Engine seized?

[prichard]

I have an old beater MTD with a Briggs OHV engine. It's been sitting for 6 months or so while I got around to changing drive belts. Now it won't start. I was thinking junk battery since no one sells a decent battery for these things any more. Well, battery that I know works and jump won't start it.

So figure maybe starter's weak. Before I invest in one I pull the plug and spin the flywheel by hand and it only rotates about 120 degrees.

Valve maybe stuck? I can see one rocker and push rod moving when I remove valve cover but not the other. I'm thinking engine needs to rotate further fir other one to move.

Any ideas short of tearing the whole engine apart?

 

[tomplum]

From your description one of your valve guides has come loose and migrated forward holding the valve from opening. Or you have dropped a push rod for a similar reason. So if you tipped your mower to install your belts, the engine could be hydraulically locked- meaning the cylinder is filled full of oil / fuel.

 

[prichard]

From your description one of your valve guides has come loose and migrated forward holding the valve from opening. Or you have dropped a push rod for a similar reason. So if you tipped your mower to install your belts, the engine could be hydraulically locked- meaning the cylinder is filled full of oil / fuel.

I tipped motor maybe 15 or 20 degrees.

So, I'd have to pull head?

 

[4570Man]

Pull the spark plugs and see if it rotates.

 

[ericm979]

If you pulled the plug and rotated it then the motor's not "hydro locked"- the oil or fuel would have come out the plug hole.

If the crank is stopping then it's probably not a stuck valve. They usually stick open not closed, and it'd be very stuck indeed to keep the crank from turning (and the pushrod would be bent now and valve train damaged from the force of the starter motor). I'd remove the head and the pushrods then try to rotate the crank. If it stops then it's nothing to do with the valve train and the next step is to pull the cylinder if it's a separate cylinder engine.

Sometimes you can gently rotate it against whatever's stopping it and feel/listen for where the blockage is.

 

[RandyT]

Something interfering with the piston should turn almost 360 degrees not 120. I would be looking to more external issues, like something jamming the flywheel or something underneath preventing engine from turning like a belt, pulley or some other obstruction. Also could be an issue where the flywheel magnets for the charging system are jamming against the stator.

 

[prichard]

Duh!

I started out thinking it was the belts I just replaced then saw slack in the primary one and decided that wasn't it. I think I started chasing ghosts. I'll go back to investigating belts.

Something interfering with the piston should turn almost 360 degrees not 120. I would be looking to more external issues, like something jamming the flywheel or something underneath preventing engine from turning like a belt, pulley or some other obstruction. Also could be an issue where the flywheel magnets for the charging system are jamming against the stator.

 

[jaxs]

There is no standard reason for this happening but if we are supposed to guess,I say valve seized in guide and piston might have bent valve on first attempt to start. I would do more investigating before tearing into the engine. When rotating flywheel by hand and it stall's,how many degrees will it rotate in opposite direction and does other rocker move?

 

[prichard]

There is no standard reason for this happening but if we are supposed to guess,I say valve seized in guide and piston might have bent valve on first attempt to start. I would do more investigating before tearing into the engine. When rotating flywheel by hand and it stall's,how many degrees will it rotate in opposite direction and does other rocker move?

I agree there's no standard reason and was looking for guesses. It is the same 120 degrees rotation in both directions.

I think randy nailed it and got me back on track.

Thank you all for ideas, hopefully I can look at it more later today.

 

[prichard]

Nothing on the bottom end binding. All belts remain free turning. There's a fair bit of acorn shells under the cowling so I think next step if to pulls the flywheel and see if mice or whatever left a surprise for me in the alternator.