Non-valk issue need suggestions please!

[Brian]

Hello everyone,
I am trying to help out a veteran friend who just had knee surgery and out on disability insurance. His John Deere LA 105 lawn tractor starting back firing really bad at mid-throttle and up. He is no mechanic with engines other than the normal routine maintenance. So here I am with a 19.5 hp Briggs and Stratton single cylinder horizontal engine back firing after completing and replacing the following:

1. Compression test ran up to 150psi.
2. Replaced head gasket and checked valve operation anyway
3. adjusted valve lash to spec's
4. Replaced ignition coil setting air gap to the recommended dimension that came on a card with the part. The card was actually the right thickness but used feeler gauges anyway.
5. Replaced carb
6. owner replace the air filter
7. new spark plug
8. Brigg and Stratton service line person suggested I check to see if the muffler was plugged. I removed it and ran it with just the short pipe. No change. Muffler is back on now.

I should also mention that the spark plug when removed comes out covered with black soot after running a short time.

All suggestions and comments are appreciated. This engine is making me feel like an idiot. 

Thank you,

[John Schmidt]

Black soot tells you it's running rich so the backfiring may well be unburned fuel passing through the exhaust and igniting.

[Harryc]

Does this machine have a seat switch? In other words it's a switch that will make if the operator sits in the seat. If that switch is flaky it will cause backfires. You can jumper across it's wiring to test. I used to be as much into JD lawn tractors as I am now Valks.

[Hook#3287]

I used to be as much into JD lawn tractors as I am now Valks

Nice upgrade Harry 

[oldsmokey]

Valve function was observed and valve lash set (was much adjustment needed?), were valve seats and faces inspected while head was off?  Certain it's a backfire through exhaust and not popping out carb.?
How many hours on machine. Run a spark tester inline with plug for a visual.

[Firefighter]

Sounds to me like ignition is on and off at higher rpm, I would disconnect the safety system first, may be one wire coming from all the safety switches (may be more complicated). The safety switches usually connect where ever the kill wire grounds to the engine. The seat switch is a good start as some one mentioned. Some machines have a blade switch, brake or clutch switch, even a grass chute switch. If the engine is running at half to full throttle and you turn the the key off then back on the engine will backfire just as our Valkyries do.

[Harryc]

I used to be as much into JD lawn tractors as I am now Valks

Nice upgrade Harry 

Lol ... I agree. A boat load of more fun too

[Itinifni]

Been a long time since I been into a single cylinder Briggs but I'll take a shot.

A bit more information on the engine would help, you mention that it's a horizontal engine, most lawn tractors are vertical shaft (horizontal or vertical refers to the crank orientation, not the cylinder). If you can post the engine model number most Briggs manuals can be found on line.

I assume this engine has some sort of solid-state ignition but they may still be using points.
Back in the days I used to mess with small engines I used to find point issues common, more specifically there is a fiber follower that rides against a cam lobe built into the crank to operate the points. That follower would often bind on older engines leading to misfires/backfires at higher RPM. I'm probably dating myself though.

Another common issue would be damaged flywheel keys leading to incorrect ignition timing. Have you had the flywheel off yet? If not I'd pull it to take a look.

If you can provide more information on the engine model I'll see if I can come up with other suggestions.

[Firefight100]

I had a similar problem with my twin b&s engine.  Turned out to be leaking at the intake manifold to head union.  Replaced both intake gaskets and retorqued the screws attaching manifold to head.

[Brian]

I had the head off, checked what I could while pushing the valves open with my thumb. I filled the head cavity with carb cleaner while holding it upside down, nothing leaked thru the valve seats.

Engine mod # is 31P677, on-line manual information was no help unless I missed something. This unit date of manufacture is 2010. Even called the B&S tech-line.

I do not have an in-line spark tester. Looks like a new tool coming.

The owner had tried the valve lash adjusting himself. He had it all jacked up with no lash at all. Push rods did not get damaged. I know how to set the valves as I have had both heads off my valk. When I bought this valk it was not running and had 3 bad cylinders. I hand seated all the valve faces due to carbon build up. I thought about pulling the valve stems and should have now just to make sure they really move freely. Turning the engine by hand, they appear to travel just fine.

The safety seat switch and ignition switch I never thought about as it continues either sitting on it or not.

I did not have the flywheel off but did look down the shaft at the key, appears just fine.

Backfires through the exhaust and the carb.

Thanks for the helpful suggestions and comments. I plan to check the safety system today.

[oldsmokey]

What spark plug was put in? I have had issues with "Torch" plugs idle fine, try to crank it up and it starts missing? 

[Itinifni]

I'm assuming plug is correct (heat range, reach, etc.) and carb is correct for the engine. Does it have a low oil shutdown? If so I'd start by unplugging the switch, if the switch is intermittent the problem should no longer occur.
If the issue persists check the wire for abrasion/shorting, you may have to remove some heat shields to follow it.

