QuestionI have a McCulloch MAC280B Blower Model 21-400228-03
Recently the screws under the magneto that hold the bracket for the ignition assembly came loose. I removed the magneto and tightened them. I replaced everything and set the ignition assembly to .030. The throttle cable came out of the carburetor. and I took the screws out of the carburetor and reconnected it.
Now it is very hard to crank. When I do get it cranked I have a problem when I give it full power.
I must have something wrong either with the carburetor settings or the magneto to ignition assembly gap. Any Ideas.
Thanks Clay
Answerclay
Make sure that the piston is set a TDC you can use a pencil to feel for the piston coming up on the compression stroke. Then as the piston starts to come back down that was the TDC. Now back up a little and adjust the mag at that point. You can use a match book cover to get the right somehow all us guys use that and it works.Just for your information 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 switch mode 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.
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.
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.
There are three adjustments on a typical carburetor:
* Main mixture - Bottom of Craftsman (Tecumseh) float carburetors.
* Idle mixture - Side into body of Craftsman (Tecumseh) float carburetors.
* Idle speed - Sets relaxed position of throttle plate.
Initially, carefully and gently turn the two mixture controls in until they just seat.
Note: "In" means clockwise (the way you would tighten a normal screw) and "out" means counterclockwise (the way you would loosen a normal screw).
CAUTION: do not force them - you are not trying to tighten anything - as you will damage the needles and seats which will require replacement of the needles or entire carburetor. Then back them out 1 to 1-1/2 turns. Set the idle speed screw 1 to 2 turns beyond where it contacts the throttle plate. Refer to your engine manual for specific recommendations! These settings should allow the engine to start and run, though perhaps not entirely smoothly or with great enthusiasm.
* Start the engine and allow it to reach normal operating temperature - a couple of minutes. Make sure any choke is off once it is started and running stably. This will also flush any old deteriorated gasoline from the carburetor!
* With the engine throttle control set for the maximum recommended rpm, very slowly rotate the main mixture screw counterclockwise (loosen) until the speed begins to drop off due to too rich a mixture. Then, rotate the screw very slowly clockwise (tighten) until the engine begins to cut out. Very slowly means a fraction of a turn at a time - then wait a few seconds for the adjustment to have an effect. Note the number of turns between these two positions and set the screw in the middle of this range.
* Repeat this procedure with the engine throttle control set to the idle or slow speed position but using the idle mixture screw instead.
* If there is a high speed adjustment - possibly on the throttle control itself or the throttle control bracket, it is best to set it using a tachometer. However, it is possible to do a very good job by comparing the speed by ear to an identical type engine that is set correctl
