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Garden Tractors and Mowers Vintage lawn and garden tractors, mowers, snow blowers, snowmobiles and other old machinery that is driven by small air cooled engines.

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Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back


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  #1  
Old 05-05-2019, 09:34:14 AM
Markoh Markoh is offline
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Question Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Good morning. Been working on my 1973 sears st12 i bought for $150 with a nice condition deck. Sat for 10-15 years in shed. Gas tank is goner so rigged a plastic one. Appears to have original sears tires. Cleaned carb and impulse pump. Changed oil, replaced fuel lines, spark plug and drive belt. New 420CCA
battery. Engine spins over nicely without spark plug installed. It has electronic ignition so I did ground the spark plug while it turned over. Nice spark. Installed spark plug. Engine only turns over until the compression stroke. Struggles through compression stroke and turns until the next compression stroke. Suspected timing / Woodruff key issue so removed flywheel. It came off easier than I thought it would. Woodruff key is good. (Noted a generator type set up under flywheel. Hopefully it is to charge the battery. Interesting that, after 15 years sitting, the lights work) Replaced flywheel and engine shrouds / covers. Same starter difficulty when turning over engine is still there.

Since it does turn over, has fuel and spark, why doesn't it start? I do remember at my first start attempt a puff of white came out of the exhaust. Not anymore, though.
Here's the mystery (to me). I put my hand over the carburetor to choke the engine and the starter overcomes compression stroke and turns nicely with the spark plug installed. Remove my hand from the carburetor intake and the starter struggles to turn the engine over. Does this happen because there is no air coming in to compress?
Is the starter just too weak? How to tell?
I've read there is a compression release mechanism in the Tecumseh engine that operates at low RPM's but, if it is working, why does the starter struggle? If it isn't working, is there a way to tell without disassembling the engine?
When the engine does turn over, there is a fine mist of fuel coming out of the carburetor in between strokes. The spark plug has fuel on it when when I remove it. Did internet search and found some post where blow back occurred with engine running but, after reading looooong postings on valves, compression releases, mud dauber plugged exhaust, carbon build up, etc., no one seemed to actually find the cause of the blow back.
Any ideas sure appreciated.
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Old 05-05-2019, 10:50:38 AM
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Brian Lynch Brian Lynch is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Is the engine a HH120 Tecumseh? They have a compression release on the camshaft. It bumps The ex valve open a bit on the compression stroke. If that release mechanism has went south, the only fix is a new camshaft assembly. Best part is you can change it without removing the engine from the tractor. Buy a good used or NOS cam, sump cover gasket and oil seal before hand. Make sure you get the correct parts for the engine by using the model, type and serial number to look up. I have a 1971 SS12 that had the same problem. I replaced the cam in mine and it only took about 1-1/2 hrs start to finish. That included flushing out the crankcase with solvent. It's a fantastic tractor. It never ceases to amaze me as to what it will pull. It has the 3 point cat 0 hitch on back. It gets used to plow, pull, mow, push snow, etc. A real workhorse. Get yours fixed and you will use it more than you can possibly imagine. I wouldn't sell mine on a bet.
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Old 05-05-2019, 02:46:05 PM
Markoh Markoh is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Thank you for the response Brian.
Not sure if it is a HH120. Where would that be on the engine?
The model number is 143.612102. It is a 1973 and I’ll bet it has a compression release.
Thank you for the information and encouraging word about changing the cam. I was a-feared of that but believe I can do it now.
Before I dive into changing the cam, shouldn’t the engine start if I have fuel, spark and I can spin it fast enough? Would the failure of the exhaust valve not raising for decompression prevent starting if I can spin it fast enough?
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Old 05-05-2019, 04:43:19 PM
CharlieB CharlieB is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Yes, the engine should start, even if the compression release isn't working. Getting the engine started might even limber-up that stuck compression release.
You might want to try a shot of starting fluid. If you get her to pop, that will tell you to suspect the carb. There are critical areas in those carbs that are impossible to reach for cleaning. I used to spend hours trying. Then I learned that ebay has replacement carbs for cheap.
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Old 05-05-2019, 05:28:43 PM
Markoh Markoh is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Shot carb cleaner that says it is highly flammable into carb. No start.
Will buy starting fluid next trip into town.
It has the original exhaust so I put my hands over the exhaust and it is definitely blowing air out under pressure.
For a moment or two it seemed like it wanted to start but maybe it was wishful thinking.
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Old 05-05-2019, 09:19:10 PM
Bruce Dorsi Bruce Dorsi is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Before tearing anything apart, you could check the valve clearances.

If someone has ground too much from the end of the exhaust valve, the compression release may be minimized or ineffective.

