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Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back


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  #21  
Old 05-16-2019, 10:06:29 PM
Markoh Markoh is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Measured as instructed.

The intake valve clearance is .012 and the exhaust valve clearance is smooth .023 and tight .024.
Since it has fired off but not run, would you button it up and wait for the new carburetor or is it clearly head off, valves out, lap in valves?[/QUOTE]
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  #22  
Old 05-17-2019, 08:23:04 AM
I like oldstuff I like oldstuff is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Some of the Tesuckems as we called them when I was in he biz can be finicky. Are you sure ignition timing is correct? The points must be set exactly right.

Before you pull the head, try the leakdown or at least a way to jam air pressure into the cylinder.
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  #23  
Old 05-17-2019, 08:52:21 AM
Markoh Markoh is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

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Originally Posted by I like oldstuff View Post
Some of the Tesuckems as we called them when I was in he biz can be finicky. Are you sure ignition timing is correct? The points must be set exactly right.

Before you pull the head, try the leakdown or at least a way to jam air pressure into the cylinder.
Fortunately this model has electronic ignition or maybe not so fortunate. I know how to set points but know nothing about this electronic ignition system on the Tecumseh except it has two pins that trigger spark. I read one pin is for starting and the other is for running?
If needed, is there a way to check the timing?
As for leak down test, I am hesitating to spend the 55 bucks for a tester for a one time use.

---------- Post added at 08:52:21 AM ---------- Previous post was at 08:48:44 AM ----------

Now that Iím thinking about it, I have a vacuum tester I can rig to an old spark plug and possibly check leaking that way.
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  #24  
Old 05-17-2019, 01:50:17 PM
CharlieB CharlieB is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

The engine has compression and intake suction, so the valves and rings are sealing. I still think the problem is fuel. Here's why:

This engine has a wasted spark ignition. The plug fires once per every engine revolution, even though spark is only needed once every second revolution. The wasted spark occurs at the transition point between the exhaust and intake strokes. In theory, there is no fuel in the cylinder to ignite at that point in the combustion cycle, but if the previous combustion cycle failed to ignite an insufficient charge of fuel, that un-burned fuel will be available for the wasted spark to ignite, causing blow-back on the intake or exhaust stroke. Since the fuel is not under compression at that point, it is easier to ignite than it would have been in the combustion chamber. The fix is to supply a proper fuel supply so that it ignites on the power stroke.
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Old 05-17-2019, 02:57:01 PM
Andrew Mackey Andrew Mackey is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

I would check your valve specs again. Typically, valve clearances are checked with the engine at TDC Compression. A clearance of .020 seems to be too much, and excessive exhaust clearance will not allow the engine to breathe correctly - IE kick out thru the carb as gasses are not fully purged due to excessive clearance. On older engines .008 and .010 were common, 10 and 12 were pushing it. I would pull the head and watch the exhaust valve. At cranking speed you should see it lift slightly at about mid stroke of the piston. No movement means either excessive clearance or the compression release is not working.
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  #26  
Old 05-17-2019, 06:53:58 PM
Markoh Markoh is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

[QUOTE=CharlieB;1574944]The engine has compression and intake suction, so the valves and rings are sealing. I still think the problem is fuel.

Geez-oh! That is a genius explanation. Too genius to be just lucky. Makes me wonder if you designed something for engines. Not many have that knowledge and the experience.
Firing off at the wrong time would explain the starter issue.
Time to test the hypothesis.
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  #27  
Old 05-17-2019, 08:50:21 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Thank you Andrew.
Checked valve specs in repair manual Bruce sent. For the HH-120 engine the specs are intake .010 and is exhaust .020.
Checking at TDC makes sense. I will do that.
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  #28  
Old 05-18-2019, 05:13:49 PM
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Smile Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Quote:
Originally Posted by Andrew Mackey View Post
I would check your valve specs again. Typically, valve clearances are checked with the engine at TDC Compression.
Thank you for the suggestion. Checked valve clearance at TDC. Intake is better at .010 and The exhaust is better at .021 so both are within limits.
Iíll check back in when I get a new carburetor hooked up.
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  #29  
Old 05-19-2019, 02:10:08 PM
Tracy T Tracy T is online now
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

