JD LX178 blowing black smoke

J

jsean

Joined
Dec 30, 2019
Messages
260
Location
USA
This is a 1992 LX178 I’ve had for a couple of years. Today while mowing, I switched from going reverse to forward. Just as I started going forwards, it randomly spat out black smoke and shut off. When I restarted it, it runs really rough and blows out black smoke.

Oil level looks fine. I’ll check the antifreeze tomorrow morning once it’s cooled down. Oil and coolant are checked before use each time and both were level and looked normal today as well.

I replaced the Mikuni carb with an Amazon chinesium one and no change. It’s down on power, idles rough and wants to die to low idle. PTO engages the blades barely. Throttle is adjusted correctly per FSM.

Not sure what’s going on here. Any thoughts and guidance on troubleshooting would be greatly appreciated.
 
Black smoke sounds like it has suddenly started running excessively rich. Check your fuel cap and make sure the little hole in the middle isn't plugged and also check the in-line fuel filter.

I'd check the carburetor and make sure nothing has come loose inside. My LX178 has a problem with fuel dilution because the carburetor is worn out and the float doesn't always engage the needle seat and allows fuel to flow when not running. This is caused by the fuel cap not venting and allowing pressure to build. It doesn't help that it's such a huge PITA to get to the carburetor for adjustment or removal.

And as always... CLEAN/REPLACE THE AIR FILTER and use the little foam pre-filter.
 
FowVay said:
Black smoke sounds like it has suddenly started running excessively rich. Check your fuel cap and make sure the little hole in the middle isn't plugged and also check the in-line fuel filter.

I'd check the carburetor and make sure nothing has come loose inside. My LX178 has a problem with fuel dilution because the carburetor is worn out and the float doesn't always engage the needle seat and allows fuel to flow when not running. This is caused by the fuel cap not venting and allowing pressure to build. It doesn't help that it's such a huge PITA to get to the carburetor for adjustment or removal.

And as always... CLEAN/REPLACE THE AIR FILTER and use the little foam pre-filter.
Click to expand...
Those are some great points. I swapped the carb out purely to see if it’d make any difference. As of now, I don’t think it’s necessarily the carb that’s the problem since it runs exactly the same between the Mikuni and Chinese one.

I wonder if I can run it for a few seconds without the gas cap and then again without the air filter? I have the foam filter - the main filter is still very clean and the foam filter is a bit dirty but I didn’t think it was awful.

Another thing I noticed is that it’s got a thin layer of oil/fuel in the air intake manifold up into the air filter cover box. Does that indicate anything?
 
jsean said:
How do I verify that this is indeed the problem?
Click to expand...
With a compression gauge. I assume this is a twin cylinder Kawasaki engine?

Over time, the cooling fins get blocked by debris, gets worse if there is any sort of oil weep around the fins, then one of the cylinder heads overheats and the exhaust valve seat looses its interference fit with the head. Sometimes it doesn't come all the way out, just loosens up, which will make that cylinder have low compression.

A yearly part of owning a Kawasaki engine is removing the metal surround and cleaning the fins of debris.
 
Thastinger, these engines are V-twin, liquid cooled. They don't have cooling fins but rather a fairly large radiator that sits atop the engine with a fan mounted underneath the radiator.

Jsean, I think the thin layer of oil in the intake tract is simply from the crankcase vent. Mine has that too.

Definitely run it (no blade engagement or excessive dust production) with the air filter off and the gas cap loose. Do each one separate to see if one thing makes a difference.

I think somethings restricting the intake air.

How much did you pay for your Chinese carburetor? I need to replace mine too and I hate spending the cash on the Mikuni. My mower is 31 years old and it owes me nothing so I try to keep it running. John Deere is impressive with how they still provide parts for these things.
 
thastinger said:
With a compression gauge. I assume this is a twin cylinder Kawasaki engine?

Over time, the cooling fins get blocked by debris, gets worse if there is any sort of oil weep around the fins, then one of the cylinder heads overheats and the exhaust valve seat looses its interference fit with the head. Sometimes it doesn't come all the way out, just loosens up, which will make that cylinder have low compression.

A yearly part of owning a Kawasaki engine is removing the metal surround and cleaning the fins of debris.
Click to expand...
I clean up the metal mesh and the plastic vent cap that sit atop the radiator prior to each mow. Is it spotless with 0 fuzz? Not really - but it’s clean for the most part. I doubt that the marginal loss in the cooling surface area is causing an issue, but I could be wrong.
 
