Tecumseh HM100 generator hot misfire fixed

[lee]

I have a 1992 Coleman Powermate 5000W generator with a Tec HM100 engine that saw very little use until recently when the power went out for two days after Hurricane Matthew. I put a little gas in the generator and let it run until the tank is dry every year or two. This time when the neighborhood was dark and I realized the power wasn't going to come back on anytime soon, I wheeled it out onto the driveway, connected the cable disconnect box, filled it with gas, and pulled the rope. It started on the first pull and ran great. I lit up the whole house 2 fridges, microwave, TV's etc. When I went out several hours later I noticed it had a little misfire and it didn't matter whether it had a load on it or not. When I restarted after refueling it was misfiring badly so i shut it down for the night. The next morning I pulled the original J8C plug and it looked fine. I put a new J19LM in and it fired right up and ran great for an hour or two and then it started misfiring again. I let it run that way until the power came back on that afternoon. After getting internet service back I ordered a new coil/module. For some reason I thought the min coil to flywheel gap was .008 so I used a feeler gauge under each leg of the coil to set the gap. It fired right up and ran for several hours under load without missing a beat. I shut it off and let it sit and heat soak for 5-10 min and it started right up and ran perfectly the whole time. I'm guessing the darlington switching transistor in the primary winding was failing due heat. It took a long time to heat up with the air from the flywheel blowing on it. But after shut down when the engine was hot, upon restart it would barely run at all. No telling how long this problem existed as I never ran the generator more than a half hour or so. So if any of you have an old generator like this it would probably be a good idea to run it with a good load for an hour or so to let it heat up and then shut it off for a few min to let the coil get really hot. If it starts and runs without misfiring you're good to go. Thank goodness the power came back when it did because the generator I thought was good turned into a boat anchor due to the bad coil. Question1 how common is this problem as I don't see it mentioned on the forum at least for this engine. Question 2 Do you think .008 air gap is too close? I found out later Tec specs call for .012. I'll redo it if it might cause problems being set too close even though it runs great now. thanks, Lee

[bgsengine]

Question1 how common is this problem as I don't see it mentioned on the forum at least for this engine.

Not that common, but common enough some shops keep a coil or two on the shelf...

Question 2 Do you think .008 air gap is too close? I found out later Tec specs call for .012. I'll redo it if it might cause problems being set too close even though it runs great now. thanks, Lee

Generally .008 - .014 would be a decent range for most any ignition coil
HOWEVER - the closer it is to the manufacturer's specs however, the better

- The angle at which the magnetic fields affect the coil can change ever so slightly (as in umpteenths of a degree) depending on the air gap, and that can affect trigger patterns (ignition timing) but the difference between a .008 and a .014 air gap would be very very hard to even measure outside of a precision laboratory

- The strength of the magnetic field can also be affected by that same air gap (which is more of a concern, actually)
- too close, and the thermal expansion of parts can allow the flywheel to hit the coil armature legs
- Too far away and the magnetic field may be so weakened as to affect ignition stability and consistency of the trigger signal, as well as the voltage (remember, the coil is a step-up transformer, turning a half a volt induced by the magnets to 15,000 volts or more in the secondary - reduce the input voltage, reduces the output voltage)

[lee]

I was thinking the same thing that .008-.012 would be in the ball park. I went with the closer gap as I was thinking the flywheel magnets may have weakened in this 24 yr old engine and that might be part of the problem? I took a flat blade screw driver and held it over the magnets and I felt a pretty good pull, but have no idea how to test flywheel magnets. The only problem with flywheel magnets I've run into is when they fly off and break. How common is it for a flywheel magnet to get weak enough to cause a misfire?

[bgsengine]

I was thinking the same thing that .008-.012 would be in the ball park. I went with the closer gap as I was thinking the flywheel magnets may have weakened in this 24 yr old engine and that might be part of the problem? I took a flat blade screw driver and held it over the magnets and I felt a pretty good pull, but have no idea how to test flywheel magnets. The only problem with flywheel magnets I've run into is when they fly off and break. How common is it for a flywheel magnet to get weak enough to cause a misfire?

Very uncommon. rule of thumb: if you can take a large (about 8-10" 1/4 to 5/16 wide blade) scre4wdriver with it held perpendicular to flywheel , magnets attract the screwdriver and when you let go, the screwdriver stays standing vertically (in fact, most flywheel magnets could hold same screwdriver horizontally) then magnets are fine.. Only magnets I see "fly off and break" are internal (under flywheel) magnets used most commonly for charging systems. but those are tyically held in place with an epoxy while most flywheel mahnets I know of (for ignition purposes) are almost all cast into the flywheel(for aluminum flywheel) or fixed into a casting and staked or riveted into place on Cast Iron flywheels

[wristpin]

I regularly work on flywheel magneto engines that are well over fifty years old and have yet to find one with an ignition problem caused by weak flywheel magnetism.