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working on this 10 horse Tecumish....
Sat for years without starting. got it running. rebuilt carb. (needle and seat), float adjusted properly.
it runs good but has a pretty bad miss about every one to two seconds, plugs aren't fouling, and don't look lean, the burn looks good. The problem doesn't go away if i add choke.
verified good and constant fuel flow to carb( i've tried the original rebuilt carb, and put a known good HM100 Aftermarket carb on it, problem persisted), air filter is new.
Its a little finicky on the start too.
I have constant blue spark in my glow tester and the tester with 1/4" gap. Fuel is my non ethanol. fresh. i've tried 3 different plugs, Champion and NGK. Flywheel key is not sheared in the slightest.
I think my next step will be to check the valve clearance. I'm unsure of the hours on this unit.

Pulled the breather cover off, checked the valve clearance.
The specs I found online were .008 Intake and .012 exhaust
I found the intake exactly at .008, and I couldn't even fit the .012 into the exhaust, nor the .008.... or a .003" there is 0 zero clearance exhaust side. And yes I made sure I wasn't on the ACR bump.

Exhaust valve spun super easy in the head and appeared slightly open even, possibly. in the closed position. ground the valve, achieved a tight .012" clearance and stopped at that, lapped the intake and exhaust valve, rechecked clearances. all good.
just waiting on a head gasket.

Well, the dern thing still has a bad miss. valves are right on spec, flywheel key is good, it has fresh fuel, and is getting plenty of fuel to the carburetor.... does the same with the rebuilt and known good aftermarket carb i pulled off one of my tecumseh 100 engines. 3 different spark plugs, coil regapped... good blue consistent spark.
hmmmm

If the valve guides are worn that can cause a random miss.

Valves didn't seem too sloppy in the guides.

Should I hit it with a bigger hammer?

What is the spec number?

Check the type of plug. Tecumseh has been out of business for more than a decade. Back then, all mowers ran a non-resistor type sparkplug. These days... due to advances in ignition design... about the only thing you can buy is plugs with built-in resistors. And that will trip you up, plus make it hard to start.

NGK puts an "R" in their plug number when there is a resistor present. Example: BP7ES versus BPR7ES. Not sure about the others.

Scrubcadet10 said:

Valves didn't seem too sloppy in the guides.

Should I hit it with a bigger hammer?

Click to expand...

Works for me.

Rivets said:

What is the spec number?

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159402S

Born2Mow said:

Check the type of plug. Tecumseh has been out of business for more than a decade. Back then, all mowers ran a non-resistor type sparkplug. These days... due to advances in ignition design... about the only thing you can buy is plugs with built-in resistors. And that will trip you up, plus make it hard to start.

NGK puts an "R" in their plug number when there is a resistor present. Example: BP7ES versus BPR7ES. Not sure about the others.

Click to expand...

The plug in it was a J17LM, but all i had was a J19LM.

Seeing you are using a hotter plug, try reducing the gap from .030” to .028”.

Rivets said:

Seeing you are using a hotter plug, try reducing the gap from .030” to .028”.

Click to expand...

Will do

I would think that a 1-2 second miss would be hard to confirm as loss of spark when looking at a spark tester.
Appears you have done about everything but swap out the mag.
I'm assuming it's the electronic magneto engine and not the old model with points/condenser.
You mentioned it don't start easy. Them HM100's with manual only rope crank are not an easy start especially in cold weather and with the non-modded low emissions non-adjustable carb they usually hunt and surge for about 30 seconds in cold weather when first started.

Scrubcadet10 said:

Well, the dern thing still has a bad miss. valves are right on spec, flywheel key is good, it has fresh fuel, and is getting plenty of fuel to the carburetor.... does the same with the rebuilt and known good aftermarket carb i pulled off one of my tecumseh 100 engines. 3 different spark plugs, coil regapped... good blue consistent spark.
hmmmm

Click to expand...

Check the govener

Would you believe me if I said: "They all do that." Ha!
I would do two things. Take a file to the gap on the spark plug. Both the center electrode - the tip should have square corners, and the side electrode - if there is any rounding, it should be make flat where it faces the center electrode. Most plugs can be faced so they will work as new, especially in the old engines where the mixture is not leaned out by dictate from on high.
Second thing would be to run the thing in the dark to see if you have spark jumping to ground via the secondary wiring. By now, the wires are old enough for a slug of Old Burnside Raw Whiskey, and likely have had some degradation of the insulation and are primed to leak those milliamperes as fast as they can. Be sure also that the ceramic is clean on the outside as any sort of deposit can lead to the spark taking a shortcut. If it has points, use some emery paper folded and trapped using the spring tension to clean off any deposits or oxide. I still have the LAV running on points and I got it used in 1992. Have new points down in the mower parts box. I did clean them once since I got it. Maybe about the time I adjusted the valves when it would not start when warmed up. It seems to start on the first pull if I take off the air filter. New filter media on the way. It does not have a choke I can fiddle with and I do not know how it 'knows' to enrich itself when cold...
tom

Rivets said:

Seeing you are using a hotter plug, try reducing the gap from .030” to .028”.

