Kawasaki FS541V running very rich

CrowTRobot · Aug 12, 2020

Hi all!

For the past few months I've been struggling with a very rich running condition on my 2009 John Deere X300R mower. The engine is a Kawasaki FS541V with 500 hours on it. The carburetor is a Walbro model that has only a low speed mixture adjustment. While the engine is running at full throttle it will often shut off suddenly and backfire out the exhaust. When this happens it can be difficult to restart, and when it does a cloud of black smoke blows out of the exhaust with a strong smell of raw fuel. If I remove the air filter I can see a little gas in the intake tube just ahead of the choke. The spark plugs have a sooty, black coating indicating a rich condition. Here are a few of the things I've tried to mitigate this problem with no lasting success:

Checked for sticking choke (it's not)
Replaced air filter & foam pre-cleaner
Cleaned & rebuilt the original carburetor (new needle & float, the seat was non-replaceable)
Replaced the original carburetor with a new carburetor
Checked ignition coil gaps and tested spark
Performed a valve job (resurfaced old valves, cleaned the seats, lapped them in & adjusted per factory service manual).

In spite of all this it refuses to lean-out and run properly. The behavior is very erratic. Sometimes it backfires within 10 minutes of a cold start while other times you can mow for over an hour before it happens. I'm at my wit's end trying to diagnose this, so I am hoping ya'll can give me some ideas on what to check next. Any thoughts?

Scott87 · Aug 13, 2020

Have you done a compression test-might be low on compression.
Look into the coils a little. RTFM maybe you can measure the resistance on them and see if they are out of spec. Kawi's are kinda known for coils that lose or have a crappy spark when the engines get hot.

CrowTRobot · Aug 13, 2020

Mac-s Lawn & Snow said:
Have you done a compression test-might be low on compression.

No, but I know this engine has a compression release. Will that affect my ability to get a good compression reading?

Scott87 · Aug 13, 2020

A quick google search says yes and to do a leak down test. I'm not familar with this engine, can you see any components of the comp. release and disable it?

CrowTRobot · Aug 13, 2020

I can’t do anything about the compression release without cracking the engine open. It’s built into the camshaft. I found specs for the coils in the service manual, so I’ll check them tomorrow. Leakdown test might have to wait for the weekend.

BigFish · Aug 13, 2020

I would suspect the coil/s.

CrowTRobot said:
When this happens it can be difficult to restart, and when it does a cloud of black smoke blows out of the exhaust with a strong smell of raw fuel. If I remove the air filter I can see a little gas in the intake tube just ahead of the choke. The spark plugs have a sooty, black coating indicating a rich condition.

Most of that "rich"condition is a result of the fuel solenoid not shutting off the gas to the main jet on abrupt shutdown.
When it stops like that, the key is still on and the solenoid is energized, resulting in the above.

CrowTRobot · Aug 13, 2020

That makes sense. If I understand correctly, you suspect that the coils work normally up to some point where, maybe due to heating, they begin to malfunction and the engine loses spark. When I tested the spark previously it was on a cold engine, so I think I'll run it up to temp, wait for it to cut-off, and test it again. I'll also pull the coils and test them with a meter as described in the service manual.

CrowTRobot · Aug 13, 2020

A followup question: if this is coil-related and the engine is shutting off completely doesn't that mean both coils have to be failing at the exact same time? If only one coil failed I'd expect the engine to continue running on one cylinder.

Scott87 · Aug 13, 2020

Maybe a inline spark tester would be better way to check and see if you are losing spark if its cutting out regularly. It took me a while to follow what Bigfish was laying down, but its all clear now. Take the fuel solenoid out and check the condition of the plunger and that it is connected to the solenoid.

CrowTRobot · Aug 14, 2020

I pulled the coils this evening and measured the resistance according to the service manual, and they both checked OK. I reinstalled them, set the air gaps, and then I tested for spark. Zip. Zero Zilch. Bupkis. Nothing from either coil. I'm wondering if my kill wire is grounded out somewhere. Gonna check that tomorrow.

