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Running on one cylinder with both sides getting spark and fuel. When running the cylinder opposite fuel pump makes no change when pulling spark plug but engine dies when pulling plug wire on the fuel pump side. On the dead cylinder I removed the valve rocker cover and found both push rods bent with the upper intake aluminum one broke in two.

When I manually push on the intake valve it goes in with no restrictions. However when pushing in on the bottom exhaust valve it only opens a small amount and seems to hit something. Is this normal? (Just found the exhaust valve guide is restricting the valve movement). Rotating the crank by hand while placing a screw driver in the plug hole shows the piston moving up and down.

I straightened the exhaust pushrod and inserted it onto both valve tappets and each is moving when the crank is turned by hand. (I do not intend to reuse this pushrod)

Other than replacing a cam (compression release broke) in my Deere D105, I have never been inside one of these engines. So ..... what is going on inside with the crank or cam on this engine, if anything? Is it worth going further into the internals? It is on a Snapper Model 2690872 from 2011 and the tractor is in great shape.

Last notes, when I was given this tractor, BOTH cylinder heads were completely covered with mouse nests under their shrouds so I know the cylinders were probably overheating. Also, I just went back and checked and found the exhaust valve guide is what is limiting the movement of the valve. It has come out. I am hoping all that is necessary is cylinder head work.

Thank you in advance,

Lee A

If your push rods are bent it's because something got between the piston and the valves or somehow the valve timing got out of wack. Never seen this and had an engine still run. You'll need to pull the head and valves / push rods etc. and inspect completely and also try and determine why it happened in the first place.

The exhaust valve guide has moved out place. Multiple causes among which the cylinder head overheated or carbon on the exhaust valve stem. You can't simply drive back the valve either as it will be loose and just move out again.

Just did one a couple weeks ago where a DIYer had installed a carburetor that had too lean of metering jets. Not only the intake push rod broken and the exhaust but had a blown head gasket. You will drop the sump and find the other half of the intake push rod and replace the cylinder head.

New complete cylinder head is around $170 list.

And if the camshaft is moving the lifters than the broken push rod done internal damage to the engine which you will only know what damage when open up the crankcase. But doubt it is damaged as the engine is running on the other cylinder.

StarTech said:

The exhaust valve guide has moved out place. Multiple causes among which the cylinder head overheated or carbon on the exhaust valve stem. You can't simply drive back the valve either as it will be loose and just move out again.

Just did one a couple weeks ago where a DIYer had installed a carburetor that had too lean of metering jets. Not only the intake push rod broken and the exhaust but had a blown head gasket. You will drop the sump and find the other half of the intake push rod and replace the cylinder head.

New complete cylinder head is around $170 list.

And if the camshaft is moving the lifters than the broken push rod done internal damage to the engine which you will only know what damage when open up the crankcase. But doubt it is damaged as the engine is running on the other cylinder.

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Thank you. I edited the original post. The valve tappets are moving free and that both cylinder heads were heavily covered in mouse nests so I know it had to of over heated. All parts of both pushrods are accounted for so I do not think I don't need to do anything other than head replacement on the one side.

You got lucky on the push rods then. I wasn't.

FWIW
Both cam lobes are the same
Both valves are the same length
Both valve guides are installed to the same depth
So you can usually see that one valve is lower than the other at either full lift or closed position
On top of that the exhaust valve head runs at yellow heat which is above forging temperatures so will bend very easy in use
Guide needs to be "Staked" back in the correct position as replacement guides are not available
Lots of You Tube videos on how to do this
Also check the valve seats as they can come loose as well
Generally the exhaust guide shifts & the inlet seat gets loose nut that is not a hard & fast rule

bertsmobile1 said:

FWIW
Both cam lobes are the same
Both valves are the same length
Both valve guides are installed to the same depth
So you can usually see that one valve is lower than the other at either full lift or closed position
On top of that the exhaust valve head runs at yellow heat which is above forging temperatures so will bend very easy in use
Guide needs to be "Staked" back in the correct position as replacement guides are not available
Lots of You Tube videos on how to do this
Also check the valve seats as they can come loose as well
Generally the exhaust guide shifts & the inlet seat gets loose nut that is not a hard & fast rule

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Bertsmobile-
Have you had good long term results with peening valve guides that have moved? I have peened a valve seat before and it lasted, but never a valve guide.

