Tecumseh Flathead Ignition Troubleshooting and Parts Replacement
Article by Mark Trotta
In this installment of the Tecumseh flathead overhaul we'll be diagnosing the ignition system, setting the magneto armature air gap, replacing points and condenser, and adding a simple kill switch.
Crankshaft, connecting rod, valve seats, cylinder bore, piston and rings are covered here
Sheet metal restoration and gas tank repair are covered here
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How To Check For Spark
On small engines, it takes very little RPM (about 350) to produce spark. To check, remove spark plug, reconnect coil cable, hold plug against the motor to ground it, and pull the rope-start.
An easier way to test for spark is with an ignition spark tester.
Simply clip one end of the tester to the spark plug wire, and the other clips onto a ground (usually cylinder head). As the flywheel turns, watch for spark in the clear circle on the tester tool. On this old Tecumseh, there was none.
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Flywheel Removal
To get to the ignition points, the flywheel needs to be removed. A three-jaw puller or specialty tool is best for this procedure.
CAUTION: Do not try to pry off a 50 year-old flywheel with a screwdriver. Aside from being very old, it's made of soft aluminum and damages easily.
TIP: Put the keyway somewhere it won't get lost.
Early Tecumseh motors have two magnets which sit flush with the inside of the flywheel. An easy testing procedure for checking these is to hold a small hand-held magnet near them. One should attract it and one should repel it.
Replace Tecumseh Points and Condenser
Early Tecumseh points are housed in a small metal box in the magneto unit. The condenser mounts to the stator bracket. Replacement procedure for both is straight-forward, and is the same for most vintage Tecumseh engines.
If you still have no spark after replacing the points and condenser, a bad coil may be the culprit. Note that the spark plug wire is not serviceable. It is part of the ignition coil, so if suspect it's bad, you'll need to replace the coil.
Magneto Removal
For restoration purposes, a total strip-down of this vintage small engine required removal of the magneto unit. Ordinarily, it would not be removed unless it's bad. Once removed, they are difficult to re-align.
The stator and coil assembly is held on by two bolts in elongated slots. The slots are there so the stator can be moved to adjust the armature air gap (see below).
Before removing the stator, make a note of the bolt indentations. This will help re-assembly. You should be able to tell where the coil was mounted before by the bolt washer marks.
There is a removable breaker point cam which slips on and off the crankshaft. This oblong cam opens and closes the points. Rarely do these need replacing, but check for damage and wear. Notice there is an arrow on one side, it should face out towards you.
Armature Air Gap
If you are replacing or re-installing the magneto unit, you need to position it close enough to the flywheel to get a proper signal from the flywheel magnets, but not so close that it's rubbing against the coil. Trouble is, it's hard to see because it's under the flywheel.
The space between the flywheel magnets and the coil pickup should be .0l0" to .012". On the other side of the flywheel (180 degrees) the gap will be about .0125" (1/8").
Back in the day, small engine mechanics placed a matchbook cover between the flywheel and the magneto coil, re-installed the flywheel, then pulled the matchbook cover out. Although this may sound nostalgic, a feeler gauge is quicker and more precise.
Once the new points were installed and set at .020" gap, the engine now had spark.
Before the flywheel was put back on, the engine was painted white.
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Simple Kill Switch
On magneto-powered small engines, the ignition is always on. When you press a kill switch button, it grounds the ignition and stalls the motor. Almost any kind of style switch will work.
On this go kart, a two-pole toggle switch was installed. One wire is grounded to the frame, and the other wire goes to the engine. When you throw the switch, the motor stalls.
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Related Articles:
Tecumseh Flathead Engine Build - Part One
Tecumseh Flathead Engine Build - Part Two
Tecumseh Flathead Carburetor and Air Filter Upgrades