Symptoms are a little puzzling, backfire through the carb is typically an intake valve hanging open or ignition timing issue. Through the exhaust is usually carburation.

Higher RPM, maybe a sticky valve (carbon) or valve float (weak/broken spring). I don't remember ever seeing either on a small engine.

I found parts and owner's manuals online, no luck with service manuals.

Good luck

[Brian]

I'm assuming plug is correct (heat range, reach, etc.) and carb is correct for the engine. Does it have a low oil shutdown? If so I'd start by unplugging the switch, if the switch is intermittent the problem should no longer occur.
If the issue persists check the wire for abrasion/shorting, you may have to remove some heat shields to follow it.

Symptoms are a little puzzling, backfire through the carb is typically an intake valve hanging open or ignition timing issue. Through the exhaust is usually carburation.

Higher RPM, maybe a sticky valve (carbon) or valve float (weak/broken spring). I don't remember ever seeing either on a small engine.

I found parts and owner's manuals online, no luck with service manuals.

Good luck

Looks like you are new to group, welcome! I plan to remove the head and actually remove the valves for a better inspection of the seats and stems. When the owner picked up the new carb for me I double checked the part # with the Briggs site as the replacement is different. The carb was listed for this engine.

I went thru the safeties and wiring, the only thing that I found was few really loose plug connections to the ignition coil and the shutdown backfire solenoid on the carb. I used a analog multimeter to verify voltage to both, the needle stayed right around 12 volts with no fluctuation at all while running. The back fire issue did reduce a little to where it will move and cut grass but not at full run position. I did not see a low oil switch, but will go take another look again. Thanks.

I am also thinking about checking the pressure off the fuel pump before I remove the head. This is the only part not replaced. I am being told that this should be putting out 2 psi and that 8 psi will over pump the float valve.

Thank you,

[Brian]

I'm assuming plug is correct (heat range, reach, etc.) and carb is correct for the engine. Does it have a low oil shutdown? If so I'd start by unplugging the switch, if the switch is intermittent the problem should no longer occur.
If the issue persists check the wire for abrasion/shorting, you may have to remove some heat shields to follow it.

Symptoms are a little puzzling, backfire through the carb is typically an intake valve hanging open or ignition timing issue. Through the exhaust is usually carburation.

Higher RPM, maybe a sticky valve (carbon) or valve float (weak/broken spring). I don't remember ever seeing either on a small engine.

I found parts and owner's manuals online, no luck with service manuals.

Good luck

Looks like you are new to group, welcome! I plan to remove the head and actually remove the valves for a better inspection of the seats and stems. When the owner picked up the new carb for me I double checked the part # with the Briggs site as the replacement is different. The carb was listed for this engine.

I went thru the safeties and wiring, the only thing that I found was few really loose plug connections to the ignition coil and the shutdown backfire solenoid on the carb. I used a analog multimeter to verify voltage to both, the needle stayed right around 12 volts with no fluctuation at all while running. The back fire issue did reduce a little to where it will move and cut grass but not at full run position. I did not see a low oil switch, but will go take another look again. Thanks.

I am also thinking about checking the pressure off the fuel pump before I remove the head. This is the only part not replaced. I am being told that this should be putting out 2 psi and that 8 psi will over pump the float valve.

The new plug was purchased from the dealer for this mower.

Thank you,

[Brian]

What spark plug was put in? I have had issues with "Torch" plugs idle fine, try to crank it up and it starts missing? 

Champion, took out a First Fire. No help with the Champion. Thanks,

[Brian]

Valve function was observed and valve lash set (was much adjustment needed?), were valve seats and faces inspected while head was off?  Certain it's a backfire through exhaust and not popping out carb.?
How many hours on machine. Run a spark tester inline with plug for a visual.

Oldsmokey, I am planning to remove the valves today for a closer inspection and check the fuel pump pressure too. Thanks.

[Brian]

Been a long time since I been into a single cylinder Briggs but I'll take a shot.

A bit more information on the engine would help, you mention that it's a horizontal engine, most lawn tractors are vertical shaft (horizontal or vertical refers to the crank orientation, not the cylinder). If you can post the engine model number most Briggs manuals can be found on line.

I assume this engine has some sort of solid-state ignition but they may still be using points.
Back in the days I used to mess with small engines I used to find point issues common, more specifically there is a fiber follower that rides against a cam lobe built into the crank to operate the points. That follower would often bind on older engines leading to misfires/backfires at higher RPM. I'm probably dating myself though.

Another common issue would be damaged flywheel keys leading to incorrect ignition timing. Have you had the flywheel off yet? If not I'd pull it to take a look.

If you can provide more information on the engine model I'll see if I can come up with other suggestions.

I pulled the flywheel, shear key is just fine. The only thing under the flywheel is the stator.