Click to expand...

No change

At this point it’s time to look for stupid, but I’m drawing at straws here. Does your carb have a black plastic cover over the idle jet? If so have you removed the idle jet and made sure the jet is open in both directions? Or, swap out the jet from a different carb. What color is the spark plug? Is it showing a rich (black) or lean (white) running condition? Seeing that adding choke makes no difference, I’m wondering if we need to lean out the carb a little, by lowering the float level. Finally I’d try a resistor plug and see what happens. Yes, I’m looking like a nincomepoop here.

Does your carb have a black plastic cover over the idle jet? Yes, jet has been removed and cleaned.
What color is the spark plug? Tan-ish brown.
I’d try a resistor plug and see what happens: Tried a resistor, BR2LM.

Rivets said:

Yes, I’m looking like a nincomepoop here.

Click to expand...

You should see me.

Does it miss at idle? WOT? loaded? Unloaded? Sprayed brake kleen around the intake manifold? Will coil jump a 1/4" gap in air?

I would hookup an online neon spark tester and remove the air cleaner and muffler. Start the engine and let it idle. If it is a spark problem it should be observable at the spark tester. If it is an exhaust valve problem it will pop in the exhaust when it misses. If an intake valve problem it will pop back through the carb when it misses.

Do you think it's something that we could hear if you posted a video?

VegetiveSteam said:

Do you think it's something that we could hear if you posted a video?

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Took one but you can't hear it in the video.

I did find an air leak, around the throttle shaft. Spray it with WD and it bogs every time.
Hammer, No miss under load.

Scrubcadet10 said:

Hammer, No miss under load.

Click to expand...

Eliminates coil and valves.
Sounds like an air leak

I'm sure it's hard to tell through a picture, how's my float level look?
In person it has a slight, slight, downward angle.

That’s how I set 95% of my carbs.

I've seen quite a few of them plastic floats look like that, non adjustable and the carb still operate great. At first I tried installing another jet, then another float and still same. It's usually on them single cylinder Nikki's that I see such.
I eventually just accepted that it's a normal thing and not causing the the lean condition the Nikki's are know for where they run better with a slight choke.

Summary: I do not think that is float not being level is what is causing your Tecumseh issue.

Well, now im back to thinking "maybe i need to clean the carb a little better" I now have the original rebuilt carb on it, since i determined the one i had on the shelf didn't change anything, or was bad, even though i;ve used it before and did fine...... fired it up, it's doing the usual miss....
the choke on this carb has the spring on the back side of the lever that catches the notches on top of the shaft, so I SLOWLY started to close the choke, pulling it where it couldn't catch a notch, and i found a sweet spot almost to half choke, where it smoothed out, i held it there by hand for 5 minutes, never a miss or deviation in RPMs. which is weird because the plug shows a good burn (Not any white to it). If i close the choke much past this point, it dies, and if open it anymore, it starts missing again. I got the WD40 spray out and sprayed down the manifold and carb and no apparent leaks.
I'm going to pull the main nozzle out, and clean it better, but here's a question., I notice this nozzle has to O rings on it, do you think it's possible for these o rings to be bad, and cause my apparent lean issue, even though it doesn't surge or anything, it's got a sweet running spot with the choke at very certain point.....?

Forest#2 said:

I've seen quite a few of them plastic floats look like that, non adjustable and the carb still operate great. At first I tried installing another jet, then another float and still same. It's usually on them single cylinder Nikki's that I see such.
I eventually just accepted that it's a normal thing and not causing the the lean condition the Nikki's are know for where they run better with a slight choke.

Summary: I do not think that is float not being level is what is causing your Tecumseh issue.

Click to expand...

this particular one is adjustable as the part where the needle clips on is metal.

Maybe this Tecumseh carb guide will be of some help. http://www.tecumsehcarburetorguide.com/how-to-guide/

Scrub, does your carb have a brass screw on the side?

VegetiveSteam said:

Scrub, does your carb have a brass screw on the side?

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Yes, it's been Removed and cleaned. It's non adjustable.

