Engine Knock After HeadGasket Replacement (Lawn mower)

Matthew K

Matthew K

Line Up and Wait
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Broke Engineer
I know lawn mowers are almost child play compared to those on here with an A&P, but you have to start somewhere I suppose.

Okay, so about a month ago I replaced a blown head gasket on my lawnmower (John Deere Zero Turn). I haven't used it much since then, considering it's not quite spring yet, but I have noticed a knock that I don't remember being there before. However, I can't hear it unless I'm relatively close to the back of the engine.

Considering I had to take the cylinder head off, I do suppose there is room for error on my part for new noises to occur.

One thing, I may not have the intake and exhaust valve timing perfect, so it may be possible the exhaust valve is opening with a little un-burned fuel still in the cylinder. Would this cause the knocking to occur? Short of that, anything else that could be causing it related to me replacing the head gasket? Or maybe me just now noticing old noises?

Thanks.
 
Lots of stuff could do that.

What I would suspect is that the bottom end is not in very good shape. Especially, if the engine has been overheated, the cylinder walls will scuff and the piston will fit loosely. If this is the case, the engine will knock much worse cold than warm, but that's not a positive diagnosis. With the head off, you should have had a really good look.

If you suspect a valvetrain problem, the knock will occur at HALF RPM (if it's a 4 stroke engine). Any valvetrain problem will do that.

If you really mean it's detonation (i.e., not rhythmic), your spark timing is suspect or you may be burning oil.

But before panicking too much, is there a valve/rocker adjustment you have to do as part of assembly? But that's more of a tap than a knock.

And take a stethoscope to the case and see where the knock is coming from. You can almost certainly distinguish rod knock from valve tap that way.
 
MAKG1 said:
Lots of stuff could do that.

What I would suspect is that the bottom end is not in very good shape. Especially, if the engine has been overheated, the cylinder walls will scuff and the piston will fit loosely. If this is the case, the engine will knock much worse cold than warm, but that's not a positive diagnosis. With the head off, you should have had a really good look.

If you suspect a valvetrain problem, the knock will occur at HALF RPM (if it's a 4 stroke engine). Any valvetrain problem will do that.

If you really mean it's detonation (i.e., not rhythmic), your spark timing is suspect or you may be burning oil.

But before panicking too much, is there a valve/rocker adjustment you have to do as part of assembly? But that's more of a tap than a knock.

And take a stethoscope to the case and see where the knock is coming from. You can almost certainly distinguish rod knock from valve tap that way.
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To my knowledge the engine has never been overheated, and I'm the only one who has every run the lawn mower since it was bought brand new. As far as the cylinder walls go, it looked to be in just about perfect condition, no red flags. Their is a rocker arm adjustment, that's what I meant about the timing possibly not being perfect, not sure if it's a tap or a knock, I'll have to listen to it a little more. I don't have a stethoscope :D
 
Matthew K said:
To my knowledge the engine has never been overheated, and I'm the only one who has every run the lawn mower since it was bought brand new. As far as the cylinder walls go, it looked to be in just about perfect condition, no red flags. Their is a rocker arm adjustment, that's what I meant about the timing possibly not being perfect, not sure if it's a tap or a knock, I'll have to listen to it a little more. I don't have a stethoscope :D
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You can use a tube stuck in your ear, but cheap automotive stethoscopes cost around $5, and are surprisingly useful tools for this type of diagnosis.

Rocker arm adjustment has nothing to do with valve timing. It's how far the valves are pushed in and out, not when. Valve timing would come into play if you're not sure you got the camshaft and chain/gears/belt (whatever it has) installed correctly. When adjusting rockers, make sure you use the correct procedure, as getting them too tight is a lot worse than (a little) too loose.
 
How old is the fuel? Perhaps its detonation now that your now making compression and perhaps the fuel is old from sitting? Otherwise I would look for mechanical noises from the valve train.
 
What type of engine is it? A side valve Briggs is pretty dirt simple and there isn't much you can adjust mechanically with the valve train.
 
How old is the engine? If it is old, run hard and put away wet, this could be piston slap. However I think this is unlikely.... -Skip
 
G-force said:
How old is the fuel? Perhaps its detonation now that your now making compression and perhaps the fuel is old from sitting? Otherwise I would look for mechanical noises from the valve train.
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The fuel is about 4 months old if that...so there shouldn't be detonation related to fuel age.
Sac Arrow said:
What type of engine is it? A side valve Briggs is pretty dirt simple and there isn't much you can adjust mechanically with the valve train.
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Well, now that you mention it, its a side valve Briggs and Stratton :D
Skip Miller said:
How old is the engine? If it is old, run hard and put away wet, this could be piston slap. However I think this is unlikely.... -Skip
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Engine has less than 100 hours. Not the original engine however, the original gave out at 50 hours. This new engine has been what I'd consider very well taken care of in terms of regular maintenance, I have no reason to believe it is piston slap.
MAKG1 said:
Rocker arm adjustment has nothing to do with valve timing.
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Like @jaybee said, I'm pretty sure it does...
Joey4420 said:
No stethoscope, you can also you a long screwdriver and put the handle to your ear.
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Thanks, I'll make sure to try it tomorrow.
 
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