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Hey All. I'm new to the forum so please excuse me if I mess up on any posting rules...

I have a Husqvarna YTH24v48 with a Briggs & Stratton Intek V-Twin engine model 44N877-0003-G1 170815YG. I've been living with a persistent afterfire from the exhaust port on cyclinder 2 since August 2019. The lawnmower was purchased with a damaged steering assembly from a thrift store with almost no hours (9-11) on it. The engine seems to run strong under load but I can't seem to find the cause of the afterfire.

The issue seems to be almost identical to the following post on this forum and youtube video:
When the engine runs on only one cylinder the afterfire goes away and it doesn't matter which cylinder. When both spark plugs are connected the afterfire returns only to cylinder 2. I have comfirmed the afterfire with the muffler off. The only difference between cylinder 1 and 2 is that cylinder 2 has higher compression by about 8%.

I have been through a bunch of engine tests, which I'll list at the end, but I haven't been able to see anything but a healthy new engine. I finally took the carburetor off for inspection based on the forum post above having the wrong carburetor. The only thing I found was slightly damaged float bowl gasket (698781) on a round section that did not seal anything. I replaced the gasket just in case, but it still continues to afterfire. The carburetor was very clean and nothing else seemed to be restricted or damage. I cleaned the carb anyway but nothing has changed. I plan on inspecting the fuel delivery, but I'm pretty much stuck except for invasive exploratory disassembly of the engine (cylinder head, camshaft.)

I would appreciate any help with this issue because I'm losing sleep over this. The only thing that stops the popping
and afterfire is running on one cylinder. Thanks in advance.

[Engine Tests Performed]
static compression test
dynamic (running) compression test
leak down test
cylinder contribution test with tachometer
ignition spark tester check
ignition diode check
ignition armature corrosion cleaning
ignition armature gap check
spark plug condition and gap check
ignition wire leak check
swap ignition coils
smoke test intake
smoke test exhaust cylinder 2
visual inspect exhaust valve chamber cyl 2
PB Blaster LMT Cylinder 2
Sta-Bil Fix It
Seafoam in Gas
PB LMT through Intake
Valve lash adjusment .005 Int/Exh
Valve guide check
Bent Push Rod inspection
Valve movement check with both covers remove
Flywheel sheer key inspection
New spark plugs, air filter, fuel filter
Clean emulsion tube with wire and carb cleaner
Clean main jet with wire
Blew transfer tube with compressed air
Cleaned air inlets of carb with wire and carb cleaner
Blew carb air inlets with compressed air
Checked backfire solenoid
Inspected float bowl needle
Clean idle circuit with wire and carb cleaner

If the problem goes away when the kill wires are removed then one of the diodes in the kill wire is bad.
Replace them both
the pigtail is about $ 50 ( Aus) the diodes are 20¢ .

Hi and Welcome to the LMF! You sound quite knowledgeable
and thorough. Sounds like folks will be asking you about
things soon.

Welcome from east central Illinois,

Walt Conner

Thanks. I'm just a diy with a few acres to cut. I still have a lot to learn.

I did run the engine with the diode pack removed and the problem remained. I double checked the diodes with a meter according to the shop manual (273521) and they both passed. I cleaned and re-gap the armatures just to be sure and it still afterfire. I swapped the coils and the afterfire remained on cylinder 2.

The only strange thing I experience was with connecting to the kill switch terminal with a tachometer. At low speeds the needle held steady, but at high rpms the needle would fluctuate down wildly with cyl 2 backfire on either coil. I had to connect the inline spark tester (run on one cylinder) and I was able to get a steady reading on the active cylinder. I think I just needed a better tachometer that reads from the secondary coil.

Any theories on why the carb (single barrel) is the culprit? I don't see any signs of running lean on the spark plugs. Cylinder 1 has a normal tan color, but cylinder 2 has a little burnt on oil on the threads and a normal grey color. The orientation of the spark plug show it's coming from the intake valve...

Just curious if you could borrow a 'known good carb' that would
fit that engine and try it just to eliminate the carburetor?

I think we have a communication problem here
Diode pack ?
there are 3 places you find diodes on mower with a B &S engine
1) on the alternator either as a single diode or a rectifier
2) on the kill wires between the two magneto coils
3) on the PTO circuit , usually hidden under the dash

bertsmobile1 said:

I think we have a communication problem here
Diode pack ?
there are 3 places you find diodes on mower with a B &S engine
1) on the alternator either as a single diode or a rectifier
2) on the kill wires between the two magneto coils
3) on the PTO circuit , usually hidden under the dash

Click to expand...