FowVay said:
Thastinger, these engines are V-twin, liquid cooled. They don't have cooling fins but rather a fairly large radiator that sits atop the engine with a fan mounted underneath the radiator.

Jsean, I think the thin layer of oil in the intake tract is simply from the crankcase vent. Mine has that too.

Definitely run it (no blade engagement or excessive dust production) with the air filter off and the gas cap loose. Do each one separate to see if one thing makes a difference.

I think somethings restricting the intake air.

How much did you pay for your Chinese carburetor? I need to replace mine too and I hate spending the cash on the Mikuni. My mower is 31 years old and it owes me nothing so I try to keep it running. John Deere is impressive with how they still provide parts for these things.
Click to expand...

So I ran out and tried running it without the gas cap on. Made no difference. Tried to run it for a few seconds without the air filters, no change. Now I did noticed that it was spewing out a bit from the intake manifold. I tried to take a picture - it’s a decent amount of fuel coming out as a spray and mist (not sure if the picture is clear - it’s the steam coming out of the center intake manifold hole).

I noticed that when I pull the plug wire on the right cylinder it simply cranks and won’t start. When I pull the plug on the left cylinder, and the right cylinder is plugged in - it starts as it has since last evening.

So I removed the left side plug and it was coated in fuel. I started the mower with the left plug out and it seemed to run better for a few seconds and then settled back to the black smoke/rough idle/low power. I didn’t do this the gas cap loose or the air filter off though - but I don’t think it’d make much of a difference?

Could a faulty coil on the one cylinder cause my issues? Got a premium Chinesium one off Amazon on the way to test my theory.

The Chinese carb was $23 off Amazon, longevity is TBD. The replacement Mikunis are really expensive and not what I’m going to splurge given the constant minor issues I have on my LX.
IMG_8653.jpeg
 
To check for a bad coil, just swap the coils side to side. If the problem moves with the coil, then that's it.
Gosh... after everything that you've already done.... I hope it isn't something as simple as a failed spark plug.
 
Last edited:
I have a '96 LX 178.

Sometimes the choke linkage gets "stuck"... or the throttle cable where it attaches to that metal plate near the carb and plastic intake.

I raise the hood and take a flat head screwdriver and poke each one towards the 'driver's side of the machine.

It frees it up when I do that.

The symptoms are the exact same as what you are describing when it happens to me. (Black smoke... barely running... low power). Typically happens the 1st few mows of the season when the machine has been setting a while.

It's worth a try.
 
mrsilv04 said:
To check for a bad coil, just swap the coils side to side. If the problem moves with the coil, then that's it.
Gosh... after everything that you've already done.... I hope it isn't something as simple as a failed spark plug.
Click to expand...
Funny you mentioned spark plugs - I didn’t even think about them. Went and just swapped them out. No dice, still running like crap. Maybe smoked a tad bit less? But still definitely low on power and stalls out when I lower the throttle about half way through.

Here’s what the old plugs look like (left and right as on the engine):

IMG_8655.jpeg
 
genynnc said:
I have a '96 LX 178.

Sometimes the choke linkage gets "stuck"... or the throttle cable where it attaches to that metal plate near the carb and plastic intake.

I raise the hood and take a flat head screwdriver and poke each one towards the 'driver's side of the machine.

It frees it up when I do that.

The symptoms are the exact same as what you are describing when it happens to me. (Black smoke... barely running... low power). Typically happens the 1st few mows of the season when the machine has been setting a while.

It's worth a try.
Click to expand...
When you mean towards the driver, you’re referring to pushing it towards the back of the mower correct? I just tried tapping/gently pushing all the linkages towards the back of the mower - no change. Interestingly enough, I got a coolant leak now. Any thoughts why it randomly popped a coolant leak by the radiator?

Another thing to note is that it cranked for a long time after the spark plug change to get started. And just as it was catching, I think there were a couple of puffs of white smoke (couldn’t tell it was dark and overcast outside), which then reverted back to the black smoke and the rough idling.

IMG_8657.jpeg

O
 
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Oh, and a general note, when I pulled the old spark plugs I took a look inside. On both sides, the pistons looked wet and grimy. Not steam cleaned like a head gasket leak. But it was black, a bit sooty and shone in the flashlight of my phone.

I’ve never inspected a piston before on a car or a lawn mower, so not sure if that means anything?
 
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