Scott87 · Aug 14, 2020

Can you disconnect the kill wire and give 'er a try? Pull the air cleaner and give it a snort of starting fluid

BigFish · Aug 14, 2020

Mac-s Lawn & Snow said:
It took me a while to follow what Bigfish was laying down, but its all clear now. Take the fuel solenoid out and check the condition of the plunger and that it is connected to the solenoid.

Nope.
When the motor shuts off abruptly, the key is usually left on, energizing the solenoid and allowing the motor to draw in fuel while it spins down, loading up the cyl. and muffler.

During a normal key off shutdown, the solenoid will stop the fuel flow thru the main jet, thus preventing loading up / afterfire , AKA backfire , thru the muff.

Could also be a bad kill relay, or related wiring/ safeties.

Scott87 · Aug 14, 2020

BigFish said:
Nope.
When the motor shuts off abruptly, the key is usually left on, energizing the solenoid and allowing the motor to draw in fuel while it spins down, loading up the cyl. and muffler.

During a normal key off shutdown, the solenoid will stop the fuel flow thru the main jet, thus preventing loading up / afterfire , AKA backfire , thru the muff.

Could also be a bad kill relay, or related wiring/ safeties.

Thanks for the further explanation :mechanic:

hort101 · Aug 14, 2020

CrowTRobot said:
I pulled the coils this evening and measured the resistance according to the service manual, and they both checked OK. I reinstalled them, set the air gaps, and then I tested for spark. Zip. Zero Zilch. Bupkis. Nothing from either coil. I'm wondering if my kill wire is grounded out somewhere. Gonna check that tomorrow.

Agree with other guys after you check rhe grounds in my opinion maybe a safety cutting out
Have read a few times seat for example cutting out if you hit a bump

CrowTRobot · Aug 15, 2020

I had the same thought about the seat safety switch, so I disabled it a couple of months ago. It didn't help.

I futzed around with the kill wire this evening and checked it & the ignition switch with a continuity tester. At one point I'm pretty sure I had a grounded condition when the switch was in the run position, but it was very brief. Most of the time it seemed to work normally. I tested the spark again and voila! Both coils were sparking again, so I guess I must've tickled something just right. It cranked right up, so my wife started mowing the yard. Less then 10 minutes later the engine shut off abruptly, so I disconnected the kill wire from the ignition switch. It started right up again, and she mowed for another 45 minutes without any problems.

I'm going to leave the kill wire disconnected for a while. If we can get through the next few mowing sessions without incident then I'm going to assume that the ignition switch needed to be cleaned or replaced.

Thanks for the assistance. I'll post a follow up when I have something to report.

VegetiveSteam · Aug 19, 2020

CrowTRobot said:
A followup question: if this is coil-related and the engine is shutting off completely doesn't that mean both coils have to be failing at the exact same time? If only one coil failed I'd expect the engine to continue running on one cylinder.

Yes that is what it means and it is very unlikely. That being said it doesn't rule out something else causing the ignition to ground out causing both coils to stop firing. How does it run just before it shuts down? Does it run fine and then just shut down like you turned the key off?

CrowTRobot · Aug 19, 2020

VegetiveSteam said:
Yes that is what it means and it is very unlikely. That being said it doesn't rule out something else causing the ignition to ground out causing both coils to stop firing. How does it run just before it shuts down? Does it run fine and then just shut down like you turned the key off?

In my previous post I mentioned that I suspected the ignition switch was faulty and allowing the kill wire to ground-out when in the "run" position. I disconnected the kill wire from the switch, and since then the mower has been used for more than 2 hours without any problems.

CrowTRobot · Aug 26, 2020

The engine ran solidly for another 1.5 hours yesterday with the kill wire disconnected from the ignition switch. At this point I’m certain I’ve got a faulty ignition switch. I’m going to remove it to see if I can disassemble it and check the contacts. Maybe they just need to be cleaned. Failing that I’ll just order a new one.

Thanks again for the advice!

Justin Hightower · Aug 27, 2020

I was going to recommend key switch or even the PTO switch as safety features usually run through it

Kawasaki FS541V running very rich

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