Scrubcadet10 said:

Yes, it's been Removed and cleaned. It's non adjustable.

Click to expand...

With the engine running crack that screw loose just a bit and see if that helps anything.

Scrub, do you have any micro drills, size 76-80.

I've got a HM100 on a gen engine. I kept a eye and ear open for the old fully adjustable jet carbs and finally stumbled across one.
The OEM non adjustable carb would cause the engine to hunt and surge when the engine was warming up and worse during temp's below 30 degrees F and if the engine went to running low on fuel it would really hunt and surge. With the adjustable jet carb when the tank run low on fuel the engine just died instead of doing the long period of hunting and surging and voltage going up and down. Not a good thing when loads are connected.

Just a hint about en riching the non-adjustable carb so as to not need a run with choke. ((if all else in the carb is ok)
What I done:
After I seen that the old fully adjustable carb worked great I went to work on the Al Gore carb and enrichened the removable idle jet little bit at time using the tiny micro bits from fleece bay and I also had to get the micro chucks for the bits. I had to do the drilling jet enlargement very carefully by hand so as to not twist off a bit. Them non-adjustable carbs are leaned out and they also use the idle jet area during intermittent and high speeds.
Anyway the enrichment of the OEM carb did the trick. Mine was like yours also, when cold it ran better with some choke and also during cold weather. I did check before I started enlarging the jet that a OEM replacement jet was available in case I needed to go back.
It was an easy mod once I started because the jet could be removed with the carb in place re-sized with drill bit by hand and then re-installed and test. It's the screwdriver slot jet under the little plastic cover. I throwed the plastic cover away. I heard a rumor that the color of the plastic cap was a hint as to the OEM size of the jet if it needed replaced. I think maybe the same carb was used on the 8hp but non-adjustable were jetted different but not sure, but the adjustable jet type carbs were same for the 8-10.
I now have two good Carbs for the HM100.
I also converted this hard to manually crank in cold weather engine to electric start and added the 3amp stator. In cold weather it takes a good at least 275CCA battery to crank good. Larger battery is better.
Tecumseh really got some bad raps for the issues you and I have experienced with them hunting and surging, especially the later models with the lean carbs.

Rivets said:

Scrub, do you have any micro drills, size 76-80.

Click to expand...

No sir, that's something I've neglected to purchase.

I've got a HM100 on a gen engine. I kept a eye and ear open for the old fully adjustable jet carbs and finally stumbled across one.
The OEM non adjustable carb would cause the engine to hunt and surge when the engine was warming up and worse during temp's below 30 degrees F and if the engine went to running low on fuel it would really hunt and surge. No a good thing when loads are connected.

Just a hint about en riching the non-adjustable carb so as to not need a run with choke. ((if all else in the carb is ok)
What I done:
After I seen that the old fully adjustable carb worked great I went to work on the Al Gore carb and enrichened the removable idle jet little bit at time using the tiny bits. I had to do the drilling jet enlargement very carefully by hand so as to not twist off a bit. DO NOT use a drill, hold both the jet and the chuck by hand and back the drill bit back out often so as to not twist off and to clear the bits/pieces.
Anyway the enrichment of the OEM carb did the trick. Mine was like yours also, when cold it ran better with some choke and also when during cold weather. I did check before I started enlarging the jet that a OEM replacement was available in case I needed to go back.
It was an easy mod once I started because the jet could be removed with the carb in place re-sized with drill bit by hand and then re-installed and test. It's the screwdriver slot jet under the little plastic cover. I throwed the plastic cover away. I heard a rumor that the color of the plastic cap was a hint as to the OEM size of the jet if it needed replaced. I think maybe the same carb was used on the 8hp but not sure
I now have two good Carbs for the HM100.
I also converted this hard to manually crank in cold weather engine to electric start and added the 3amp stator. In cold weather it takes a good at least 275CCA battery to crank good. Larger battery is better.
Tecumseh really got some bad raps for the issues you and I have experienced with them hunting and surging, especially the later models with the lean carbs.

Like Forest mentioned if you try to open that hole in the idle restrictor you normally only need to take out a tiny tiny bit. It's very easy to go too far. Cracking that screw loose with the engine running will let fuel go around that restrictor and let you know if enlarging that hole is necessary. If you crack the screw and it immediately runs worse, which in theory it should if everything else is okay, I'd look elsewhere before trying to enlarge that hole. If you crack that screw loose and the engine immediately runs better then enlarging the hole is probably your fix.

Okay, well, it's definitely running lean, ran it this evening and it's muffler is glowing........
this time closed the choke and it made no difference in its running...
i think the ghost of Chief Tecumseh is haunting me.

