Sabo lawnmower with Briggs & Stratton engine won't start when warm

I have already performed the basic adjustment for the bimetallic arm.

When the engine is cold, the arm is at the rear and the choke is closed. When it is warm, the arm is at the front (pushed forward by the bimetallic element) and the choke is open.


The engine starts up without any issues when cold.
When it's warm, however, it won't start.
But if I close the choke manually, it does start.

Here are two videos:
One shows the warm-up phase, where you can see the bimetallic arm moving (I edited the video to keep it short).
The other video shows that it doesn't start when cold unless I give it a little help.
Cold start:
Cold start:


Warm start:

What is the problem?
I'm really at a loss here.
 
If I did not know the history you provided I would say this from the videos......On the cold start video Nothing looks off or unusual other than the bimetallic thermostat seems a bit loose as it bounces side to side. In my experience they are a bit firmer. Are you sure it is installed correctly with the gaskets on both sides of it? I am only saying that does not look !00% normal.

In these situations you want every thing to be like factory baseline to rule out subtle effects and possibly human error in the last inspections. Because this has been a long and difficult problem for you. It seems you have exhausted all of the likely causes. At this point you can not say something is close enough and ignore it. You want to make it like from the factory.

On the warm start I would say it did start because on the second start attempt the choke was wide open when it caught. It did not start quickly but it started. It may be best to have a half choke at that temp the engine is at in that video. it seems it is not getting enough gas or too much air. Did you mean to say the engine was hot or warm here? There is a difference.

It is hard to say how warm is warm and how hot is hot from here. In theory a warm engine should be at ~half choke, a hot engine should have no choke.

Did you pull the plug and inspect it for how the engine is running... Does it appear lean or does it appear rich or normal. This can be a clue and you want to get everything back to factory base line normal. Lean suggests it may be a fuel or carb or air intake leak issue. Running lean, runs hot and may open the Thermostate sooner than optimum. I would investigate this next.

In general, typical trouble shooting responses would be: ( In no particular order of probability) a weak spark, air leak, valve adjustment needed, gas issue, Is the carb 100% clean, Carb float set too low, Is it the correct carb and jet size? You have confirmed all of these? Right? Sometimes a second look is justifiable when you are stuck.

If all else fails it may be the Thermostat is malfuntioning and needs replacing. I think that is unlikely as I have never seen a Briggs choke thermostat break and it appears to be rotating with heat applied. I have seen them abused and still work as they should. The Kohlers on the XT series engines are less robust and I have seen them malfunction.

I hope this is helpful.

Good Luck
 
Thanks a lot for all the tips.
It seems there are quite a few things that could be causing the problem.
I let it warm up for about 5 minutes. The outside temperature was pretty warm, too, so I’d say it was definitely up to operating temperature. You could actually hear it crackling or clicking, and the bimetallic strip was sitting quite tight against it.

The hardest part for me would be checking and—above all—adjusting the valves. Could that really be the cause? Generally speaking, I think it runs well once it’s actually running.
From what I’ve read, they can’t be adjusted directly but have to be ground down. I don’t know if I can do that myself, but I know someone who should be able to.
Are there specific specs for this engine?

I removed the spark plug and installed a new one; the ignition coil is new, too. The spark plug was producing a spark, though I’m not sure if it’s strong enough. In my opinion, it could be a bit stronger (though it might be normal as is).

Regarding the potential leak, I sprayed brake cleaner around the intake port, but the engine’s behavior didn’t change.

I cleaned the carburetor in an ultrasonic bath with Tickopur R33 for at least 30 minutes. I split this into two sessions and blew out the jets and passages with air in between. The fluid turned a bit greenish near one of the holes.
I’ve ordered a replacement carburetor and plan to install that next. I didn’t go with an OEM part, though; I hope that wasn’t a mistake.

I could also replace the thermostat. Unfortunately, the cost of spare parts is starting to add up, but I don’t really have any other choice now.

Someone told me that if you remove the spring attached to the plastic arm that operates the choke, you’ll never get the engine running again. Could that be true? After all, the spring just hooks into place.
 
I see you are testing here. Wanted to point out, not to run the engine long without the top shroud. It directs cooling air around the block and cooling fins. Don't want you to burn the engine up.

2 minutes running without the shroud is too long. I would say at the 1 minute mark she needs cooling. These engines warm up really fast.
 
Raiden said:
Are there specific specs for this engine?
Click to expand...
Yes there are specs. Yes that engine can have the valve clearance checked. On an engine this old, betting the valves have never been looked at. Guessing it needs a full de-carboning as well. All this and a lot more in your engine manual. Lap the valve in. Lap head and area where the head sits on the block.
 
Raiden said:
I removed the spark plug and installed a new one
Click to expand...
WHERE did the plug come from specifically?
Raiden said:
the ignition coil is new, too
Click to expand...
This will require testing. Tossing on new parts means zero these days. Test what you have BEFORE buying new parts.
Raiden said:
The spark plug was producing a spark, though I’m not sure if it’s strong enough. In my opinion, it could be a bit stronger (though it might be normal as is).
Click to expand...
Need an open gap style spark tester. One that you can adjust or alter/change the gap size to stress the coil out. Also test for spark in low lighting conditions. Allows you to see the spark better plus doesn't pollute the proper color or Kelvin temp of the light frequency. Summation provides a better color recognition of the spark.

Recommend buying non-resistor plugs too. Provides more spark energy TO the gap.
 
Raiden said:
Regarding the potential leak, I sprayed brake cleaner around the intake port, but the engine’s behavior didn’t change.
Click to expand...
Where and when did everyone use brake cleaner for anything other than brakes? Stop buying that junk. It's very hard on the lungs. If you can smell it, your lungs are full of it as well. Think about it.

You can use free water in a squirt bottle and do the very same thing. Way cheaper and less caustic than brake cleaner.
 
Raiden said:
Someone told me that if you remove the spring attached to the plastic arm that operates the choke, you’ll never get the engine running again. Could that be true? After all, the spring just hooks into place.
Click to expand...
Leave the spring installed and leave it alone. Your friend needs to read more.
 
Raiden said:
I could also replace the thermostat. Unfortunately, the cost of spare parts is starting to add up, but I don’t really have any other choice now.
Click to expand...
To me it is working as normal. I see all that flopping around. Fact is that arm is not on bearings and won't move smoothly.
 
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