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I have a Lawnboy 10730, with a Kohler XT650 engine. Five years old. Seems to have developed hydro-lock, whereby I can't pull the starting cord. I remove the spark plug, and I can pull the cord fine fine (though nothing comes out of the plug hole - I would have expected some oil to shoot out). I replace the spark plug, and I can't pull the cord again. What's the fix???

If nothing comes out the cylinder with the spark plug out, I’d be looking at the valves. Remove the spark plug and valve cover. While rotating the flywheel by hand watch the valves to confirm they are opening. Let us know what you find.

Can you feel any compression when the plug is out and you pull the cord ? This will expose a lot

Thanks. I managed to get it started, and have wired the safety handle to keep it going for a while. Is that likely to eventually clear out the problem? It's running smoothly, with no smoke coming out.

Bertrrr said:

Can you feel any compression when the plug is out and you pull the cord ? This will expose a lot

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No, it pulls easily with the plug removed.

Well, that's depressing. After getting it started, and letting it run smoothly for 15 minutes, when I try to start it back up, the hydra-lock is still there. You'd think that running it for a while would clear out any problem, but that's evidently not the case.

While it's locked as you say , pull your plug and use a straight rod and see what position the piston is in, Curious if it's BTC or TDC.

Not sure what that means. Please explain carefully. What do I do with a rod?

Does not sound hydrolock as the term indicates there must be a liquid involved. It more likely a mechanical failure such as the ignition being loose, engine brake not releasing, or internal problem.

BTW BDC is Bottom Dead Center and TDC is Top Dead Center. Both refers to the piston position.

As for the rod you would inserting it thru the plug hole to find where the piston is sitting in the cylinder.

But, as I said, I was able to get it to run, and it ran smoothly for 15 minutes. You'd think a mechanical failure wouldn't come and go, and specifically go away when I pull the spark plug. Tried that several times. Spark plug in, CAN'T pull cord. Spark plug out, CAN pull cord.

You got to understand you are going from no compression load with the plug out to a compression load with the plug in. This changes to whole dramatics of the rotating engine components. I have even seen recoil starter to jam because compression load. If you have worn crankshaft bearings the crankshaft can shift position under compression load.

Since this is a 21" walk behind install the plug (leave plug wire off to prevent accidental starting) and try rotating the engine with the engine brake off with the cutting blade.

When this first happened, I assumed there was a jam at the blade, so I turned the mower on its side, saw there was no jam, and rotated the blade by hand. The blade rotated just fine.

Stick a straight rod like a coat hanger or welding rod through the plug hole and feel where the piston is , if you can roll the flywheel while doing it you can watch the piston position by seeing the rod go up and down

StarTech said:

Does not sound hydrolock as the term indicates there must be a liquid involved. It more likely a mechanical failure such as the ignition being loose, engine brake not releasing, or internal problem.

BTW BDC is Bottom Dead Center and TDC is Top Dead Center. Both refers to the piston position.

As for the rod you would inserting it thru the plug hole to find where the piston is sitting in the cylinder.

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Note sure if this info is useful, but right now, with the motor "stuck", I remove the spark plug, and the rod, when inserted reaches 2.0 inches below the plug seat. I suppose I could jostle it a bit, and get the motor started, and then try it again after that. Do I roll the flywheel by rotating the blade?

Bertrrr said:

Stick a straight rod like a coat hanger or welding rod through the plug hole and feel where the piston is , if you can roll the flywheel while doing it you can watch the piston position by seeing the rod go up and down

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Can't sit here over the net and hold your hand. You got know to work on an engine before we can help you. There basic things that you must know first and it appears that you have no idea of what you are doing.

OK, sounds like we're done. I was hoping for some useful advice here. Ther seems to be some reluctance to giving that.

No, the problem is that you are posting as if you don't have foggiest idea of how to troubleshoot an engine.

It kinda makes sense that would be something loose if you are able to get it to turn over by jostling it don't it? We can't see what is going on on your end.

I do aerospace engineering, not gas engine engineering. That's why I'm here, because I don't know how everything about how to troubleshoot a gas engine and I thought I might get some help. I haven't really heard any good explanations for my problem, though. I'll head over to a different forum, and likely just bring the thing in for repair.

Well the troubleshooting very similar to gasoline 4 cylinder horizontal opposed Piper Tomahawk engine mechanical wise...except it a much larger engine. When things comes loose they tend to bind things up. Just don't want it happening at 5,000 ft but does sometimes. My engine once started missing like mad at 4,000 ft over a major city with no place to land. That when it get very scary. At lease I had a 16:1 glide ratio and I made to the local jetplex for an emergency landing. I was already down to below Angles 1 and the local elevation is 630 ASL. Not a lot left to spare.

