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I've got a Craftsman M270 mower that's 1 year old, and recently been having trouble keeping it running. It's extremely hard to start, but after a couple minutes of cranking it'll usually start sputtering until it fires up properly. It'll then run flawlessly for around 8 minutes or until I hit a solid object or tall grass. Once it stalls it'll send white smoke out of the exhaust, and if I start cranking it directly after it stalls it'll continue smoking from the exhaust for probably 10-15 seconds. If I continue cranking it after stalling it backfires from the carburetor every few revolutions, sometimes small pops, sometimes big enough flames to see from the back of the mower. I've been working on this mower for a couple of weeks now and here's what I've checked/done:

Engine:

• The compression reads at 140psi
• I changed the oil. I made sure to check if the crankshaft was split in half(wouldn't be the first time) while I had the engine apart, and it was fine.
• The pcv line & muffler aren't clogged.

Fuel:

• I've cleaned the carb & soaked it in carb cleaner for a couple of hours multiple times, the main jet, pilot jet, and emulsion tube aren't clogged.
• The float is allowing fuel to flow properly.
• It'll run the full 8 minutes with or without the fuel cap so I doubt it's the problem.
• The auto choke is closed when the engine is cold and open when the engine is warm. The governor is able to open and close the throttle.
• I did notice rust in the bottom of the bowl from when water accumulated in the tank after leaving the mower outside for a while.
• Fuel is flowing from the tank to the carb with no problem.

Valve timing:

• I've checked and reclearanced the valve rockers to 0.005".
• I pulled the engine from the mower and inspected the cam, it's in time and the lobes are in the proper locations.
• The push rods aren't bent.

Ignition timing:

• I've replaced the spark plug
• The coil tested right
• I reclearanced the coil gap to 0.005", there's a bright white spark from the plug.
• The flywheel key isn't sheared so I'm pretty sure the spark timing is right.

The only consistent thing I've noticed from this mower is after heating the tip of the plug with a torch and throwing some gas into the cylinder through the spark plug hole, it seems to fire up pretty reliably, however that only works while the engine is cold. Putting literally anything flammable into the carb throat makes it send 6 inch fireballs back through the carb. I've never had an engine be this finicky and I'm almost out of things to try. I'm wondering if the coil is failing once the engine has warmed up, or maybe if the rust I saw in the carburetor bowl could have something to do with it. Any information anyone has is very appreciated.

The cause is likely to be either the timing is off, or way too lean a mixture. Most mowers are .010" on the air gap but I doubt if your .005" contributes to the problem. Spray a little flammable carb cleaner around the intake manifold and see if it changes the way it runs.

• I pulled the engine from the mower and inspected the cam, it's in time and the lobes are in the proper locations.

How do you know this.

hlw49 said:

• I pulled the engine from the mower and inspected the cam, it's in time and the lobes are in the proper locations.

How do you know this.

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bent, burnt or sticky valves - note intake must be open or timeing off when flamess come out carb - period. the other factor is a little harder to determine - a migrated valve guide that prevents the valve from proper travel or the rocker arm from proper travel, as the guide can migrate in or out.

8X65MUA 159cc Engine?

A couple possibles:
Blown head gasket between cylinder and push rod galley. I know compression is reading good but still possible.
Ignition coil's electronics going bad throwing off spark timing.

When you checked the flywheel key was it smooth on all 4 sides? You may have possibly hit a small stump and distorted the key as it tried to shear and knock it out of time. Try a new key..

As stated I agree that you have a lean mixture problem, which can be caused by many things. Coil air gap set at .005” isn’t helping your problem, as when the engine warms up you need a long spark to fully burn all fuel. I would be removing the shroud and when you get the engine running spray all gasket areas and the entire intake manifold with carb cleaner. If you see the RPMs change you now have identified an air leak. Plastic manifolds do have a tendency to crack or warp as the heat up.

sgkent said:

Spray a little flammable carb cleaner around the intake manifold and see if it changes the way it runs.

