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Engine: Kohler SV-740-s on a Cub Cadet GT1554 (2006 model).

History before failure: No trouble starting and using mower one evening (normal ops).

Shutdown was normal; it did not die while running. Would not start next morning.

(Not a gradual degrading of starting or performance.)

Found the oil was 1/2 quart low, and filled it. Suspected oil low pressure cutoff

switch, but filling did not help.

Gasoline is fresh. Checked for water, found none. No gasoline was added shortly

before failure. Tank is 2/3 full.

Battery is charged fully to 12.78v. Battery is only two months old.

Engine "cranks" strong. New starter motor two months ago.

Choke and throttle linkages connected and working normally.

Both spark plugs are new, and the spark is strong, bright white. Some folks speak of

"blue" spark being the hottest, and necessary. But, I work on a lot of engines and I

can't say that I ever see a true "blue" spark. When I see orange spark, I worry about

it being too weak.

Both spark plugs clean and dry. No indication of fuel getting into the cylinders. No

oil on plugs. Have not been able to make the plugs wet with all my cranking.

New fuel pump installed, just because I had one on hand. No help. Pump does push fuel

to the carb, as it should.

Tests done with fuel fill cap loose, to rule out air vent issue.

Carb idle adjust screw removed and compressed air blown down that port.

Carb top plate removed and compressed air blown through all ports.

Carb bowl does fill normally after emptying (emptied each time I removed the solenoid

for troubleshooting).

Fuel Shutoff Solenoid: Checked good 12v to solenoid with ignition on. Removed

solenoid and visually checked that it actuates when energized. Reinstalled, and

listened to the distinctive click when ignition switched on.

Fuel placed directly into carb throttle throat, and still not even a pop. Checked

plugs immediately after, and they were still dry.

Fuel placed directly into cylinders through plug port, and not even a pop.

I don't have a Leak Down Tester, but I can hear the compression releases at the end

of each cranking (via starter, or hand cranking the flywheel). Also, the "finger"

test for compression verifies a positive exhaust, and a positive intake of air, with

sufficient force that low compression can probably be ruled out.

I am running out of ideas, and leaning toward timing being the issue, because it has spark, but maybe not at the proper time (at end of the compression stroke).

I have not pulled the flywheel yet to check for a sheared key. However, there is no

indication of flywheel slipage when manually turning the flywheel. When I turn it,

the pistons are definitely following and seem to go through their normal 4-stroke

cycle with no slipage of the flywheel on the shaft.

Safety switches have been checked (seat, brake, PTO). If any one of these fails, I do not believe the engine would even crank. Correct?

Fuses checked okay. There are only two that I have found, mounted one each on either

side of the steering column.

I cannot check the stator output without a running engine. But, with a fully charged battery, I think there would be a hot enough spark, even if a stator failed.

Okay, with that history, what do you think the problem is, or where would you look next?

I would go after the flywheel key. You will not have any extra movement in the flywheel even if it is damaged. I suspect you will find it sheared. Why, sometimes things happen and no one knows. Have seen this before, not truly uncommon.

Thanks for your opinion. There was no catastrophic event that would support the sheared key theory. But, I agree that mechanical failures happen on their own timeline. It was probably weak from multiple prior impacts, and finally failed with the simple force of the starter engaging.

sdguinter said:

Thanks for your opinion. There was no catastrophic event that would support the sheared key theory. But, I agree that mechanical failures happen on their own timeline. It was probably weak from multiple prior impacts, and finally failed with the simple force of the starter engaging.

Click to expand...

A catastrophic event is not required to shear a flywheel key. I had a Briggs & Stratton single cylinder 17.5hp (if I recall correctly) simply backfire and it sheared the key completely. The flywheel moved approximately 20 degrees on the crankshaft. Turns out there is a new and improved flywheel key that is stronger than the standard aluminum key and is recommended for this particular engine.

Lawnranger said:

A catastrophic event is not required to shear a flywheel key. I had a Briggs & Stratton single cylinder 17.5hp (if I recall correctly) simply backfire and it sheared the key completely. The flywheel moved approximately 20 degrees on the crankshaft. Turns out there is a new and improved flywheel key that is stronger than the standard aluminum key and is recommended for this particular engine.

Click to expand...

Lawnranger,

I just finished pulling the flywheel, and the key was NOT sheared. Also, it looked like a steel key, not aluminum.

After re-assembly, I rechecked the spark, and it still sparked white. So, I gave it a shot of starting ether in the carb throat. Still not even a hint of trying to fire or backfire.

So, now I am in need of a new theory for why she won't fire. I am wondering just how "hot" the spark has to be, and what would cause a weak spark. Any ideas along those lines?

Thanks.

The three things an engine needs to operate properly are proper air/fuel ratio, compression and sufficient spark at the right time. You are the eyes and hands here so you will have to tell me which one is missing.

To test for sufficient spark, you can take an old, working spark plug and clip the side electrode off and use that as your spark checker. You will have to find a way to ground the threaded part of the plug but by doing so you will cause the coil to put out more voltage to jump the larger gap. If your coil can jump the larger gap then spark should be sufficient.