But I would recommend you do take the engine into small engine repair shop.

I am just thinking out side the box. But maybe a cause of a bad bearing on crankshaft that is allowing the crankshaft to tilt in a bearing and allowing the flywheel or something else to bind on something else which won't be noticeable with the plug in but would be under compression.

Had a Honda GC160 a few years ago where the top bearing failed allowing the flywheel to hit the ignition module. Currently have a Vanguard 31 hp in the shop that has a bad crankshaft bearing allowing the flywheel to jam on the ignition modules

First check the oil level and that it does not smell like gasoline.
Disconnect the sparkplug wire, strap down the the engine brake bail at the handle. Turn mower on side and try to turn engine with the blade. Can't turn? Turns gritty? Turns smooth?

Don't give up on this forum there are a lot of smart guys here and give good advise, you're problem is unique in that it does not add up logically but Lots of times this happens, my mind set is to keep an open mind when trouble shooting a problem. ( like you installed a new plug but the new one is bad ) you just never know.

Valves are out of adjustment.

Looks like it has a compression release due to parts break down. Take the valve cover off spark plug out and turn the engine over by hand. Watch the valves and see if the bottom valve bumps open just before TDC. If not it has a bad compression release.

Didn't listen the first time to check & watch valve movement. Hummmm... common sense??

Danl99 said:

But, as I said, I was able to get it to run, and it ran smoothly for 15 minutes. You'd think a mechanical failure wouldn't come and go, and specifically go away when I pull the spark plug. Tried that several times. Spark plug in, CAN'T pull cord. Spark plug out, CAN pull cord.

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I had same exact on my LT1000 tractor mower with Kohler. My small engine guy replaced the carb and problem solved.
He said the carb was dripping and causing hydrolock and he was correct.Signs leading up to this were longer than usual cranking to start it AND excessive smoking for approx 45 seconds after initial start.All better now with new carb.Charlie

Danl99 said:

I have a Lawnboy 10730, with a Kohler XT650 engine. Five years old. Seems to have developed hydro-lock, whereby I can't pull the starting cord. I remove the spark plug, and I can pull the cord fine fine (though nothing comes out of the plug hole - I would have expected some oil to shoot out). I replace the spark plug, and I can't pull the cord again. What's the fix???

Click to expand...

This engine has a compression release built into the camshaft. Sounds like its not working, possible broken spring.

Defective crankcase breather, maybe. Hydrolock may be occurring on the downstroke of the piston due to high crankcase pressure. Overfilled oil? Does the crank move at all or does it act like it's seized? Compression test, maybe?

Charlie, I once had hydro lock on my Troy-Bilt tractor from gasoline. I could not get the seat for the needle valve out to replace it so just installed a valve between the gas tank and carb. Have not had a problem since and the engine does not seem to use more gasoline than it did before.
The Kohler website pictures of the XT650 appear to show a fuel hose from the tank to the carb. There might be a way to install a valve in it to verify that it is not a leaking needle valve. Although I would think that unlikely since the OP said he was unable to restart the engine after it ran for 15 minutes.
Is it even worth fixing with an internal issue like crank bearings, etc?

First, check the oil level and make sure it's not too low or way over the full mark. Also smell the dipstick to make sure it doesn't smell like gas and that the oil is not thinned out from fuel dilution.
Now, take the three bolts off and remove the shroud from the top so you can actually see the engine brake to make sure it is fully disengaging when you squeeze the safety stop bail on the top handle.
I get a number of mowers in a year where people say they can't pull the rope or it's hard to pull and it's simply because either their cable has broken, the plastic holder come out of the handle, or something has stretched or bent so it is not disengaging the blade stop.
While it is possible to start most of these engines once they've been used a couple of years with the friction break against the flywheel, it makes it much harder to pull to get to that point of starting.
Yours is a little different because the XT series has a wire in the graphite shoe or whatever it is and that's what the kill switch is also but often that fails and these things will sit there with the handle released and the blade break grinding on the flywheel for 10 or 20 seconds if not longer because it's not making enough electrical connection to actually shut off the ignition.
Regardless, take that shroud off which is very simple with the 3 10 mm nuts unless of course one of the studs to try to tries to come out with a nut which happens about 15% of the time but you should be able to get the shout off.
Then remove the spark plug wire just for safety, and then squeeze the top handle or tie it somehow so it's in the start mode.
At this point make sure you can spend the flywheel easily by hand.
I had one just come in last month that said it was locked up and you couldn't pull it and it was actually the drive belt had slipped up above the pulley and was wedged into the bottom of the engine and it was darn near impossible to pull.
If this is fine, put it back together and you probably are looking at a compression release or valve adjustment issue.
I will say this is not a common problem as the XT's do not have a high rate of problems with compression release problems or valves being out of adjustment but both of those will certainly give those symptoms.

