Backfiring through the exhaust

[Auto Doc's]

How many hours on the mower, what year is it? Is it parked outdoors, any sign of vermin? Sounds like an mis to me not a backfire. Does it get worse when it’s warmed up? If so probably a coil. Start easy,
obvious, before chasing your tail. I’d type more but the pop ups on this site make it ridiculous sometimes.
very annoying.

Hello Dpmulvan,

Here is how to control the pop-up issue:

On the top right corner of the window, you will see 3 dots. Click there and go to more tools on the drop-down menu.

Pick internet options, a block with internet properties will open, On the top tabs will be a privacy tab, click on that and another box will appear.

At the bottom is pop up blocker. Click on that and it will resolve your issue.

 

[outdoorpowermike]

A backfire is a pop sound coming out of the carb bore, not really the muffler. What you have going on is either a problem with the muffler coming apart internally or a exhaust valve or seat problem. If you do a good clean carb like you said I would check for these issues.

 

[Auto Doc's]

According to the engine number, this is around 25 years old I'm guessing.

The one thing I notice that has not been mentioned is the exhaust. The muffler can usually be unbolted from the bottom of the chassis and slipped off the 2 exhaust tubes.

Try that and see if the engine sounds louder, but the odd miss/ backfire is gone. The muffler could have a rotted out internal baffle.

The other possibility is the exhaust pipe flange gaskets on the head are rotted and burned-out allowing air to be sucked into the hot exhaust flow. I've only had it happen once over many years. The engine seems to run normal, except it has an annoying exhaust note.

The exhaust flange nuts on the heads are nearly impossible to get to without loosening and blocking the engine up slightly for clearance.

 

[Lostviking]

Check and see

407777 0188 B1

On a service call, started out, this thing ran like crap. Lot's of back firing through the exhaust. After about 10 minutes of it running & mowing, it seemed to clear up. It was still backfiring, but in spurts, it would completely clear up for several seconds. Maybe even a couple of minutes.

So I thought maybe it was just some junk in the carb that made it's way through. A couple of weeks later, customer calls and says it still running bad and that it wouldn't even stay running. So I picked it up and brought it home. (like a dummy, I didn't even try to crank it there)

At home, I crank it, and it's still got a little back fire issue. But nothing major, like it was the first time I cranked it. I throw on a set of new plugs (RC12YC). Of course that didn't change anything. But it eliminated the possibility of a bad plug. I ran it for about 10 minutes and the plugs were spotless.

So I guess the question is what could make it run like crap one minute. I don't want to do nothing to it, return it to the customer, only for to start doing it again.
Like I said the only real symptoms now is the slight exhaust backfire. I remover the air filter so I could see down in there, and didn't see any carburetor back fire.

if you have a burnt Exhaust Valve or possible Lazy Valve spring!!

 

[golfergordy]

I had some backfiring (years back) with a Honda walk behind mower. Checked the valve clearances and found them out of spec. Re-adjusted the valves and no more backfiring.

 

[Forest#2]

Any tips on removing a plug wire while the engine is running.

I know that sounds like a rookie question. But the first and only time I've done it (decades ago), I got the fire knocked out of me, and never tried it again.

I prefer to ground the tip of the spark plug by sliding back the insulator rather than pull the wire
but if I need to pull I use

I use the scissor type ideal fuse pullers 34-002 that are about 7 inches long.
You can see them on flea bay

 

[RevB]

Any tips on removing a plug wire while the engine is running.

I know that sounds like a rookie question. But the first and only time I've done it (decades ago), I got the fire knocked out of me, and never tried it again.

Grab the cap and pull it off...won't get shocked unless you want to stick your finger in the end... The voltage isn't enough to each around the insulated cap. Many of the really old cables just had a bare metal clip that attached to the plug. If you have that I'd be surprised.

However, in retrospect Forest#2 says he prefers to ground the plug tip....that may not be possible, some connectors like the NGK are are solid plastic affairs. But his advice does bear scrutiny and made me reconsider just pulling the cap off as this may damage the coil transistors since the High Voltage has to dump somewhere.....better to pull the cap while not running and use an old spark plug that is grounded to dump the energy into a spark as was intended as the engine is started again...or just pull the existing plug and ground it. The engine would start more easily than having to fight a dead cylinder's compression.

 

[RevB]

I prefer to ground the tip of the spark plug by sliding back the insulator rather than pull the wire
but if I need to pull I use

I use the scissor type ideal fuse pullers 34-002 that are about 7 inches long.
You can see them on flea bay

You made me question my advice to just pull the cap....see post #57. Thanks!

 

[PTmowerMech]

Is that .06mm or .06"? .06mm = .0023"
Intake should be .004" Exhaust should be .006" Engine must be cold for setting valves

.02 isn't exactly right. But it's not enough to really be concerned about. It' wasn't a sloppy .06.

 

[Forest#2]

Grab the cap and pull it off...won't get shocked unless you want to stick your finger in the end... The voltage isn't enough to each around the insulated cap. Many of the really old cables just had a bare metal clip that attached to the plug. If you have that I'd be surprised.

However, in retrospect Forest#2 says he prefers to ground the plug tip....that may not be possible, some connectors like the NGK are are solid plastic affairs. But his advice does bear scrutiny and made me reconsider just pulling the cap off as this may damage the coil transistors since the High Voltage has to dump somewhere.....better to pull the cap while not running and use an old spark plug that is grounded to dump the energy into a spark as was intended as the engine is started again...or just pull the existing plug and ground it. The engine would start more easily than having to fight a dead cylinder's compression.

this may damage the coil transistors since the High Voltage has to dump somewhere.

Correct not a good thing to pull a plug wire on some ignition systems.
Also
If you have one of these inline spark testers it will also allow grounding the the ignition while the engine is running if the spark plug boot is the molded type and cannot be slide back on the ignition wire for access to the tip of the spark plug.
OREGON SPARK TESTER Repl Briggs 19051 19368