I ran it with the kill wires removed and it continued to backfire. I did not mess with the alternator rectifier(regulator) or the pto circuit.

DaveTN said:

Just curious if you could borrow a 'known good carb' that would
fit that engine and try it just to eliminate the carburetor?

Click to expand...

I have an old nikki double barrel that's running rich on a Murray.

gre1252 said:

I ran it with the kill wires removed and it continued to backfire. I did not mess with the alternator rectifier(regulator) or the pto circuit.

Click to expand...

Swap the coils left right & see if the problem shifts with them

bertsmobile1 said:

Swap the coils left right & see if the problem shifts with them

Click to expand...

he wrote "I swapped the coils and the afterfire remained on cylinder 2."

my general engine thoughts on this are to swap plugs and see if the problem swaps. You won't be the first person to get a bad plug to stick it in to replace a bad plug.

I have to wonder if something is going on in the valve train or intake. Doing a compression test at low RPM can't really test a moderate speed misfire. I remember having a 6-cyl Jeep head completely rebuilt at a reputable shop when I first got to Sacramento. It ran fine for about 10,000 miles then it developed a mis-fire on a couple cylinders. I finally took it to a guy with a scope who told me based on the waveform it was something with the valves. I pulled the head even though I thought he was wrong and sure enough, two cylinders had worn the new false guides so badly the valves rocked. They were randomly not seating properly. The shop gave me my money back, I found a new head for it off a totaled low mile, and life was like new again. You could have a weak valve spring, or a worn guide that isn't picked up in a low speed compression test. Likewise any air dilution can make a mixture so lean it misfires, or if the mixture is too rich it can misfire and then burn in the exhaust when air reaches it there, and the next exhaust flame hits it.

Look for leaking exhaust gasket on cyl 2. Sounds like air or fuel is passing into the cylinder/exhaust and popping. Vacuum leak on cyl 2?

Is valve lash proper?

Flywheel key okay?

You said all the above was good. Smoke test normally has nearly 0 pressure. Wondering if this was a proper check?

Now it's cam inspection time. Maybe a dead lobe in #2 cyl?

Remove valve covers. Look to see all valves move the same with engine on and off.

maybe the cam lobe suggestion might have some value. The higher compression on that cylinder might be caused by the valve having less duration and lift.

bertsmobile1 said:

Swap the coils left right & see if the problem shifts with them

Click to expand...

I tried swapping the coils and the afterfire did not move. I double checked it with the muffler off. Only cylinder 2 is popping out the exhaust.

slomo said:

Look for leaking exhaust gasket on cyl 2. Sounds like air or fuel is passing into the cylinder/exhaust and popping. Vacuum leak on cyl 2?

Is valve lash proper?

Flywheel key okay?

You said all the above was good. Smoke test normally has nearly 0 pressure. Wondering if this was a proper check?

Now it's cam inspection time. Maybe a dead lobe in #2 cyl?

Remove valve covers. Look to see all valves move the same with engine on and off.

Click to expand...

I smoked cylinder 2 from the muffler pipe and I could not see anything leaking with a bright light. I also smoked the intake from the air filter connection, while plugging the vents with my fingers on the back, and there was no visible leaks. I use a diy paint can smoker which has worked on cars in the past.

Valve lash is set to both .005 in. as recommended by the shop manual at 1/4" past TDC on cold engine. I watched all of the valve movements with both covers removed carefully while feeling the piston head movement in the cylinder. Everything looked the same on both sides. I measured the exhaust port spring compression when fully open with a caliper and both sides measure the same. I doubt there's any cam lobe issue. I thought the camshaft compression release might be at fault, but the dynamic compression test number shows numbers in the 170-180 range when shutting off the engine from high rpms. The counter weight is definitely moving on the camshaft.

Yes, the list of test above came back normal except for a damaged float bowl gasket (698781) in the clean carburetor. One of the little circle gasket on the side of the main gasket in the middle was mangled and creased. However, on this model of carb it does not seal anything. The space above and below that section of gasket is blank plastic or metal. I replaced the gasket, just in case, but the engine still pops and backfires out the exhaust.

does the carb have two outlets, are there two carbs, or is the manifold bifurcated?

Did you swap the plugs?

