HM100 woes

[VegetiveSteam]

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.

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]

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.

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?

 

[ChrisBFRPKY]

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.

 

[Scrubcadet10]

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.

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

 

[VegetiveSteam]

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.

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.

 

[Forest#2]

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.

 

[Forest#2]

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

 

[VegetiveSteam]

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.

 

[Forest#2]

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.

 

[Scrubcadet10]

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