VegetiveSteam said:

Like Forest mentioned if you try to open that hole in the idle restrictor you normally only need to take out a tiny tiny bit. It's very easy to go too far. Cracking that screw loose with the engine running will let fuel go around that restrictor and let you know if enlarging that hole is necessary. If you crack the screw and it immediately runs worse, which in theory it should if everything else is okay, I'd look elsewhere before trying to enlarge that hole. If you crack that screw loose and the engine immediately runs better then enlarging the hole is probably your fix.

Click to expand...

No change loosening the screw.

How far did you back it out? Maybe go a little bit more. Backing out that screw should richen things up.

VegetiveSteam said:

How far did you back it out? Maybe go a little bit more. Backing out that screw should richen things up.

Click to expand...

I went all the way out
It would seem to me loosening this screw would lean it out more. It's not a tapered adjusting screw. And it's just a direct hole into the low-speed circuit.

Scrubcadet10 said:

I went all the way out
It would seem to me loosening this screw would lean it out more. It's not a tapered adjusting screw. And it's just a direct hole into the low-speed circuit.

Click to expand...

Yes. It's a fixed restrictor screw. There is no adjustment. You just screw it in all the way for normal operation. It should bottom out in the hole and fuel should go through the restrictor hole cross drilled in the screw. When you loosen the screw, for test purposes only, that should let normally unwanted fuel go around the restrictor screw and cause the mixture to richen up. I don't think I've ever seen no change when that screw is loosened. It wasn't uncommon to see someone loosen that screw and find that the engine ran better. They would then take a welding tip cleaner and end up ruining the restrictor.

VegetiveSteam said:

Yes. It's a fixed restrictor screw. There is no adjustment. You just screw it in all the way for normal operation. It should bottom out in the hole and fuel should go through the restrictor hole cross drilled in the screw. When you loosen the screw, for test purposes only, that should let normally unwanted fuel go around the restrictor screw and cause the mixture to richen up. I don't think I've ever seen no change when that screw is loosened. It wasn't uncommon to see someone loosen that screw and find that the engine ran better. They would then take a welding tip cleaner and end up ruining the restrictor.

Click to expand...

And you already answered this but I'm stumped as well so I'll ask if the cross hole and the end hole are for sure open?

I don't know if your HM100 uses the same carburetor as the HH100 models but on the cast iron HM100, HH100 and several other models, if you removed the center nozzle from the carburetor when you cleaned it, you were screwed. Some of the fuel passages were drilled with the factory nozzle in place so if you removed it, there is virtually no chance of ever installing it back into that exact same position. This was a common headache with most Tecumseh carburetors. Any time you rebuilt the carburetor you had to install a replacement "Service nozzle" which you probably couldn't find because they haven't made those for quite some time now. Perhaps yours is different, I hope so because it sounds a lot like you're having those same kind of issues. Stens may possibly have a service nozzle? I don't know, they used to have a lot of stuff for the older engines. Either way, I'd install a K series Kohler carb in place of the Tecumseh carb. It's an easy swap, I have the Chinese made Kohler K series carbs installed on a HH100, HH120, and a HM100. I plan to install one on an OHV160 when I get around to working on that one.

ChrisBFRPKY said:

I don't know if your HM100 uses the same carburetor as the HH100 models but on the cast iron HM100, HH100 and several other models, if you removed the center nozzle from the carburetor when you cleaned it, you were screwed. Some of the fuel passages were drilled with the factory nozzle in place so if you removed it, there is virtually no chance of ever installing it back into that exact same position. This was a common headache with most Tecumseh carburetors. Any time you rebuilt the carburetor you had to install a replacement "Service nozzle" which you probably couldn't find because they haven't made those for quite some time now. Perhaps yours is different, I hope so because it sounds a lot like you're having those same kind of issues. Stens may possibly have a service nozzle? I don't know, they used to have a lot of stuff for the older engines. Either way, I'd install a K series Kohler carb in place of the Tecumseh carb. It's an easy swap, I have the Chinese made Kohler K series carbs installed on a HH100, HH120, and a HM100. I plan to install one on an OHV160 when I get around to working on that one.

Click to expand...

i did not remove the main nozzle during the original cleaning and rebuild.