Check the exhaust valve clearance while the intake valve is fully open. ...I don't remember the proper clearance from memory, but someone else may know the recommended setting.
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Old 05-06-2019, 07:34:54 AM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Thank you for the response Brian. Is the only way to check valve clearance would be by removing the head?
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Old 05-06-2019, 10:26:58 AM
Bruce Dorsi Bruce Dorsi is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

The HH120 is a flat-head engine.

The valve clearances are checked at the lower end of the valve stems, which are located behind the flat plate behind the carburetor.

The specs are: Intake valve clearance = .010" (.009"-.011" is acceptable)
Exhaust valve clearance = .020" (.019"-.021" is acceptable)

Measure the exhaust clearance while the intake valve is fully raised.

Measure the intake clearance while the exhaust valve is fully raised.

By any chance, is your engine # 143.612012, not #143.612102?

I am e-mailing a Tecumseh parts list and Tecumseh Repair Manual to you, so check your e-mail.
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Old 05-06-2019, 10:52:27 AM
Tracy T Tracy T is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

hard to diagnose over the net, but it sounds like you may have two things going on. #1 it sounds like it is flooded, sparkplug is wet. #2 starter is not spinning fast enough and lacks power. have your battery checked, I have had brand new ones fail. if it checks out the starter may be on its way out. I have had the magnets come loose inside the starter, they are just glued in and can be repaired with some good glue like 3m weather strip glue. be careful that glue can get messy!
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Old 05-06-2019, 06:58:10 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Thank you, Brian, for the info. Nothing else is working. Will have to check the valves next. Checked model number for the engine and it is 143.622102. Does that affect the effectivity of parts or repair manuals?

---------- Post added at 06:58:10 PM ---------- Previous post was at 06:51:15 PM ----------

Thank you, Traci. I'll check the spark plug again. It wasn't wet until I smothered the carb with my hand and I noticed it doesn't blow back until I smother the carb. So, it isn't getting enough gas until I choke it and floods when I do? I did shoot carb cleaner into the carb that says it is highly flammable without any effect. Bought a spark tester. Better check to see if that is ok to use with electronic ignition.
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Old 05-06-2019, 07:31:01 PM
mmcdonald mmcdonald is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Let me start by saying I am no small engine or Tecumseh guru.......(and it is late and I'm beat) BUT if you're getting air coming out of the carburetor then you have an intake valve open(or burned) on the compression stroke(or possibly exhaust). Maybe I'm too tired but an intake valve being open at the wrong time(or all the time) seems the most obvious answer to me. If there is a problem with the decompression it should be on the exhaust valve which would not cause the intake valve to be open I don't think??? I would pull the plate as Bruce suggested and while you're at it checking clearances watch the movement of the valve train as you rotate the engine. If it was multiple cylinders I would suggest comparing the valve movement to another but that won't work on this one. You might blow or suck(mechanically, humanly or otherwise) on the manifold(not the carb) with the intake valve in a closed position of the cam and see if you have air going past it. If a light bulb comes on I'll post again. Good luck.
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Last edited by mmcdonald; 05-06-2019 at 07:34:15 PM. Reason: more
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Old 05-06-2019, 08:34:54 PM
Tracy T Tracy T is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

i know the last poster was tired, but he gave sound advice. a compression gauge would answer several questions. they are not expensive and you may even be able to rent one. advance auto rents tools now, i have rented some tools and my experience is you pay what the tool cost and if returned in good condition you get your money back. pretty much free use of the tool. Oriles has a similar plan.
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Old 05-07-2019, 12:23:56 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Thank you Bruce for the pdf. files.
It seems to me narrowing down to an exhaust valve issue. Considering it has sat for so long a sticking valve would be a reasonable issue.
I'll pick up a compression tester this afternoon.
I can see the valve closest to the flywheel (I assume would be the intake valve) through the spark plug hole so will watch it as I turn it over. If memory serves from working on old cars, just because I can see it goe up and down doesn't mean it isn't sticking.
I am one of "those" who have to write or talk to think.
It would have been good to do a compression test in spite of deciding it had good compression because it was so difficult for the starter to turn the engine over with the spark plug installed and a new car size battery hooked up.
Stop assuming, start testing would be a good motto.
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Old 05-08-2019, 05:44:23 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Watched intake valve move up and down through the spark plug hole and it occurred to me to try one more thing before disassembling to measure valve clearance in case there was some gunk making the valve stick. I turned the motor over until the exhaust valve was all the way open. Shot some Liquid Wrench down the valve shaft and let it sit.

