Tecumseh Points Ignition Repair
Article by Mark Trotta
Pull after pull, this old Tecumseh motor showed no sign of spark. Time to check the points.
Small Engine Spark Tester
There's several ways to test for spark on a small engine. I have a Briggs & Stratton spark tester which I find quick and easy to use. Clip one end of the spark tester to the spark plug wire and the other clips onto a ground (usually cylinder head).
Check For Spark
With points ignition, it takes very little RPM (about 350) to produce spark. This is easily done by pulling the rope start. As the flywheel turns, watch for spark in the clear circle on the tester tool.
If you see a spark, your ignition is working properly.
On early engines such as this H25, the flywheel needs to be removed to get to the ignition points. Do not try to pry off a vintage small engine flywheel with a screwdriver. Aside from being very old, it's made of soft aluminum and damages easily. A three-jaw puller (or a specialty tool) is best for this procedure.
TIP: Put the keyway somewhere it won't get lost.
How A Magneto Works
The magneto type of ignition is the simplest spark-making system out there. It requires only minimum parts; breaker points, condenser, a coil and rotating magnets.
When you pull the starter rope on a magneto-fired small engine, two magnets inside of the flywheel (sometimes outside) revolve around a stationary coil. The current created travels into the ignition coil where it gets stepped up from low to high voltage. From there it travels to the points, which act as an on/off switch for the spark. After that, the voltage goes through the ignition wire, igniting the spark plug.
Once the engine is running, the flywheel keeps rotating, and this cycle is repeated over and over. The magnets keep passing the coil and the spark plug keeps firing.
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.
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. 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.