Tecumseh HM100 driving me nuts!

[mowfixer]

I have been repairing small engines for quite a long time. This Tecumseh HM100 on a 6 Kw geberator is driving me crazy. It has an occasional very noticeable miss at random times. When it misses there is no fire from carb or out exhaust. Sounds like ignition.

Here is what I have done so far:

New Plug
New coil
New flywheel key and checked flywheel for cracks etc
Checked flywheel magnets with screwdriver test and it snatches screwdriver nicely.
New carb, filter fuel lines and fuel shutoff in tank,
Lapped valves checked seats and adjusted valve clearance.
Cylinder walls still have nice cross hatch pattern on them
cleaned carbon out of head and off piston
Checked exhaust for carbon but OK
tried 2 different carbs and 2 coils with no change and set idle speed to factory specs
Using a resistor plug gapped at 30.
Seems to run a bit better with non resistor plug but not much
Set governor linkage

Spark is bright blue

In almost 30 years of working on engines I have never seen one like this. I do admit I tried to stay away from tecumseh motors most of that time as I really hate them.

Only thing left is a 45 cal shot to its head and that may be soon.

Any ideas of what I have missed. Gun is loaded and ready

Thanks

Mowfixer

[KE4AVB]

Tecumseh engine were already gone before I got starter here but it sounds like you might have bare spot on the kill wire touching metal frame at random times. Others with more experience will chime in later as check in. Note it is the off season here in the US so forum is quite.

[bgsengine]

I have been repairing small engines for quite a long time. This Tecumseh HM100 on a 6 Kw geberator is driving me crazy. It has an occasional very noticeable miss at random times. When it misses there is no fire from carb or out exhaust. Sounds like ignition.

Here is what I have done so far:

New Plug
New coil
New flywheel key and checked flywheel for cracks etc
Checked flywheel magnets with screwdriver test and it snatches screwdriver nicely.
New carb, filter fuel lines and fuel shutoff in tank,
Lapped valves checked seats and adjusted valve clearance.
Cylinder walls still have nice cross hatch pattern on them
cleaned carbon out of head and off piston
Checked exhaust for carbon but OK
tried 2 different carbs and 2 coils with no change and set idle speed to factory specs
Using a resistor plug gapped at 30.
Seems to run a bit better with non resistor plug but not much
Set governor linkage

Spark is bright blue

In almost 30 years of working on engines I have never seen one like this. I do admit I tried to stay away from tecumseh motors most of that time as I really hate them.

Only thing left is a 45 cal shot to its head and that may be soon.

Any ideas of what I have missed. Gun is loaded and ready

Thanks

Mowfixer

Lot of work thrown at it.. but what about fuel quality? bit of water in gas (doesn't even have to be visible) can be enough to cause a random misfire.. and just because it is "fresh from the gas station" doesn't mean it it's good.. also try changing the octane of the fuel (try a bit of premium perhaps?) and if the carburetors are the non-adjustable fixed-jet type, it's well within the realm of possibility that the fuel mixture is just a touch too lean.. (or too rich)

[mowfixer]

I only use non ethanol fuel on all my small engines and the fuel in it is pump fresh. It has one of those emission carbs on it and I was thinking of looking for a fully adjustable replacement. I see one available that will drop right one with one exception. It has a tube coming out for a primer bulb but everything else is the same. Snow blower carb I suspect but not sure what I could do with primer pipe other than plug it.

I have seen many small engine problems in the years I have worked with them but this one is strange. I wonder if there might be an issue with the flywheel magnets. They attract a screwdriver nicely but not a very scientific test. A flywheel is an expensive part to replace if I end up as I am now.

I forgot to mention. This engine starts the first pull of the cord, hot or very cold it starts first pull.

I also disconnected low oil switch thinking it might be causing the issues bot no joy there either.

Thanks

Mowfixer

[KE4AVB]

As BGS suggested new fuel doesn't it is water free. I more than once have purchased fuel to only get a lot water in it. And as said only it only a little bit to cause a misfire. I suffer through a similar problem with a Briggs engine about 5 yrs ago on a personal mower when I final seen about a tablespoon full of water in the gas tank that every once a while enter the carburetor.

I even purchase 5 gallons for my TB Horse tiller to only get over a quart of water with the purchase. Boy that a fun day to pull that several hundred pound tiller out of heavy tilled soil just to clean the carburetor and fuel tank.

[mowfixer]

I think I may have found it. I put my air adapter into spark plug hole and pressurized cylinder. Hear air escaping out exhaust port and when I rotated engine there was no resistance. Pulled head and looked at exhaust valve and seat. Seat looked fine but when I lapped the exhaust valve it had too narrow of a lap line. I re-lapped it until I had a nice wide line. Temp put head back on and when I put air in cylinder if I tried to crank engine to TDC I could not spin it once the valves closed. When I put in air motor turned which it didnt do before.

Seat looks good and now valve is properly lapped. Re did valve clearance and when I get new head gasket I believe it might be fixed.

Thanks for the help. Got me motivated to look a little deeper

Mowfixer

[Skywatcher]

Hi Mow Fixer

A few years ago, I was regularly running into almost new Tecumseh engines on snow blowers that would miss regularly. Found that the valve clearances were not set correctly from factory, exhaust valve wasn't even closing properly on one unit. Tecumseh specs say .008" to .012" valve clearance on all side valve engines. Once the valve stem ends were ground to get correct clearances, the engine ran perfectly. I took to setting the intake to .010" and the exhaust to .012" so the valves had room to wear in before any problems started surfacing again. All the best,

Sky

[KE4AVB]

Sky, That's what I do on all L-head (side valve) engines as well as OHV knowing the direction they wear to creating problems. Otherwords L-head set wider gap and OHV to the narrower gap.