Totally off topic. Lawn mower troubleshooting

[Bigred0216]

So, I have been tinkering around on my roommate's riding lawn mower.
It is a Briggs & Stratton vertical shaft 16 horsepower single cylinder, if that makes any difference.

I was told that while mowing last summer, the mower backfired, stopped running, and would not restart after that.

the following are the symptoms

Will not start

Good spark

seems to turn over too fast

Carb is in good working order and was cleaned

while cranking, fine mist of fuel is being expelled from carburetor.

My best conclusion so far is that the intake valve is burned.
My theory is that fuel and air is being drawn into the combustion
chamber normally during the intake stroke.

But as the intake valve closes and the piston comes back up to tdc on the compression stroke, the compressed fuel/air mixture is escaping past the intake valve, through the intake port and back out the carburetor.

If anyone here has any other conclusion based on the information provided, please feel free to share.

 

[rustybronco]

Like anything else, take a compression test and compare the numbers to what the specs call for.

I'd be worried about the seems to turn over too fast... sounds like little to no compression.

 

[marvinsc]

Like anything else, take a compression test and compare the numbers to what the specs call for.

I'd be worried about the seems to turn over too fast... sounds like little to no compression.

Yes it does

 

[Bigred0216]

I agree that it probably has very little or no compression.

Couldn't the low/no compression problem be caused by a burned valve? If the valves are not sealing properly, there will not be good compression, correct?

 

[Guest]

My vote would be for the flywheel woodruff key, either shearing or nearly so.

 

[Bigred0216]

My vote would be for the flywheel woodruff key, either shearing or nearly so.

care to elaborate?

 

[barnbiketom]

STUCK exhaust valve.


trust me lol

smack the engine with a BFH.. if you hear a CLUNK then I was right..

 

[Bigred0216]

STUCK exhaust valve.


trust me lol

smack the engine with a BFH.. if you hear a CLUNK then I was right..

i pulled off the valve cover and and rotated the engine by hand. the valves are moving normally. if the exhaust valve was stuck open, the valve stem would not remain in contact with the rocker arm (except, of course, during valve overlap when the intake and exhaust valves are both fully closed).

 

[Guest]

care to elaborate?

Lose or damage the key, lose the timing.

 

[barnbiketom]

a sheared key would NOT make it loose compression...

 

[barnbiketom]

a sheared key would NOT make it loose compression...

you need to take a REAL compression reading with a Gauge.... it 'seems?" to turn over too quickly..


lets find out!

 

[Bigred0216]

I have not had the pleasure of playing around with small engines before.

from what I understand so far, the engine has gear-to-gear timing.
So if the woodruff key shears, or partially shears, it will jump timing because there is either nothing driving the camshaft or the camshaft is so far retarded in relation to piston travel that the valves are opening and closing way too late. Am I warm? I guess it would help to understand the theory of operation of this engine before I go trying to repair it lol.

 

[rustybronco]

If the woodruff key sheers it would still have spark (unless the engine still uses points), but the spark timing would be way off.

Logical if someone hit something large enough with the mower blade.

 

[TooManyToys]

A key usually shears when the blade hits an immovable object. Loss of correct timing but compresson would still be OK. Need to check compression first and see where that goes before I would work on pulling the flywheel.

 

[Bigred0216]

We are talking about a riding lawn mower that uses a belt to drive the mower deck, not a push mower that uses the blade as the flywheel. would the belt not slip on the pulley before damaging the key?

 

[Guest]

A back fire can shear a key .

 

[barnbiketom]

i pulled off the valve cover and and rotated the engine by hand. the valves are moving normally. if the exhaust valve was stuck open, the valve stem would not remain in contact with the rocker arm (except, of course, during valve overlap when the intake and exhaust valves are both fully closed).

I'm willing to bet it has a burned valve..

get a cheepo compression gage!!!

another possibility is a bad head gasket!!!

 

[bonanzadave]

Got a borescope ? Or a cheap dental camera ? Pull the plug and look at the piston. Hole ?

 

[barnbiketom]

The Key that we are speaking of is the FLYWHEEL MAGNETO KEY and has button to do with valve timing or compression... merely ignition timing.
Also. Some movers have no magneto.. so instead of confusing you.. I would rather know what motor it is etc.
You can't guess or just replace parts Willy Billy. Its easy to repair and diagnose correctly.

Check Co.pression with a Gage if it seems low.

And don't rule out a flooded condition making the rings cleaned of oil and no compression that way

 

[Dave_17954]

sounds to me like a sheared flywheel key. backfiring usually is a sign of bad timing. a possibly partially sheared key would make it back fire and now a no start means a completely sheared flywheel key. What happens is it would still fire under a partial shear but not all of the fuel would be burnt and and some would get dumped into the exhaust which would then cause it to explode/backfire. A backfire can cause a shear to the key and I did have a neighbor shear his key on a rider when just mowing. its the same reason anti-fire selonoids were installed on the bowls of carbs. when you turn off the key you turn off the electrical power to the engine but what still happens if you shut it off at high speed is the piston would still have compression pulling fuel into the cylinder and then pushing it out into the muffler. raw gas and hot muffler equals a big boom and I'm sure we all been there done that