When you swapped the coils over, did you try it with the kill wires removed ?
I ask this because you can get feedback ( which is what the diodes are supposed to prevent ) from one to the other
The fact that you got a faultering tach reading on one side & not the other points towards a bad magneto.

Also have a good look at the rockers
the cam grinds are identical , the followers are identical & the valves are identical
So if the cam is good both pairs of rockers should sit parallel at firing time & move the same amount.

If the head overheats then one of the guides ( usually the inlet ) can slip in the head so the inlet rocker will be out of place .

sgkent said:

does the carb have two outlets, are there two carbs, or is the manifold bifurcated?

Did you swap the plugs?

Click to expand...

The carb has only one throttle plate. The part number is 597128.

Yes, I tried swapping the plugs back and forth with the coils and nothing changed.

So brother, this is puzzling. Again flywheel key just a micro tick off? You said it was checked already.

Could be excess fuel or air, SOMEHOW, getting into the muffkin, maybe dripping into it??

Carb needle and seat pressure tested to 7psi, holds for 30+ minutes?

Think your exhaust port needs pressure tested with 20-40'ish psi. These car evap system smokers are 1psi and under. Think they are 1psi MAX pressure.

does the carb have only one outlet, and then the intake manifold bifurcates into two cylinders?

slomo said:

sgkent,

My ancestors were Colonial Scot and Native American.

Should be Colonial Scout isn't it? Simple typo?

Click to expand...

Colonial Scottish, and Cherokee (probably Pamunkey), and also one of the Plains Tribes on my Grandmom's side although I did not learn about that until a year or so ago because her mom and dad divorced, and the Plains was on his family's side. Her father's family reached out to me a few years back when they realized their shared father had her in a prior marriage. The Scottish, and Native Americans didn't always get along that well during the 1600's and 1700's. So whom do I root for when they are fighting each other, my Scottish side or my Native American side? And there was some Scouting in there too during the War of Independence. My gg-grandmother was secretly raised by her grand uncle, Gen. Francis Marion, The Swamp Fox, when her father, (his nephew) was killed by the British during that war.

sgkent said:

Colonial Scottish, and Cherokee (probably Pamunkey), and also one of the Plains Tribes on my Grandmom's side although I did not learn about that until a year or so ago because her mom and dad divorced, and the Plains was on his family's side. Her father's family reached out to me a few years back when they realized their shared father had her in a prior marriage. The Scottish, and Native Americans didn't always get along that well during the 1600's and 1700's. So whom do I root for when they are fighting each other, my Scottish side or my Native American side? And there was some Scouting in there too during the War of Independence. My gg-grandmother was secretly raised by her grand uncle, Gen. Francis Marion, The Swamp Fox, when her father, (his nephew) was killed by the British during that war.

Click to expand...

So below your message body you have a blue section about your ancestors. That is what I was talking about.

My ancestors were Colonial Scot and Native American. I get really confused in old westerns whom to root for.

bertsmobile1 said:

When you swapped the coils over, did you try it with the kill wires removed ?
I ask this because you can get feedback ( which is what the diodes are supposed to prevent ) from one to the other
The fact that you got a faultering tach reading on one side & not the other points towards a bad magneto.

Also have a good look at the rockers
the cam grinds are identical , the followers are identical & the valves are identical
So if the cam is good both pairs of rockers should sit parallel at firing time & move the same amount.

If the head overheats then one of the guides ( usually the inlet ) can slip in the head so the inlet rocker will be out of place .

Click to expand...

Yes, I removed both kill wires when the coils were swapped. The tach did act up on both sides with the kill wires removed. Putting the spark tester inline was the only thing that stop the needle from swinging at high rpms on either side.

I made markings on a plastic straw at or near tdc and bdc with the valves either open or closed. At the same piston position, on the opposite cylinder, the valves looked identical. Nothing felt out out of place from what I could feel with the straw on the piston and what I could see with the rocker arms and compression of the springs.

I thought it might be a sticky exhaust valve guide which is why I tried the blaster lmt in the cylinder first. I probably needed to spray the guide from inside the valve cover and exhaust port.

slomo said:

So brother, this is puzzling. Again flywheel key just a micro tick off? You said it was checked already.

Could be excess fuel or air, SOMEHOW, getting into the muffkin, maybe dripping into it??

Carb needle and seat pressure tested to 7psi, holds for 30+ minutes?

Think your exhaust port needs pressure tested with 20-40'ish psi. These car evap system smokers are 1psi and under. Think they are 1psi MAX pressure.

