Precise Engine Repair Notebook

Convert your Tecumseh 8hp - 10hp engines to electric start

Many of the applications that our powered by Tecumseh HM80 or HM100 engines can be converted to 12 or 120 volt electric start without to much trouble. You must have a flywheel with a ring gear, starter motor, 12 volt battery, solenoid and starter key switch. I will explain below how all these go together to make your engine either a 12 or 120 volt electric start. Make sure your blower housing has a bulge in it for the starter drive to go through or you will need to modify it. Also make sure the engine block has 4 mounting holes to mount the starter.

120 volt starter

120 volt start 33329E Tecumseh or 33-738 Oregon: If you are converting to 120 volt electric start for a snow blower or other application, you must have a ring gear on the flywheel or replace the flywheel as instructed below. The 120 volt starter is self contained electrically, so you do not need to do any of the electrical work you would do for a 12 volt start. Just mount the starter and push button box.

The first thing is to remove the old flywheel and replace it with a new one with a ring gear P/N 611091 if needed (which is now no longer available from Tecumseh, try google to locate one). This flywheel is for engines with magneto coils that are mounted externally above the flywheel (newer engines). You can use a two jaw puller or with the flywheel nut loosened a couple turns, pry on one side of the flywheel lightly (screwdriver behind it) and tap it on the side opposite from where you are prying. If it does not come loose, turn it 180 degrees and do this again. Most of the time, they will pop off as they are on a tapered shaft. The flywheel nut must be torqued to 40 ft lbs. Be careful how you hold the flywheel, as you can easily break fins or damage teeth on it. A strap wrench P/N 670305 is best for this, but not everyone has one of these.

12 volt starter

Next mount the starter with the 4 bolts supplied and put the flywheel cover back on. Most of these engines have an area for the starter drive to go through, if not you will need to fabricate one. If you are doing a 120 volt conversion, mount the push button box, plug it into 120 volts and you are ready to crank it.

12 volt start 36680 Tecumseh or 33-712 Oregon: The next part of the conversion involves mounting the battery, solenoid and key switch. You must decide the best place and way to do this. I am going to recommend Oregon solenoid P/N 33-334 and Oregon key switch P/N 33-386., You may use a small engine or automotive battery as long as it is 12 volts. Mounting it will depend on how much you need to move the unit. Be sure the B+ cannot touch a metal ground such as the generator frame.

The starter key switch has some extra terminals, which you may want to use if you want the switch to also shut the engine off or run accessories. Use 16 gauge wire for connections to the switch.

12 volt key switch

The P/N 33-386 Oregon key switch connections are listed below:

  • B ... B+ from the positive battery connection.
  • S ... Connect to small terminal on solenoid.
  • M ... Connect to magneto stop terminal if you want the engine to stop by turning off the switch.
  • G ... B- connection which is to a good engine ground.
  • L ... Use this terminal for 12 volt lights or accessories.

The P/N 33-334 Oregon solenoid connections are listed below:

Start solenoid
  • Large terminal ... Connect B+ cable from battery, 8 gauge or larger wire.
  • Large terminal ... Connect to starter motor, 8 gauge or larger wire.
  • Small terminal ... Connect to starter switch S terminal, 16 gauge wire.
  • Small terminal ... Connect to B- or good engine ground, 16 gauge wire.

Be sure the B- connection is connected to a good engine ground and connect the solenoid and switch grounds here also. Turn the key to crank and it should turn over. This flywheel system does not have a battery charging circuit on it, so you will need to trickle charge the battery occassionally, depending on how often you start it - Bruce Perrault

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The Precise Engine Repair notebook is maintained by Bruce Perrault