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I have a small riding mower with B&S single-cylinder OHV engine, which I've owned from new (about 15 years).

A year or two ago it developed a problem that seemed insignificant - the engine would turn over slowly before firing, almost as though the starter motor was tired. Nothing went wrong until one day, during use, the engine simply cut out. I took it apart and found the alloy pushrod had bent and snapped (not sure which came first).

I removed the cylinder head, inspected the engine, found nothing much wrong, fitted a new pushrod and plastic guide, put it all back together and did another season's work without issue.

This year, I dragged the mower out of hibernation, used it for half an hour and then exactly the same happened - the engine stopped, and the new pushrod has snapped.

Does anything look amiss in here, or has the guide pushed up?

Thanks.

That head looks different than all the ones I have seen...... It does look like it is out of place though...... Try to find one like it and compare it ..... Or go to a shop and get a second opinion...

It won't be long before someone else will chime in.............

Brother in law had a Poulan rider with a Briggs OHV engine. Lost power a couple months after he bought it, took to dealer, loose valve adjustment. Tech told him those needed adjusted every year on those mowers. He didn't listen, next year the engine quit, push rod broke and jammed the crankshaft. So I guess just get used to adjusting the valves on a regular basis? Or maybe put a jam nut on the rocker adj. nut.

tom3 said:

Brother in law had a Poulan rider with a Briggs OHV engine. Lost power a couple months after he bought it, took to dealer, loose valve adjustment. Tech told him those needed adjusted every year on those mowers. He didn't listen, next year the engine quit, push rod broke and jammed the crankshaft. So I guess just get used to adjusting the valves on a regular basis? Or maybe put a jam nut on the rocker adj. nut.

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Mine is a British-market equivalent of the Poulan, so I guess it's the same engine.

I never adjusted the valves for the first ten-or-so years but lately have checked them regularly.

Something I forgot to mention was that when the engine stopped this time, the valve clearances seemed to have closed up - so much that, with a new pushrod in place, there's no gap for the shim.

Aluminum pushrod is getting abrasive wear. Something is contaminating the oil causing it to war the guide and pushrod. What does the oil look like when you change it. Possible partially blown head gasket causing carbon deposits in the oil. You can also use the steel rod instead of the aluminum rod if you want, and will not cause any issue other than will take longer to wear thin.

ILENGINE said:

Aluminum pushrod is getting abrasive wear. Something is contaminating the oil causing it to war the guide and pushrod. What does the oil look like when you change it. Possible partially blown head gasket causing carbon deposits in the oil. You can also use the steel rod instead of the aluminum rod if you want, and will not cause any issue other than will take longer to wear thin.

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Head gasket looks fine (was renewed with the last pushrod) and the oil appears (looks and smells) unremarkable.

I'll certainly try a steel rod instead - thanks.

DRWDRW said:

Mine is a British-market equivalent of the Poulan, so I guess it's the same engine.

I never adjusted the valves for the first ten-or-so years but lately have checked them regularly.

Something I forgot to mention was that when the engine stopped this time, the valve clearances seemed to have closed up - so much that, with a new pushrod in place, there's no gap for the shim.

Click to expand...

That's usually a sign of the valve receding into the head. In some repair manual, there should be a maximum installed height for the valve spring.

The collet keeper on the exhaust valve looks boogered up,
At first glance.

I'll re-read this all again.

So which valve's pushrod is dying?

Measure the height of the valve guides, they should be the same.
What is happening is the pushrod is dropping into the engine then the cam comes around & breaks it
The push rods jump off because the valve lash gets too big
The plastic guide that is there need to be replaced , where is the other one ?
Do not reuse the old head gasket.
There is way too much oil in your head so most likely the valve stem oil seal on the inlet valve is gone ( none on the exhaust ).
Were both of the lash caps on the end of the valve stems ?
If one dropped off
1) it will be in the engine somewhere
2) puts the rocker at an angle where it can hit the valve spring which can cause the push rod to drop off.

Fish said:

So which valve's pushrod is dying?

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It's on the inlet side. The exhaust has the steel rod.

bertsmobile1 said:

Measure the height of the valve guides, they should be the same.
What is happening is the pushrod is dropping into the engine then the cam comes around & breaks it
The push rods jump off because the valve lash gets too big
The plastic guide that is there need to be replaced , where is the other one ?
Do not reuse the old head gasket.
There is way too much oil in your head so most likely the valve stem oil seal on the inlet valve is gone ( none on the exhaust ).
Were both of the lash caps on the end of the valve stems ?
If one dropped off
1) it will be in the engine somewhere
2) puts the rocker at an angle where it can hit the valve spring which can cause the push rod to drop off.