Did compression test. After four turns it has 65-70lbs. Seems low so I shot a bunch of WD40 into the spark plug hole to see if compression would increase and it actually fired off once or twice and white smoke came out the exhaust. So repeat and it fired twice again. I can repeat it but that is all I'm getting. This only happens if I can get it to spin rapidly by smothering the carburetor. When I smother with the carburetor with my gloved hand the glove gets fuel all over it. Other than fuel is clearly in the carburetor, not sure what that indicates. If I attempt to start it without smothering the carb, I get the blub-blub slow turn over and white smoke comes out the carburetor.
If you are wondering if I am avoiding disassembling the engine, you are right. I think I am ruined by having had had a nice shop to work in where I worked but may have to do the shade tree mechanic thing.

I am wondering if I should clean the carb again.

I now have problem with starter that I didn't have before. The starter turns the engine over 3-4 times and then it becomes disengaged and just spins until I release the ignition key.

The ignition switch has become intermittent so I'll order another.
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Old 05-08-2019, 07:40:47 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

WD 40 isn't an engine fuel.
For about 15 bucks, you can remove the carburetor variable from the equation.

https://www.amazon.com/Tecumseh-CARB...=df0&hvadid=31

https://www.ebay.com/itm/CARBURETOR-...-/162950990135
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Old 05-08-2019, 08:38:15 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

You mentioned having removed and cleaned the carb once already, is that correct?

If you had the carb off, any chance you have the governor linkage connected improperly? Throttle plate should be open when engine is not running.
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Old 05-10-2019, 11:52:05 PM
Ogrebeast64 Ogrebeast64 is offline
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

I would definitely recommend making sure the valve faces and seats are not pitted/burned as others have stated. You should NOT be getting any blowback through the carb at all.

I would pull the head and the valves, thoroughly clean the valve stems, ports, lap them in properly, and check for the proper clearances before reinstalling the springs.

Another thought. If the crankcase vent goes through the carb, then disconnect that tube and try cranking again. If the blow back disappears, then you have excessive blow-by past the rings.

EDIT: I've been looking through my 17th edition of the small engine service guide here, and the numbers you gave(143.612102) just aren't adding up. The codes listed in my manual stop at 143.61112 and skip to 143.614012. The latter number refers to an ECH90 engine, while the former number refers to an H30 engine.
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Old 05-11-2019, 08:15:16 AM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

If you suspect leaky valves do a leakdown test on it. If you don't have access to a formal tester you can do it the crude way. Make an adapter for the spark plug hole out of an old plug. Make it to accept an air hose.

Get the engine to tdc on the compression stroke and devise a way to lock the crank so it won't turn. Hook up the air at say 40-50 psi.

Listen at the oil filler for rushing air. Weak rings.
Listen at the carb for air. Bad intake valve.
Listen at the muffler. for air. Bad ex valve.
Listen around the head for air. Bad head gasket.

There ya go, three things tested by one 30 second test.

With a formal tester the gauge is calibrated in percent. The standard for these testers is to set them to 0 which provides 60 psi of air pressure. Connecting it to the engine then shows the amount of leakage with ~10% being a general borderline condition. Well made engines will often show only 2%.
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Old 05-11-2019, 04:16:58 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

Thank you CharlieB. A new carburetor at 15 bucks is a no brainer. Hello Amazon.

---------- Post added at 03:54:31 PM ---------- Previous post was at 03:49:13 PM ----------

Thank you Scooter Bob. Checked governor for free movement after the first time I move the carburetor. I’ll check governor linkage to ensure throttle plate is open

---------- Post added at 04:14:18 PM ---------- Previous post was at 03:54:31 PM ----------

Thank you Ogrebeast for your suggestions. With the low compression readings and blow back, the valves are the most likely suspect.
No crank case ventilation through carburetor.
Your research is correct. The model number is 143.622102 not 143.612102 making it a HH120.

---------- Post added at 04:16:58 PM ---------- Previous post was at 04:14:18 PM ----------

Thank you Oldstuff. Too many years since doing a leak down test. Forgot all about that. Neat trick with a spark plug. I will get a leak down tester next time I’m in town.
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Old 05-16-2019, 06:49:39 PM
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Default Re: Sears ST12 Tecumseh starter difficulty mystery and carburetor blow back

The valve clearances are checked at the lower end of the valve stems, which are located behind the flat plate behind the carburetor.
The specs are: Intake valve clearance = .010" (.009"-.011" is acceptable)
Exhaust valve clearance = .020" (.019"-.021" is acceptable)

Measure the exhaust clearance while the intake valve is fully raised.
Measure the intake clearance while the exhaust valve is fully raised.

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?
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