As for leak down test, I am hesitating to spend the 55 bucks for a tester for a one time use.
if you have compressed air, aircompressor or air tank this should cost $10-$15. fitting thats made to screw into the sparkplug hole the air hose connects to it. any auto parts store should have it, tool is used to replace valve stem seals on a OHV engine without removal of the head. I would just about bet you could pull it off with a common blow valve.
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  #30  
Old 05-22-2019, 09:42:46 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

My carburetor number is p.n. 631251A.
Will a p.n. 631068A work on my Tecumseh?
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  #31  
Old 05-23-2019, 11:14:44 AM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Great idea! Thank you
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  #32  
Old 05-24-2019, 08:16:56 AM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Quote:
Originally Posted by Markoh View Post
My carburetor number is p.n. 631251A.
Will a p.n. 631068A work on my Tecumseh?
Or, Does anybody know what the difference is between those two part numbers?
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  #33  
Old 05-24-2019, 01:02:23 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Hmmmm.
Didn't realize that your model carburetor had become so hard to find.
Might have to go the PIA route after all and revive the one you have.
Lots of very tiny holes to clean out.
Here's a pretty good video:

https://www.youtube.com/watch?v=4W9Y3JC1yDY
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  #34  
Old 06-03-2019, 07:31:37 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Redid carburetor again. Cleaned all holes as shown in video. No start. No change.
Engine spins fast with spark plug removed. With plug installed it spins and stops, spins and stops, as if the starter cannot overcome the compression.
I gave up and bought a can of starting fluid. Shot it in the carb. Turned it over. Still Nothing. Nada. Zilch. Don't get it. Never had this kind of problem with an engine and I have worked on several.
Spark plug has spark. I saw it sparking for every rotation of the engine and felt it when I was holding the spark plug boot and spark plug. I felt it tingle my hand and my leg where my leg was touching the tractor. No laughing allowed.
I went through the trouble shooting checklist in the repair manual. Checked all the wiring with ohm meter. All checked good.
The condition of the starter, as long as it turns the engine over, shouldn't have affect the ability to start, especially with starting fluid.
My next move is to disassemble the starter to check it out but I believe I am left with some sort of timing issues.
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  #35  
Old 06-03-2019, 08:25:51 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

I have a couple of these very same tractors.
Had a similar problem at one time.
Drove me just about nutty.
Solution was to remove the belts from the engine pulley.
1 of them was dragging and when the plug was installed the starter just did not have enough to roll engine and drag belt.
It started right up after I removed belts.
Then I sorted out the belt issue and the tractor has been awesome for 10 or so years,
Regards Chris
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  #36  
Old 06-05-2019, 11:01:31 AM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

By golly I will give that a shot! I did put a new belt on it. Did not check the transmission to see if it spun freely. After sitting 15 years there might be an issue there.
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  #37  
Old 06-16-2019, 08:29:20 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Took off belt and spun the engine. Have some problem. Turned engine over by hand. Turns what seems to me to be normal until compression then it is almost impossible to turn.

Took starter apart. Brushes and commutator look like they are new. Would post a picture of the starter brushes and commutator so experts could look at it just in case I missed something but haven't found the photo upload button.
Excellent condition of the brushes and commutator makes me wonder if this tractor has sat for many years because past owner couldn't get it started...like me.

Even though the starter spins the engine nicely with the spark plug removed, the only thing I can think of to do next is to remove the head and have a look.
Can't think of any reason why removing the head would help but don't know what to do next. Any last suggestions before I take the head off???
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Old 06-16-2019, 08:53:12 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

I think the compression release is the issue.
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  #39  
Old 06-17-2019, 07:57:32 AM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Quote:
Originally Posted by Elevated View Post
I think the compression release is the issue.
that's what I said way back in post #2.
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Old 06-18-2019, 09:03:47 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Hey Guys
This dog has been beat pretty bad, lets try something simple, remove the valve cover and stuff what ever you have, like several layers of beer can to fill the gap between the valve stem and lifter on both valves to 0 clearance to check if the comp. release is worn down, don't put cover back on, the motor will run fine without a cover, may leak a bit of oil, sure is a lot of clearance on both valves, i only run about 5 and 10, works for me, too small gage battery cables will also make problems.
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