[oldsmokey]

Not likely to be fuel pump. All it needs to do is flow it from tank to fill the bowl. Hardly ever need replacement, sometimes times need to be "primed" after sitting for winter. To eliminate all ignition possibilities (safety switches) you can disconnect the control wire to the coil for it to run, will not be able to shut down without grounding it. These coils are also very trouble free. It sounds like you already ruled safety switches anyhow. Was this a sudden onset or gradual issue that developed? A leak down test would be good to perform. Any evidence of a mouse nest in the past?

[Harryc]

... and the shutdown backfire solenoid on the carb. I used a analog multimeter to verify voltage to both, the needle stayed right around 12 volts with no fluctuation at all while running.

Remove the solenoid from the bottom of the bowl and make sure the plunger retracts all the way when 12v is applied. I've seen these go bad and it will cut your fuel if they don't fully retract.  Basically what they are designed to do it cut the fuel supply (by closing the float valve) when you shut the engine down (remove 12v) to reduce backfiring from too much fuel in the cylinders. If they stick it could cause a lean condition and possibly backfiring. Make sense? I've even gone as far as replacing them temporarily with a regular bowl nut. It may backfire when you shut it down, but if it alleviates the backfire while running condition, Bob's your uncle...just replace the solenoid.

[da prez]

   Back to basics. Pull the flywheel and check the timing key. Even a few thousands distortion plays havoc with that engine. Also , check for a vacuum leak. It may have a pulse fuel pump. An internal leak is hard to find. Always do the basics first and do not assume.

                                                       da prez

[sinner]

I know it is a bit obvious, but have you checked the air filter?     

[Brian]

... and the shutdown backfire solenoid on the carb. I used a analog multimeter to verify voltage to both, the needle stayed right around 12 volts with no fluctuation at all while running.

Remove the solenoid from the bottom of the bowl and make sure the plunger retracts all the way when 12v is applied. I've seen these go bad and it will cut your fuel if they don't fully retract.  Basically what they are designed to do it cut the fuel supply (by closing the float valve) when you shut the engine down (remove 12v) to reduce backfiring from too much fuel in the cylinders. If they stick it could cause a lean condition and possibly backfiring. Make sense? I've even gone as far as replacing them temporarily with a regular bowl nut. It may backfire when you shut it down, but if it alleviates the backfire while running condition, Bob's your uncle...just replace the solenoid.

Hello Harryc, the carb is new as is the solenoid as it came with it. Where can I get a bowl nut to try running without the solenoid? I though about cutting the plunger off the old one.

[Brian]

I know it is a bit obvious, but have you checked the air filter?     

Air filter is new, I have been running the motor with out it for troubleshooting.

[Brian]

   Back to basics. Pull the flywheel and check the timing key. Even a few thousands distortion plays havoc with that engine. Also , check for a vacuum leak. It may have a pulse fuel pump. An internal leak is hard to find. Always do the basics first and do not assume.

                                                       da prez

Hello da prez, I pulled the flywheel, the key is perfect, no indentations at all on the sides. Fuel pressure off the pulse pump to the carb is solid at about 2psi. Thanks,

[Brian]

update:
Went to Harbor Freight and bought a cylinder leak down tester, test showed leak down thru the intake valve. I had removed the carb intake and exhaust from the head for the test as well as the rocker arms. I tore down the head cleaning the valves, more carbon on the intake valve than the exhaust. Cleaned both with the dremel fine wire wheel. Hand lapped the valves, got a nice thin polished line through the valves seating surface and the seat. Put back together and tested again. Intake still leaking!

FYI, the tester from HF worked just fine as well as it was family discount weekend with a 25% off coupon.

[oldsmokey]

Likely need to re-cut the seats and replace the valves. Complete replacement head is likely to cost about a buck and a quarter. Was there any evidence of overheating from a mouse nest?

[oldsmokey]

Any sign of dirt ingestion from poorly-maintained air cleaner? Fine particles built up in throat of carburetor, Worn throttle shaft? Inspect valve stems and guides, if worn they will not seat properly. Was there carbon tracking down the stem of the intake valve. That would be a key indicator of too much slop.

[Brian]

Any sign of dirt ingestion from poorly-maintained air cleaner? Fine particles built up in throat of carburetor, Worn throttle shaft? Inspect valve stems and guides, if worn they will not seat properly. Was there carbon tracking down the stem of the intake valve. That would be a key indicator of too much slop.

The intake valve had a lot of carbon built up on the intake or topside of the valve, overhead valves. No indication of a nest but then again I did not see the old filter. Did not see debris in the carb throat. The owner has a new head on order. I am headed out of town today so it will be a week before I get back at this engine.

Thanks for all the comments.

[sudsman]

Ensure valve guide didn't move engine gets hot some have hade the guides move and not allow full closure on seat. tape guide down a bit and peen area around base with center punch. May or may not work. fixed on like that.