I spray some carburetor cleaner in the intake and crank the engine to check for fuel delivery problems. If the engine starts, runs and dies suddenly then the fuel system is suspect.

The final check is compression. Most engines should have over 100psi. Briggs & Stratton does not publish a specification for compression but rather they use a leak down tester which regulates compressed air into the cylinder via the spark plug hole with the engine at TDC. You simply listen for air coming out the intake, exhaust or oil fill/dipstick. A small amount of leakage is acceptable but if you hear rushing air there is a problem somewhere.

Now that you verified that you have spark at the right time, you need to make sure you have sufficient spark. I use a special spark checker that has an alligator clip on the side to attach to ground but you can make your own for cheap.

Once you verified you have sufficient spark you will need to check compression/leak down and this requires some special equipment so you may need to take it to a shop or buy a compression gauge and leak down tester. You mentioned you sprayed some starting ether and that stuff is very flammable so any little spark should have ignited the ether. Just for grins, try some carburetor cleaner and see what happens.

However, you mentioned in your original post that the plugs are not getting wet with fuel so maybe your anti-backfire solenoid is not letting fuel through the main jet??? Maybe the main jet is plugged? How long did you use the choke when trying to start the engine?

You may have one of the engines that defies all laws & logic. Kidding aside, something is missing and you will have to find out what's missing.

Would you something stupid because we are grasping at straws. As I reread your entire post, it seems that what continues to come up is that your plugs are always dry after each test. To me it indicates not enough fuel is getting to the cylinder. I'm wondering if we don't have enough compression to ignite a small amount of fuel. Would you try flooding the cylinders with fuel, put in way too much. Reinstall the plugs and try to start the engine. The reason I would like to see what happens, as the fuel is reduced we should get the right ratio for the engine to fire, if we have spark and compression. I am thinking that we may not have enough compression to ignite the fuel. Compression also means suction on the intake and if it is low fuel may not be drawn into the cylinder. I know that this is not the most logical thing to do, but we need to rule out something. I don't think our problem is how hot the spark is, but could be a problem with timing, if it is not compression.

Lawnranger & Rivets,

Yes, compression is where I am headed. From the beginning, I thought the dry plugs definitely suggested not enough suction to draw fuel into the pistons. The finger-in-the-plug-hole test indicates some compression. But is it enough? I can kluge my compression tester into a leak-down tester, so I will do that tomorrow.

Now the problem with no compression is: How did this happen overnight? The engine ran fine the evening before. 14 hours later the no-start appeared. Am I to believe that the rings just evaporated overnight?

And, why not even a pop with ether sprayed into both plug ports (which bypasses all the carb or solenoid possibilities). That would mean really low compression!

So, if the leak-down shows really low compression, I will tear it down and do whatever is necessary, because even if it started at this point, I would want to restore the power and get it ready for another 400 hours.

Good thing I have a Grasshopper and another small tractor to get the 4 acres of grass around the house and barns mowed! I can take my time with the Cub Cadet.

I'll let you know. Thanks again.

I'm thinking a sticking valve, which could occur over night. Check out this website, it just might help you.

http://www.kohlerengines.com/onlinecatalog/pdf/sm_32_690_01.pdf

Rivets said:

I'm thinking a sticking valve, which could occur over night. Check out this website, it just might help you.

http://www.kohlerengines.com/onlinecatalog/pdf/sm_32_690_01.pdf

Click to expand...

And that would show up with a compression/leak down test which has not been performed yet.

Performed the leak-down test this morning. Cylinders held pressure real well. Too well, it turns out. Meaning, a valve was not opening. Pulled the covers and found bent push rods on both intake valves. Placed order for new ones because once they're bent, they are soft, and likely to bend again. BUT, I wanted to get her running and rule out other issues, so I straightened the bent rods, re-installed, adjusted all to to proper clearance, and buttoned up everything. She started right up and ran real well.

Lessons Learned: Should have pulled the valve covers sooner for a look. It's so easy, but I was chasing all the electrical causes for a no-start.

Now for the reason, I believe, the two rods bent: The last task the tractor had was pulling a small sprayer trailer. I was doing that at very slow speed while spraying, with the throttle all of the way back in idle position. I believe I pulled her into the barn in that condition, and turned off the ignition.

Well it turns out that Kohler warns against that very condition. They say always have the throttle at mid to high RPM for shutdown. The purpose is to make sure the anti-backfire fuel shutoff solenoid activates to close the high speed jet and prevent the backfire.

So, I got lucky this time. It was frustrating to troubleshoot, but not expensive to fix. I view a bent rod as a good thing, over a hole in a piston, or broken piston rod, or a number of other things that would be much more expensive to fix.

So, case closed. Thanks for your interest and suggestions.

Glad ya found it. I had a Briggs engine bend a rod and it was still under warranty so I called Briggs on behalf of the customer and they said it was due to bad fuel leaving deposits causing the valve to stick in the guide and would not warranty the repair.