Tecumseh stopping after 30 seconds

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Flowerpots
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Tecumseh stopping after 30 seconds

Looking for some inspiration instead of exasperation:

Have a Tecumseh Lawnmower (35)  with a 3.5 hip engine and Tecumseh carb, problem , stops running after 30 seconds. Now like me , thinking usual, fuel problem, electrical problem, compression problem....so carb stripped cleaned, new needle valve and seat and for good measure a new float, still same problem.

Thought maybe air lock , so tried running without petrol cap on, no joy still stops after 30 seconds, next changed spark plug still stops, checked coil volts okay. Tested compression fine no problem with compression. So drained petrol tank cleaned and petrol pipe, cleaned carb again still no joy. It primes and fires on second pull but just dies after 30 seconds.I have an identical mower so transposed fuel tank, carb, coil onto the mower and same thing happens just dies after 30 secs interestingly my other mower works fine with all the items taken from the non runner. Thought maybe one of the valves may sticking, so checked them there fine. It gives you the impression it is running out of fuel, checked the float bowl when it has stopped and has fuel. Once it stops you can usually start it again but again dies after 30 secs. I'm thinking maybe air leak somewhere , but where I don't know because my other mower runs with the carb, coil, fuel tank and air filter housing from the non runner.

so anyone any words of wisdom 

 

Lee Smallwood
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I have limited mechanical

I have limited mechanical skills, and if all runs on the other mower then all you have mentioned is fine, so fuels getting in, it fires so spark seems ok, could be intermittent spark, but one thing I would look at is exhaust port and exhauste itself, if it gets in but can't get out it'll stop too. Could be choke related. Just giving you ideas, change the good running electrical system over to the bad machine if  possible to rule that out first. Wristpin would be the idea bloke to help, he's a wizard I'm sure. 

Flowerpots
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Thanks lee

Thanks lee

already changed over the coils so rules that out, coil on the mower that keeps on stopping works on the running mower. Carb hasn't a choke, Just push primer bubble to start.

hortimech
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First, Tecumseh never made

First, Tecumseh never made lawnmowers, they just made engines, it is further complicated by the fact that Aspera made Tecumseh engines under license in Italy. Aspera was later taken over by Tecumseh and became Technamotor before finally disappearing.

So, which engine do you have ?

A photo will help in identifying it and for a bonus, we should be able to tell you what the lawnmower is as well ;-)

Flowerpots
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Flowerpots
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Engine of the mower

wristpin
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Possibly tight valve

Possibly tight valve clearances going to  zero when hot . How soon after the first cold start does it fail before going into the 30 second failure cycle. What are the stone cold clearances?  Make sure that you check them with the piston a quarter inch down the bore past TDC on the power stroke. Otherwise the decompressor will interfere with the exhaust valve reading. 

Flowerpots
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Roughly fails all the time

Roughly fails all the time after about 30 seconds, doesnt matter if its a cold start or hot start, I would have to take the head off and valve cover to check valve clearances and look up the Tecumseh valve clearances, the other thought I had was the kill wire from the coil being faulty had this problem last summer on another mower ( that did run for longer about 3-4 mins then died) once replaced the kill wire it ran no problem. Will probably do this first t morrow.

wristpin
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Not much logic to a”faulty

Not much logic to a”faulty kill wire “, particularly on a 30 second time schedule;   it either has continuity or it doesn’t  - unless, of course, in changing it you have incidentally corrected a terminal touching ground or some such. 

Just a thought, immediately it stops check the spark.

Flowerpots
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Well, think I'm going to call

Well, think I'm going to call it a day ..took head off and valves out, nothing untoward with either head, valves or spring, clearance was with in range 0.05 - 0.10 when cold. 
Still stops after 30 seconds, it still feels like a fuel problem, but it will have to stay an enigma.

wristpin
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Looking back to your first

Looking back to your first post “coil volts ok”. Not sure exactly what that means. Have you swapped the coil with the runner? Also, you haven’t answered my suggestion to check for a spark IMMEDIATELY it stops

hillsider
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Run the engine with the air

Run the engine with the air filter and its cover removed and when the engine starts to falter either dribble some fuel into the air intake or better still puff a small amount of GT85 spray lube into the air inlet. If the engine picks up again you have a fuel problem. If it does not fire up again but continues to die you have likely answered Wristpins question re the lack of a spark.

Re my choice of GT85 spray lube other products may work but I have found some that do not fire in an engine due to their chemical make up. Products containing Ether (Easy Start to name one) will also work but I find them rather harsh when used on small air cooled engines.

Flowerpots
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There is spark immediately

There is spark immediately after it stops, checked  the ohms ( volts) reading on the coil with a multi meter and it is fine, and also swapped it with the coil on the running machine and it still stops.

Flowerpots
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Okay tried your idea, about

Okay tried your idea, about dribbling some fuel into the air intake but still dies, as I said in my original post, we also swapped the carburettors around but it still dies and interesting the carb on the one that dies works also on the good mower. We do actually fix small engines for a living so we have a lot of knowledge on 4 stroke engines, just this has us stumped.......

 

 

 

hortimech
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Just a thought, you do have a

Just a thought, you do have a blade fitted, don't you ?

As you fix these sort of things for a living, then you will be aware just how hard it is to fix things from a distance, nothing beats standing next to mower.

 

wristpin
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There is spark immediately

There is spark immediately after it stops, checked  the ohms ( volts) reading on the coil with a multi meter and it is fine, and also swapped it with the coil on the running machine and it still stops.

ohm = resistance. volts= "pressure"  - not the same!

As you have not mentioned contact breaker points, I assume that you have electronic ( breakerless) ignition.  Proper testing of an electronic system usually requires a specific test using apparatus that simulates the flywheel to trigger the coil - not something found on a multimeter.  Tecumseh used to sell a test unit which, even back in the 90s was, if I remember correctly, about £400. I only ever saw one on a service course as they were difficult to justify, even in a busy workshop shop. It was more cost effective to try a new coil ! 

Flowerpots
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Thanks, Yes I have electronic

Thanks, Yes I have electronic ( breaker-less) ignition, no points, we have tried a brand new coil this morning and still no joy. Possible could be the flywheel, but probably not going to investigate further, it's a 20 year old mower and we have spent too much time on it, even for coriosity. 
 

Thanks for everyone's comments but I think it's going to remain a mystery...

wristpin
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A pity that you are so far

A pity that you are so far away, I’d love the challenge!