Click to expand...

I've measured the heights and they're identical (which surprised me, because they didn't look it).

The plastic guide from the inlet side had popped off the plate. It was floating around in the rocker cover, along with the bit of pushrod and lash cap.

The exhaust lash cap (I mistakenly referred to them as shims) was still in place, and the clearance appeared to be normal (I didn't check it).

On the inlet side, I decided to check the clearance before disassembly - I compressed the spring and dropped in a new pushrod. It was then tight against the rocker (little or no clearance) even without the lash cap in place. This is what made me believe the valve clearance had closed up.

I recall setting the clearances a year or two ago, and noticing the rocker on the inlet valve was sitting at an unusual angle. But the clearances tightened up okay, so I assumed to be acceptable. Is this actually my problem?

I've attached another photo here - this taken prior to disassembly but with a new pushrod trial-fitted to check the clearance.

IT is sounding more & more like a loose valve guide.
Give each one a LIGHT tap using a drift to prevent damaging the end.
If the inlet moves even the tiniest amount then it needs to be repaired.

Walt has a system that seems to work fairly well, or just replace the head.

DRWDRW said:

I've attached another photo here - this taken prior to disassembly but with a new pushrod trial-fitted to check the clearance.

Click to expand...

Looks like the rocker retainer nuts are different between then intake and exhaust. Could the larger looking nut be causing binding of the intake rocker leading to excessive wear on the intake aluminum push rod.

ILENGINE said:

Looks like the rocker retainer nuts are different between then intake and exhaust. Could the larger looking nut be causing binding of the intake rocker leading to excessive wear on the intake aluminum push rod.

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One of the nuts was replaced a few years ago when the thread failed. It's the correct B&S part - or so I was told - but I could swap it to the exhaust side and see what happens?

bertsmobile1 said:

IT is sounding more & more like a loose valve guide.
Give each one a LIGHT tap using a drift to prevent damaging the end.
If the inlet moves even the tiniest amount then it needs to be repaired.

Walt has a system that seems to work fairly well, or just replace the head.

Click to expand...

A light tap didn't budge it the guides at all.

Unfortunately, a ham-fisted idiot (me) making a heavier tap with the drift did move the inlet guide and damaged the end... :ashamed:

The valve guide should not exceed 1/8 inch above the head casting. It is quit normal for the aluminum push rod to wear thin and bend when not rotated end for end periodically on this engine when adjusting valves which should be done every year per B&S but a couple years at most. Replace it when the push rod shows heavy wear. As said above, many have replaced the aluminum push rod with the steel one with no apparent problems. Note that the later version of this engine uses a different assembly support method eliminating this wear problem.

No clearance when new push rod installed - check that the push rod is seated in the recess in cam follower, tappet.

Walt Conner

Tinkerer200 said:

The valve guide should not exceed 1/8 inch above the head casting. It is quit normal for the aluminum push rod to wear thin and bend when not rotated end for end periodically on this engine when adjusting valves which should be done every year per B&S but a couple years at most. Replace it when the push rod shows heavy wear. As said above, many have replaced the aluminum push rod with the steel one with no apparent problems. Note that the later version of this engine uses a different assembly support method eliminating this wear problem.

No clearance when new push rod installed - check that the push rod is seated in the recess in cam follower, tappet.

Walt Conner

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Thanks. The thing with this engine is that the pushrod was new last season, after suffering a similar break last year; until then it had been fine for a dozen years or more.

I've now replaced the whole cylinder head with a good second-hand part, and it seems to be working perfectly. Interestingly, the rocker (which was previously at an awkward angle) is now level.

DRWDRW said:

Thanks. The thing with this engine is that the pushrod was new last season, after suffering a similar break last year; until then it had been fine for a dozen years or more.

I've now replaced the whole cylinder head with a good second-hand part, and it seems to be working perfectly. Interestingly, the rocker (which was previously at an awkward angle) is now level.

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Probably due to a problem with the covering on the metal push rod support, missing or worn thru allowing push rod to ride on the bare metal. I expect a new rub block would have cured the problem.

Walt Conner

Tinkerer200 said:

Probably due to a problem with the covering on the metal push rod support, missing or worn thru allowing push rod to ride on the bare metal. I expect a new rub block would have cured the problem.

Walt Conner

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You're right - when the previous pushrod snapped, it had worn the metal guide plate and the black plastic square. This time, though, all three parts - pushrod, plate and plastic support - were new.