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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.

Garden Tractors and Mowers

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
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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 06-29-2019, 11:38:36 AM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

https://www.smokstak.com/forum/showthread.php?t=148084
https://www.smokstak.com/forum/showt...=114607&page=2
See if any of this helps you
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Old 06-29-2019, 04:55:45 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

https://www.amazon.com/Performance-T...DTHQ06PKRRNJPB
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Old 06-29-2019, 07:12:48 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Quote:
Originally Posted by CharlieB View Post
copy and paste,
The product is easy to use and durable
• The product is manufactured in Taiwan
• Easy installation
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Old 06-29-2019, 08:43:02 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

I got outsmarted by the computer.
The ratchet mechanism link in post #56 was supposed to be accompanied by a message that said, "Engines of this size can be started with one of these ratchets, socket wrench accessories, and a 1/2" drill. The socket wrench engages the big nut that holds the flywheel on." Prevents Model T kickback.
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Old 06-29-2019, 10:13:56 PM
keith1944 keith1944 is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

The starter is of the "permanent magnet" type and will lose the magnet charge over time. It'll spin the engine fine without the plug but won't have enough energy to spin it over against the compression.
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Old 07-20-2019, 12:45:41 PM
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

first - Thank you for all the inputs!
Second - I ain’t given up. See last sentence.
Georgia ss - Go kart starter that’s a great idea.
Brian - thank you for the information and the pictures. Very useful. Not quite visualizing how the compression release works. Do you know if the exh. valve rides on the pin when in compression release mode?
Tracy T - Fly wheel housing has definitely been removed. Paint on all other engine bolts appears to not be disturbed. Starter is still on my suspect list even though it looks new on the inside.
Charlie B - Thank you for that link! Looked a good bit and didn’t find anything like that. I Did attempt to turn the engine over with my Kobalt drill motor. No dice. Couldn’t hold onto it. Came close to hurting myself. Won’t try that again.
Spent most of morning looking for a Combination spark plug compression release. Apparently no one has designed one that works.
Uglyblue - thank you for the link. Looks like similar issues. Good info. especially if I tear the engine down.
Keith1944 - didn’t know starter was a permanent magnet type. I suppose a new starter is the only answer to a weak magnet.
With .027 shim in the exhaust valve, the engine did fire off for a second or two. Didn’t continue to run it because I didn’t want to burn the exhaust valve. Without a shop and proper tools I loath getting into the engine.
My next step is to get the engine to turn over safely

---------- Post added at 12:39:24 PM ---------- Previous post was at 12:31:41 PM ----------

Part of last post was not posted....
Next attempt is to get the engine to turn over -safely- using the starter and drill motor (with ratcheting adapter) at the same time. Hopefully, together they will overcome compression.
Thank you all again!

---------- Post added at 12:45:41 PM ---------- Previous post was at 12:39:24 PM ----------

Looking at go kart starters helps me realize I need a good way to hold the drill motor from torqueing my wrist.
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Old 07-20-2019, 10:00:08 PM
Glenn Ayers Glenn Ayers is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Have ya tried a bigger .. better battery ?

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Old 07-20-2019, 11:20:02 PM
Mark Abner Mark Abner is offline
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Default Re: Sears ST12 Tecumseh Starter Difficulty Mystery and Carburetor Blow Back

Been reading along, I have restored many Tecumseh HH120 engines for suburban tractors so here is my opinion, go back to engine basics, starter stall on compression will not cause excessive fuel to blow out the carb or flood the cylinder just like faulty carb will not cause starter stall. Don't confuse ignition timing with valve timing. Ignition timing fires the plug at the proper time in relation to number of degrees of crankshaft rotation, valve timing operates intake and exhaust valves in relation to piston location. every HH120 I have done the flywheel was a bear to remove, since yours came off easy I bet someone has been in the engine. It sounds to me like your cam timing is off. Remove the side cover of the engine, you will see a punch dot on the crank gear and a punch dot on the cam gear, rotate the crank and make sure these two dots line up at some point. This way you will know for sure your cam is set correctly. Put the side cover back on, next release the valve springs and pull the intake and exhaust valves inspect the seats then lap the valves for a perfect seal. This way you know exactly that the valve timing is set correct and the valves are seating on fresh seats.
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