Diagnosis - broken compression relief valve and damaged camshaft. Replaced camshaft. Evidently not a very common issue.

Danl99 said:

Diagnosis - broken compression relief valve and damaged camshaft. Replaced camshaft. Evidently not a very common issue.

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Kohler no, Briggs single OHV yes.

sounds like your exhaust valve needs adjusting. if it is closed and not opening to relieve compression pressure, it will be hard to pull. check the engine brake first. if the engine brake is working properly, take the valve cover off, remove the spark plug, turn the engine over and visually watch both valves open and close. If it is not in the valve train, while turning the engine over feel for play in the shift to rule out bearings. Have patience with the guys in this forem. They all want to help you, however some expect you to know more about troubleshooting than you do, and that comes from a lack of patience and compassion on their part ! I hope this helps you to fixing your problem, Pete's small engine repair

Danl99 said:

I have a Lawnboy 10730, with a Kohler XT650 engine. Five years old. Seems to have developed hydro-lock, whereby I can't pull the starting cord. I remove the spark plug, and I can pull the cord fine fine (though nothing comes out of the plug hole - I would have expected some oil to shoot out). I replace the spark plug, and I can't pull the cord again. What's the fix???

Click to expand...

If no fluid is in the cylinder, it's not hydrolock. You need to first adjust the valves, then start troubleshooting other things.

I've got good service folks. Diagnosis reported above.

Have the same problem with a 2 stroke. Determined that wear from the piston TDC is cocking the piston locking it up. Suggested fix for the 2 stroke is add a thicker gasket on the base to offset the piston in a different spot at TDC. Have not tryed it yet.

Danl99 said:

I have a Lawnboy 10730, with a Kohler XT650 engine. Five years old. Seems to have developed hydro-lock, whereby I can't pull the starting cord. I remove the spark plug, and I can pull the cord fine fine (though nothing comes out of the plug hole - I would have expected some oil to shoot out). I replace the spark plug, and I can't pull the cord again. What's the fix???

Click to expand...

Might also be something wrong with the pull cord , I know sounds crazy.

Geez, guys. I TOLD you what happened. MOWER FIXED. CASE CLOSED.

DinosaurMike said:

Charlie, I once had hydro lock on my Troy-Bilt tractor from gasoline. I could not get the seat for the needle valve out to replace it so just installed a valve between the gas tank and carb. Have not had a problem since and the engine does not seem to use more gasoline than it did before.
The Kohler website pictures of the XT650 appear to show a fuel hose from the tank to the carb. There might be a way to install a valve in it to verify that it is not a leaking needle valve. Although I would think that unlikely since the OP said he was unable to restart the engine after it ran for 15 minutes.
Is it even worth fixing with an internal issue like crank bearings, etc?

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I think I will install a valve for shutoff.Especially since my mower is now doing exact same thing a season and a half later after
new carb install.Got a semi hydrolock the other day with 2-3 times tried starting and just a click, then it cranked and started and blew smoke for 30 seconds just as it did last year.Fuel tank was 99% empty so it had been dripping for a few weeks .Charlie

Danl99 said:

I do aerospace engineering, not gas engine engineering. That's why I'm here, because I don't know how everything about how to troubleshoot a gas engine and I thought I might get some help. I haven't really heard any good explanations for my problem, though. I'll head over to a different forum, and likely just bring the thing in for repair.

Click to expand...

Really guy, you need more knowledge before trying to work on this. If you can't fix a Briggs single, you need more library time.

If you think another forum is your fix, think again. Here, you have access to some of THE BEST mower repair guys on the internet. It's hard for us to help you when you don't know what we are telling you to fix or look at. The guys helping you are TOP SHELF, end of message.

crazy charlie said:

I think I will install a valve for shutoff.Especially since my mower is now doing exact same thing a season and a half later after
new carb install.Got a semi hydrolock the other day with 2-3 times tried starting and just a click, then it cranked and started and blew smoke for 30 seconds just as it did last year.Fuel tank was 99% empty so it had been dripping for a few weeks .Charlie

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I see 1/4" cutoff valves on eBay really cheap.Is 1/4"correct?? Looks just like the cutoff on my snowblower.Charlie