Tecumseh HM100 driving me nuts

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  #1  
Old 01-08-21, 02:47 PM
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Tecumseh HM100 driving me nuts

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
 

Last edited by mowfixer; 01-08-21 at 03:41 PM.
  #2  
Old 01-08-21, 03:03 PM
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This is only when the generator puts a load on it or have you taken the engine off and ran it by itself with the same miss?
 
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Old 01-08-21, 03:41 PM
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Miss is a bit less frequent under load than at idle
 
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Old 01-08-21, 04:30 PM
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It could be carb/mixture. You might try applying the choke a little. If that changes the miss or eliminates it I would look at the mixture on the carb at idle.
 
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Old 01-09-21, 12:52 AM
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I have tried 2 different carbs and they both act the same. I tried different steps on the chokes but does not help. Was reading somewhere that the Tecumseh fuel filter must have an element of a certain number of microns for proper flow. Will remove fuel filter completely for a test of that theory.
 
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Old 01-09-21, 05:35 AM
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Engines used on generators should have a method of controlling the throttle to maintain the frequency of the voltage at 60 hertz for proper operation of AC motors.Does this engine have such a control that is malfunctioning?
 
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Old 01-09-21, 07:01 AM
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This generator is not that sophisticated. It has only a governor arm, return spring connected directly to the carb throttle lever. Not a precise 60 Hz inverter generator like today, this is an old gal. What is odd is that it will start on the first pull even when air temp is 30 deg or hot start after running 30 minutes. Just has an annoying random miss. I even disconnected low oil switch thinking it might be triggering as oil sloshes around but no change.

I have two things I am thinking about. I saw Tecumseh wants a 60 micron fuel filter for proper flow. I have a generic one on it so today I will bypass fuel filter completely for a short test.

Other thing but in my many years working on these I have only seen once could it possibly be weak flywheel magnets? I did the screwdriver pull test and the magnets pulled blade to them but not strong enough to hold screwdriver (6 in long) to magnet if I let go. That would be a rare one but not cheap to replace flywheel only to not fix problem. When I worked in shop we had meter to measure magnetism but now retired no such tools.
 
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Old 01-09-21, 08:15 AM
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I would think the gas flow thru a filter is constant for 30 minutes unless the tank is full of junk. Can you try adjusting the float level to maintain more gas in the bowl? Most electronic components in the ignition module don't fail intermittently unless a wire lead breaks due to vibration. When you stated you changed the coil, did that include the ignition module? How long does this noticeable miss last? The shorter the miss, the more sophisticated the diagnostic tools need to be.
 
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Old 01-09-21, 09:23 AM
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This engine has a normal magneto electronic ignition with external mount outside the flywheel. I read the filter for the tecumseh should be 60 micron. Many are much less with more fuel restriction and I dont know what the generic filter I have is. I just bypassed the filter and the miss was less frequent. I also found if I go from stock resistor spark plug to a non resistor plug it gets better. I Opened the fuel line into a jar and there is great fuel flow through shutoff valve and into hoses. I just dont see what it can be as I have two ignition coils and they act the same.

May be due to these Chinese carbs that are all you can buy these days. I am running the same fuel in my wood chipper and power washer and they run fine so its not fuel.

Maybe I need to pull head again and do a leakdown test on the valves. I lapped them and got a nice lap ring on valves and seats looked bright and shiny.

I am about to just junk this thing and move on. I have other projects and I dont see any easy resolution to this. Miss is about once every 10 seconds and only one big miss. I can see it on my tachometer when it happens.
 
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Old 01-09-21, 01:37 PM
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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
 
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Old 01-09-21, 01:39 PM
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Disconnect the kill wire from the coil and see if the miss goes away. You'll have to choke it or pop the plug wire off to kill the engine, but let's eliminate the kill switch and wiring . Sounds like a problem in that area.
 
 

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