Small Engines (Lawn Mowers, etc.): Tecumseh governor, flat blade, phillips head

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Hi there Eric,
I have a Tecumseh 3.8 mower about 10 years old.
I have no real problem mowing the lawn with it except the only thing that I'm not sure is normal or not is that the engine never stays at the same rev range, the governor is always altering the revs even if the mower is just sitting on concrete doing nothing, the governor is always working making it rev up and down constantly. I can see the spring and the external lever always moving, but it still seems to mow the lawn fine. is this normal? is it possible that maybe i should adjust the mixture screw? please advise me of what and how I should handle this situation.

Regards,

Steven Carter
Answer -
Do you have any mixture screws on the carb?  Most Tecumsehs do not.

The surging is not normal.  99% of all surging problems are caused by a dirty carb or leaking intake gaskets.

If cleaning the carb with compressed air and ensuring the intake gaskets are sealing does not fix the problem then the next step is to check the engine RPM with a tachometer.

If the engine RPM is correct then I'd replace the governor spring and reset the engine RPM with a tachometer.

Since you have an older engine I would also check the headbolt torque.  I've found headbolt to loosen causing similar symptoms.

Let me know what you find.
Eric

Thanks for your reply Eric,
the carby has a star headed screw with a spring on it not a phillips head or flat blade. I thought this was the mixture screw? From what you say it must be an idle screw. Is this correct?
I will do as you have advised. The only thing I dont know how to do is to check if the idle speed is correct. I do own a car tune up meter where I connect the positive lead to the negative side of a cars coil and earth the earth lead to give me rpm readings on a car. how do i do it with my mower? could you please advise me.

Thanks again,

Steven Carter.
PS
do i need to buy a complete carby kit including intake gaskets before i pull it apart for cleaning? i also believe i have a mechanical governor, is this correct?
Answer -
You do have a mechanical governor.  As for the carb kit you usually only need a bowl gasket, inlet needle and seat.  Check the float, we occasionally find bad floats.

We prefer to replace the gaskets but you can use some Permatex #2 non-hardening gasket sealer to ensure the gaskets aren't leaking.

Check the headbolt torque...should be 200 inch pounds.  We've found loose headbolts, usually on the exhaust valve side that will cause weird problems that appear to be governor or carb related.

Let me know what you find.
Eric

hi Eric,
well so far all ive managed to do is check the headbolt torque and all is well there. i do have a mixture screw underneath the top plate on top of the carby which has the other very small adjustment screws. i screwed the mixture screw all the way in and brought it back out one and a half turns but it made no diff whether it was screwed all the way in or unscrewed all the way out, the engine surged as it normally does, no diff at all so i left it one and a half turns out. is this ok?
i'm about to take the carby off and clean it as you said. can you please give me step by step instructions?
i noticed there is a screw under the carby bowl, what do i do with this eric? do i need to unscrew it to get the bowl off? then if i do, does it then have to be re-adjusted?
is there an easy fix to cleaning these carbys out such as putting a carby cleaner additive in the fuel tank instead of all this trouble?
sorry for all the qs Eric, but ive never done this before.
thanking you,

Steven Carter

Answer
The adjustment on the bottom of the carb is the high speed mixture.  This could definetely cause the surging.

If the engine will run at high speed then I would adjust the high speed jet.  Try turning it out and see how the engine reacts.   If you loose track on the adjustment you can turn the high speed needle in and then back out 1 & 1/2 turns.

The auto tach will not work.  Most small engine tachs are inductive types but Briggs sells one that works off vibration; this one is a pain to use.

As for the carb kit I would just buy the whole kit.  It will come with a lot of parts, some of which you will not need but is will have all the parts to repair your carb.

Pay attention to how the governor linkages are connected before you remove the carb...draw or take a picture if needed; this will help when you put it back together.

As for the carb, it is very simple once you remove it from the engine.  There is basically a float and a few needles and a bowl gasket.  Use an air compressor to blow out all the carb orfices.   There will be orfices in the carb blow and inside the carb throat.  Blow all these out and then reassemble the carb and install it.

Let me know what happens.
Eric