Click to expand...

I pulled the large nut off and the key looked perfectly straight. I bought keys just in case, but I didn't want to pull the flywheel if I did not have to.

I should have replaced the float needle but it was $16. I don't see any issues with it flooding or running rich on the spark plugs. The needle and seat looked new. I may have to pull the carb again so I will double check with a vacuum next time.

The paint can smoker I built has a low pressure propane regulator. It might be 1/2 psi. I guess I could plug the muffler pipe and bubble test for leaks. thanks

sgkent said:

does the carb have only one outlet, and then the intake manifold bifurcates into two cylinders?

Click to expand...

Yes, the intake manifold bifurcates to two cylinders. There is only one outlet port.

My desperation tool of last resort because I need all the hair I got left is a smart little device called a COLORTUNE made by Gunsens in the UK
Basically it is a spark plug with a window in it and by watching it you can see if the window flashes yellow , white or none at all when the engine stumbles.
You can also see the spark jump so no flash & no spark = magneto failure while no flash with a spark = fuel failure .
I set the camera to video mode with sound recording on a tripod to film the colortune so I can isolate the times when it misfires and then step through the video frame by frame in that period to see what is going on.
While it is very rare , you could try running the engine with the muffler removed just in case it is a back pressure problem causing the occasional choking
If you do this then try to leave the headder pipes on .
Not that you will be running the mower for that long you do run the risk of burning exhaust valves without a headder .

gre1252 said:

I should have replaced the float needle but it was $16. I don't see any issues with it flooding or running rich on the spark plugs. The needle and seat looked new.

Click to expand...

Pull the oil dipstick and smell for gas.

Still need to pressure test the needle for 30 minutes at 7psi.

slomo said:

Pull the oil dipstick and smell for gas.

Still need to pressure test the needle for 30 minutes at 7psi.

Click to expand...

I used gumout multi-tune in the oil and blaster lmt in cylinder 2. There could have been gas. I changed the oil 2 days ago and so far no gas smells or change in the oil level.

I'm not sure how to pressure test the needle with the air vents going to the float bowl. The test I've seen is to turn the carb upside down and pull a vacuum from the inlet port.

The engine symptoms have changed and now cylinder 2 pops when running on one cylinder. I went through a few tests and the dynamic compression is way off on cylinder 2 from what I had originally. I don't know if I damaged something running cleaners (gumout/pb lmt) in the oil and cylinder. The leakdown, static compression, and intake smoke test came back the same. It could be a bunch of issues from an intake cam lobe to a scored cylinder wall.


Does the intake manifold usually get cold enough to have condensate? I saw fluid in the bottom of both intake manifold ports on the cylinder heads. It was very clean and cylinder 2's port sits downhill on the manifold...

gre1252 said:

Does the intake manifold usually get cold enough to have condensate?

Click to expand...

Not at my house with 107 temps LOL. Are you running alcohol fuel? Can you confirm water in the gas? Should be at worst just unsuspended fuel in the intake runner. This is a twin with a longer than most intake manifold.

Drain all the fuel out. Check for water in the pan.

gre1252 said:

I saw fluid in the bottom of both intake manifold ports on the cylinder heads. It was very clean and cylinder 2's port sits downhill on the manifold...

Click to expand...

Sounds like your carb is passing mega fuel to me. Maybe one of the other real experts can chime in and assist.

Maybe remove the carb from the engine. Check for excess fuel leaving the carb at no crank and cranking.

bertsmobile1 said:

My desperation tool of last resort because I need all the hair I got left is a smart little device called a COLORTUNE made by Gunsens in the UK
Basically it is a spark plug with a window in it and by watching it you can see if the window flashes yellow , white or none at all when the engine stumbles.
You can also see the spark jump so no flash & no spark = magneto failure while no flash with a spark = fuel failure .
I set the camera to video mode with sound recording on a tripod to film the colortune so I can isolate the times when it misfires and then step through the video frame by frame in that period to see what is going on.
While it is very rare , you could try running the engine with the muffler removed just in case it is a back pressure problem causing the occasional choking
If you do this then try to leave the headder pipes on .
Not that you will be running the mower for that long you do run the risk of burning exhaust valves without a headder .

Click to expand...

Interesting. I think it might be time to invest in a mini oscilloscope like the ds213.


I thought there might be a mouse (fluffy) in the muffler but it is not possible. thanks