
All common 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.
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:
1. 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!
2. 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.
3. 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.
4. 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.
5. 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.
6. (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.
6. (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.