Only ever done 1 mower guide and it is still running fine 5 years latter on
It just depends if you know what you are doing & how to do it
As I hope you know there is more to peening than smacking a punch with a hammer
I used a long fine punch on the mower head so it went in deep from both sides
People forget that you have to deform both the head & the guide to lock it in place .

bertsmobile1 said:

Only ever done 1 mower guide and it is still running fine 5 years latter on
It just depends if you know what you are doing & how to do it
As I hope you know there is more to peening than smacking a punch with a hammer
I used a long fine punch on the mower head so it went in deep from both sides
People forget that you have to deform both the head & the guide to lock it in place .

Click to expand...

I welded a valve guide one time. It held but the mower continued to only run on one cylinder.

Method two is to remove valve guide, cut a small groove(s) around it, and install c-clips, e-clips, circlips. I have not done it, but a small engine business owner told me he has used this method successfully.

TiIngot said:

Running on one cylinder with both sides getting spark and fuel. When running the cylinder opposite fuel pump makes no change when pulling spark plug but engine dies when pulling plug wire on the fuel pump side. On the dead cylinder I removed the valve rocker cover and found both push rods bent with the upper intake aluminum one broke in two.

When I manually push on the intake valve it goes in with no restrictions. However when pushing in on the bottom exhaust valve it only opens a small amount and seems to hit something. Is this normal? (Just found the exhaust valve guide is restricting the valve movement). Rotating the crank by hand while placing a screw driver in the plug hole shows the piston moving up and down.

I straightened the exhaust pushrod and inserted it onto both valve tappets and each is moving when the crank is turned by hand. (I do not intend to reuse this pushrod)

Other than replacing a cam (compression release broke) in my Deere D105, I have never been inside one of these engines. So ..... what is going on inside with the crank or cam on this engine, if anything? Is it worth going further into the internals? It is on a Snapper Model 2690872 from 2011 and the tractor is in great shape.

Last notes, when I was given this tractor, BOTH cylinder heads were completely covered with mouse nests under their shrouds so I know the cylinders were probably overheating. Also, I just went back and checked and found the exhaust valve guide is what is limiting the movement of the valve. It has come out. I am hoping all that is necessary is cylinder head work.

Thank you in advance,

Lee A

Click to expand...

Follow up - Still have issues. Cleaned and removed all carbon from the piston head and head gasket contact surface. Checked gap on both spark armatures. Installed new #1 cylinder head assembly w new valves, a new head gasket and new push rods. Cylinder walls were smooth as h*ll with no scoring at all. Adjusted both valves to .005.

Started it up and after about 45 seconds it began to backfire from #1 side. (Note: there was no backfire when it was running on only #2 cylinder only.) Shut off engine and rechecked valve gap settings and installed new spark plug. Started it up and still backfiring after 45 seconds. I do not have any history on this tractor since it was given to me by a scrapper.

Edit: The backfire or miss in coming out the exhaust not the carb.

Also when I received the mower I know the carb was removed at some point because the choke linkage was hooked up backwards.

Any suggestions on what to address next?

If it will run on one cylinder at a time , do that and see which one is causing the backfire , back fire at the muffler indicates unburnt fuel hitting the hot exhaust, through the carb would indicate an intake valve not sealing in my opinion , you said you installed new valves - did you use lapping compound and lap them in ? This is a must

Bertrrr said:

If it will run on one cylinder at a time , do that and see which one is causing the backfire , back fire at the muffler indicates unburnt fuel hitting the hot exhaust, through the carb would indicate an intake valve not sealing in my opinion , you said you installed new valves - did you use lapping compound and lap them in ? This is a must

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The valves were installed in the new Briggs head with the springs and keepers as an assembly when I received the head. I did not lap them. All I did was install the rocker arms and set the new push rod gaps to .005 after installing the new head. I just did a compression test and both cylinders are around 150psi.

Sounds like the valves are set right and compression verifies it. This back fire happens after it dies or you shut it down or does it just start running bad and backfiring all of a sudden ? It's possible the flywheel key got sheered and threw off your ignition timing , never saw one run with even a slightly sheered key but you never know.
This may be totally unrelated to your original problem as well ,
Backfiring through the muffer indicates unburnt fuel in my opinion .

Bertrrr said:

Sounds like the valves are set right and compression verifies it. This back fire happens after it dies or you shut it down or does it just start running bad and backfiring all of a sudden ? It's possible the flywheel key got sheered and threw off your ignition timing , never saw one run with even a slightly sheered key but you never know.
This may be totally unrelated to your original problem as well ,
Backfiring through the muffer indicates unburnt fuel in my opinion .