ChrisBFRPKY said:

I don't know if your HM100 uses the same carburetor as the HH100 models but on the cast iron HM100, HH100 and several other models, if you removed the center nozzle from the carburetor when you cleaned it, you were screwed. Some of the fuel passages were drilled with the factory nozzle in place so if you removed it, there is virtually no chance of ever installing it back into that exact same position. This was a common headache with most Tecumseh carburetors. Any time you rebuilt the carburetor you had to install a replacement "Service nozzle" which you probably couldn't find because they haven't made those for quite some time now. Perhaps yours is different, I hope so because it sounds a lot like you're having those same kind of issues. Stens may possibly have a service nozzle? I don't know, they used to have a lot of stuff for the older engines. Either way, I'd install a K series Kohler carb in place of the Tecumseh carb. It's an easy swap, I have the Chinese made Kohler K series carbs installed on a HH100, HH120, and a HM100. I plan to install one on an OHV160 when I get around to working on that one.

Click to expand...

I wouldn't bet my life on it but I believe the HM carb is different than the HH. I had forgotten about the nozzle on the HH but you're right. You were screwed if you removed that nozzle.

In the picture on POST 42 the center hole down the middle of the jet is the one I used the micro bits on and enlarged by hand drilling with the micro bits. It appeared I had to drill for maybe about 1/4 inch carefully before it intersected the cross drilled hole. I think I only went about 2-3 bit sizes larger. I stopped immediately when I got a good run. I gauged the jet before I started so as I would know what size if I needed to go back or replace. I thought maybe if I needed to go back I would try soldering the jet hole closed and re-drill to OEM size. Loosening that jet screw on mine made it run worse but I had to loosen it instead of just cracking it loose.

Here is couple of links I saved sometime back about Tecumseh carbs you might review:

Tecumseh carb cleaning link

Small Engine, Lawn Mower, Snowthrower Troubleshooting, Repairs and Safety

outdoorpowerinfo.com

Tecumseh carb parts link 12 2020

https://outdoorpowerinfo.com/engine_specs/tecumseh_carb_mfg_no.asp

This is why I was stumped when you loosened that restrictor screw and got no change better or worse. If you look at the attached drawing you'll see where the fuel flow path is from the bowl nut (yours might be adjustable I don't remember) up through the idle restrictor screw and then up to the venturi of the carb. Backing out that screw should let more fuel pass and cause a change in how the engine is running. It should be a noticeable change better or worse. With no change in how the engine runs when you backed out that screw makes me wonder if there could be some blockage somewhere in the fuel path between the bowl and the venturi. Just my thought and if someone wants to correct me it won't hurt my feelings. I don't mind learning.

Backing that jet out few turns on my engine made a change for the worse.
It should make some kind of a change because it allows air to be sucked in if unscrewed far enough. and the idle suction in front of the throttle butterfly might be restricted but if so the engine would not LOW idle good. Try loosening the jet screw at low idle.

If no change I would suspect a restriction in those passage ways.
You say your other OLD carb off the shelf acted same on the engine, but you did not trying the applying choke with it when you were testing.

Might install it and try the jet loosening procedure and applying a choke to it two carbs. While you are comparing them see if the L jet will interchange with each other. If so mod one and test.

I see some China clone adjustable carbs around for approx $15 for the HM100 using part number 632697 but I'm not sure they will fit yours.
China clone carb subbing is not always a good thing, kinda rolling the dice trying to trust a China made carb.

The adjustable jet carb i got had to have a different choke mechanism (taller and longer reach choke shaft because it was for a now obsolete Coleman Powermate 5K generator air filter set-up and it cost around $25 few years ago.

So I called up a buddy today that has a mower junkyard, asked him if he had any Tecumseh HM100 carburetors, or any that would work. He happened to have another old Coleman generator with a HM100, different spec#. Put gas in it, and it fired up and ran good for 10 minutes.
Took it off, brought it over to my place, installed it, engine still runs like crap.
I'm going to call the owner and let him know I'm grasping at straws at this point. And that I dont think it's cost effective for me to spend billable hours chasing my tail, is what I feel like.
I got his two other Honda gens running, so maybe he won't be too disappointed

Sometimes your choices are few and you have to move on but I'm disappointed we couldn't be much help to you on figuring this one out. I think you definitely ruled out carburetion.

I know the sputtering Tecumseh engine is gone.
Here is couple of links to videos of how a carb MIGHT can be modded to fix a sputtering hunting engine that is running lean.
This is the basics to what I did to my HM100 Tecumseh carb idle jet.
My Tecumseh carb jet was similar to the Kohler/Honda but all brass.


How to fix/mod a one barrel surging Nikki carb used on Briggs single cylinder engines link. See post number 9 for the video



The OEM jet size was .015 and he ended up at .034

https://www.mytractorforum.com/threads/scotts-by-john-deere-surging.1352937/



For a Kohler/Honda style carb surging mod. Jet size was .015 and went to .028