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Tried this today, it didn't change anything. All of the gaskets on the intake manifold are in once piece as well. There is one piece of plastic that connects to the cylinder head that I cant reach with the carb cleaner as the big metal "carburetor heat shield" is in front of it. Next time I'm working on it I'm going to see if I can remove that plastic and look for cracks or warps as Rivets stated.

StarTech said:

Blown head gasket between cylinder and push rod galley.

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I've had the head off a few times now and the gasket doesn't have any cracks or damage to it.

dana a said:

When you checked the flywheel key was it smooth on all 4 sides? You may have possibly hit a small stump and distorted the key as it tried to shear and knock it out of time. Try a new key..

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The key looked to be fine although I did notice a slight scratch on the slot in the flywheel, but I don't think it is big enough to affect timing. I filled the hole with some jb weld steel stik as a temporary way of testing if it would have an affect on how the mower ran, but there wasn't any difference.

hlw49 said:

• I pulled the engine from the mower and inspected the cam, it's in time and the lobes are in the proper locations.

How do you know this.

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It's pretty hard to see in this, but this is the best picture I took from when it was apart. There's sort of teeth located in between the lobes on the camshaft and on the inside of the plastic connecting the lobes, so I imagine if the lobes were to separate from the camshaft itself the teeth wouldn't align with each other. The time itself is marked on the cam gear & crank gear.

Something I forgot to mention in the original post, the carb backfiring mostly only occurs after the engine has warmed up, which leads me to believe it is a lean condition as well. I'll reset the coil gap to what it was before as tightening it didn't seem to help. I highly doubt the valve timing is the issue because everything functions as it should while running, and I can watch the valves open and close properly with the valve cover off. I tested the intake valve for leaks and couldn't see any, although my test wasn't done with the kinds of pressures it would actually be subjected to as the engine turned. Carb cleaner does seem to be helping it cold start whether its through the carb or the spark plug hole, so I'm considering eating the $20 and ordering a new carb just to see if it'll fix it. Thank you all for the advice on this and I'll update once I have more info.

did you reuse the head gasket? If it leaks it can let in air that causes a lean mixture. The cause of a lean mixture is that the fuel molecules are so far apart it causes the fuel flame front to burn so slowly that it extends the burn time until it can spit back out the intake. An intake valve staying open when the engine is hot would cause it too. I would recheck the valve clearance to spec again. Also look for a broken valve spring or guide out of place. Also, if a valve guide was so worn out that the valve wobbled greatly, it might not seat properly every time, causing misfires.

After tearing apart the head to look at the valves & valve guides, I think I found at least one of my problems. The entire valve system was fine, valves weren't wobbling in the guides, no broken springs or bent valves.



Some pretty significant gouging there, and another small gouge on the opposite side of the cylinder. My current theory on this is prior to the carb being cleaned it was leaning out enough to cause detonation and overheated the cylinder. I don't know why it's still backfiring through the carb after cleaning it out so if anyone has a better theory on what's going on and if it could be related to the gouging I'd like to hear it.

sgkent said:

did you reuse the head gasket?

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Yes I did and I noticed today it was deteriorating around the opening for the pushrods, so I imagine it could have been leaking there.

A new crankcase is only around $70 so I might pull the engine back apart and check if the piston would be reusable.

What I see in that bore is evidence that an engine has been run hard and hot, maybe too lean. I still think you have a problem with the fuel supply or a vacuum leak.

sgkent said:

What I see in that bore is evidence that an engine has been run hard and hot, maybe too lean. I still think you have a problem with the fuel supply or a vacuum leak.

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Agreed. The engine is now occasionally seizing so I'm going to see if this is something that can be taken care of by warranty, and in the likely case that they won't take it I'll just be replacing the mower. FYI if it weren't for your last post about the head gasket and valves getting me out there to take it apart again, I was going to start throwing money at replacement parts without ever identifying the gouges in the cylinder wall, so I appreciate you saving me the time and money there

To me those gouges are probably caused by dirt ingestion and will not be covered by any warranty. Time to look for a new unit or replacement engine.