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Thank you for the reply. The plot thickens. It backfires intermittently when running and appears to be the cylinder with the new head. The other cylinder never appeared to have a problem. Remember I received this tractor running but I have no history. A wire going to the brake safety interlock was pinched under the battery so the mower shut off then the PTO was turned on. Engine appeared to run find after I found the pinched wire. When I received it I did not know it was only running on one cylinder, cleaned it and put it in storage.

I rechecked the valve clearance and they are correct. The new cylinder head had the valve and valve springs pre installed so I did not lap the valves? Should I have done so? My You Tube research takes me to three possibilities.

First, a burnt exhaust valve. It is a new head and valves and has only run maybe five minutes, one minute at a time during trouble shooting. Too soon to burn a valve?

Second, is the cylinder running lean? I have never worked on a two cylinder before. Can one cylinder run lean but not the other? The exhaust pipe very quickly turned red!! I have just removed the carb and getting ready to clean it. Note, when I got the tractor I could tell the carb was removed before because the choke linkage was installed incorrectly.

Three, the timing is off because the key may have been moved slightly. Have not addressed this yet, only put penetrating oil on the fly wheel bolt at this time.

Looking for the needle in the haystack!!

Just worked a v-twin recently where the previous so tech put on the wrong carburetor where the jets were .05 mm each too small. IT cause the engine to destroy #2 cylinder head.

I had to rework the carburetor the engine would even off choke. Then I found the bad cylinder head with half of one push down in the crankcase.

Being your compression was good on both cylinders, I'd say your valves are ok without being lapped in, When you get the flywheel nut off you should be able to see the keyway / key profile, I've never run across one even slightly sheered and have it run but never say never in this game. You may have mentioned earlier but are both plugs gapped the same ? Just throwing stuff at the wall now but worth a shot at different or new plugs

Bertrrr said:

Being your compression was good on both cylinders, I'd say your valves are ok without being lapped in, When you get the flywheel nut off you should be able to see the keyway / key profile, I've never run across one even slightly sheered and have it run but never say never in this game. You may have mentioned earlier but are both plugs gapped the same ? Just throwing stuff at the wall now but worth a shot at different or new plugs

Click to expand...

If the muffler is glowing, there is your red alert, literally. Go through carburetor and clean thoroughly. Make sure you have good gaskets and no air leaks. That is a good start and then see how it runs.

Tiger Small Engine said:

If the muffler is glowing, there is your red alert, literally. Go through carburetor and clean thoroughly. Make sure you have good gaskets and no air leaks. That is a good start and then see how it runs.

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Thanks to everyone posting suggestions. I removed, disassemble and cleaned the carb including using torch tip wires in all the tiny ports. Internally, the carb showed absolutely no dirt at all. One of the cleanest one I have seem.

Have not installed it yet. Giving the task a couple days off to think about my next course of action.

Will post results later.

SUCCESS!! I owe everyone who replied a big thank you!! After pondering the issue for two days I pulled the flywheel. The key was sheared!! $2.18 later and it purrs. This has been a journey. I purchased this Snapper LT125 (2014) with 23.5HP from a scrapper for $25.00 last fall. It is the first V-Twin I have worked on. The following is what I went through:
1. Both cylinder heads were ENTIRELY clogged with mice nests. Never saw one this bad.
2. Purchased a battery. Found wire under the battery and moved it out when putting in new one.
3. Engine hard to start. Used starting fluid. Found carburetor installed wrong. The throttle/choke linkage was in the incorrect hole and the governor linkage installed wrong.
4. #2 Cylinder head had three stripped bolts on the valve cover. Some dummy used a power tool on it. Was able to salvage two of the bolt holes and use a bolt and nut on the forth. Set the valve clearance on #2 cylinder and new valve gasket.
5. Engine started up and appeared to run fine (I did not know it was running on one cylinder), except it died then brake pedal released.
6. Found wire going to the brake safety interlock, at one point was left under the battery pinching off the circuit inside the rubber insulation. Cut section of wire out and spliced in a piece. Tractor running and stored for winter.
7. Spring, used tractor twice and it started running horrible. No time to work on it so back into storage.
8. October, both cylinders getting spark but #1 valve cover was colder than #2. Found both pushrods inside #1 valve cover bent and broken. Installed new cylinder head and pushrods and set the valves on #1 cylinder.
9. Engine started but backfired through #1 exhaust. Re-set valves again. New plugs and it backfired less but exhaust pipe glowing red.
10. Removed the carburetor, disassembled it and cleaned everything using brake cleaner and torch tip wire. The insides were all clean but I cleaned it anyway.
11. Pulled flywheel and found key sheared and flywheel shifted 1/8". Reinstalled the carb, put in a new key and the engine purrs.

Again to all.....................THANK YOU