Backfires

[bnolte]

Many of my older pieces of lawn and garden equipment (which is most of them) seem to have problems with backfires a second or two after shutting them off. On my Cubs and my Simplicity riding mower, this results in a loud gunshot like BANG, but other than making my ears ring it seems to do little other damage. On my B&S powered rototiller however, the muffler is in close proximity of the gas tank, which is often damp with gasoline which shakes through the vent holes in the filler cap. When the rototiller backfires through the muffler like that, it has caused some small fires, and today I almost lost it, and had to resort to throwing clumps of dirt on the area around the carburetor to extinguish the fire.

I try to let my equipment cool down a little by idling the engine a couple of minutes before shutting down, but it still seems to happen. I'm pretty sure it's from raw gas getting dumped into a hot muffler, but I'm not sure what I can do about it, other than carry a fire extinguisher with me.

 

[kide]

Shout "Pull" and put an imaginary shotgun to your shoulder? ..(inside joke w/next door neighbor for the last 22 years).

Usually letting it idle down for about 20 seconds does it for me on both the 129 and my (OT) ZTR. If the machine is still doing it, I'd start by looking for overheating combined with too high a float level or other carb problems that are pushing an overrich mixture into the muffler....

 

[digger]

Kendell,
I don't care who ya are! Thanks just plain FUNNY!

 

[dcurrin]

My read on the noise aka bang. Is "hey dummy, check the air filter" any one of my cubs (7) say the same thing. I am not saying it will fix your problem but mine goes away every time.

PS I like Kens' solution. rotflmbo (thats butt)

Dudley

I have been known to put a dirty filter on.

 

[kide]

Maybe I need this in my signature line..

 

[kmcconaughey]

Kendell, that's good, I'll have to remember that one. BTW, nice animated GIF!

Last Friday one of the guys I work with took an extended lunch with me and we went out to the farm I grew up on and shot some trap. Also shot a variety of handguns and rifles. Yeah, we needed a break from work.

 

[eford]

We have a "history" of my company written in 1987. In it they used to shoot and fish on the property of our foundries(spelled write?). I wish we could still do that, but the neighborhood across the tracks might complain, and we'd have no one wanting to become a higher-up anymore....

Too many trips to the body-shop afterwards.

I've had plenty of small equipment backfire on me, all I know is;
1)rebuild the carb(all new guts including float)
2)new airfilter
3)remove and clean head
4)new sparkplug
And it wouldn't do it again.

 

[kide]

OK, just to make sure we aren't confusing the issue too much here, the most common "backfire" <u>on shutdown</u> is caused by a redhot muffler getting loaded with unburned fuel and igniting when the outflow of exhaust quits, allowing enough oxygen back in through the exhaust pipe for the fuel to ignite.. That's why letting the motor idle down for a few seconds usually will solve the problem - the muffler cools off and the fuel won't ignite... Note that the upturned muffler like a 129 has on it is especially good at collecting unburned fuel and then imitating a 20 gauge ..

 

[bnolte]

Kendall,

The "backfire" on shutdown is exactly as you describe

 

[jnaas]

My old 123 scared the crap out of me today when it let loose a bang after shutdown (second time ive ran it since I got it). Isn't that why the newer stuff has a fuel shutoff solenoid on the carb? I had a Tecumseh that I took that solenoid off of that would throw a mean fireball....Too bad it didn't burn down

 

[bnolte]

I suspect that a new air filter will only help a little, the engine on the 129 got a new filter only a year ago when I built it up, and it's done the "backfire" pretty much from the start, whenever I shut her down a little hot. I let her idle for 20 or 30 seconds before shutting down, maybe a little longer will help.

On the tiller, the situation is a bit trickier, the combination of a leaky gas cap, a somewhat dodgy carburetor, and the muffler in close proximity to both is an accident waiting to happen. I never did like the way B&S made their gas caps or ran the exhaust in such close proximity to all that fuel on those old horizontal shaft engines during the '60s and '70s.

The tiller was a resurrected giveaway that had set for many years, the fuel tank was filled with varnish and rust, and the carburetor was all gummed up. The carb got a thorough cleaning and new gaskets, diaphragm, and needle valves. Using carb cleaner, Kerosene, and a handful of nuts and bolts, I was able to remove most of the crud, but I suspect some remains, I had to re-clean the carburetor once after about a half hour of running, and I repeated cleaning the tank as well. It seems to run better than it did, but it still messes up when I muscle the tiller around much.

I'm thinking of installing a 45 degree Street Elbow to get the muffler out of the way of the whole carburetor/gas tank area, and maybe find a better gas cap. It won't solve the "backfire" problem, but hopefully it will prevent future fires.

 

[dfrisk]

Not sure how the B&S carb is setup for adjustments but I bet the engine/carb is set too lean, Lean is HOT...rich is cooler. Also I bet the idle speed is set too high so the engine spins over filling the muffler with fuel/air mixture and when it hits a glowing hot piece of steel in the muffler it explodes.
Might not hurt to let the engines idle longer also....I let My engines idle for 3-5 minutes after a long run (3-5 hours)

 

[bnolte]

I might also throw out there that lower compression from a worn engine might figure a part with the backfires as well. Being less efficient due to blowby, the engine would have to work harder to put out the same power, and as a result run hotter. A worn engine might also turn over more times once the ignition is killed as well, dumping more fuel in the muffler.



That being said, I'll try tweaking the mixture, lower the idle a bit, and be a little more patient with cooldown time the next time I give the 129 a serious workout to see if it helps.

I may give the 129 a break this week. I'm gonna borrow my neighbor's 125 which has a powered bagger attachment that rides on the back. I want to collect some grass clippings to mulch the garden.

 

[bbranstetter]

This one will back fire once in awhile. Had to adjust the high idle the other day, forgot, the folks never used premium in there stuff. Not to mention they bought it new just before I was born. It's got a couple of hours on it.

 

[bnolte]

Brian, same engine as mine, and yours is just about as old. Here is what I ended up doing with the muffler on it, I used a 45 degree elbow and a union fitting to point the muffler away from the carburetor and gas tank. I'm doubt it will prevent the backfires, but hopefully it will prevent the thing from catching on fire again if it does misfire.

 

[bbranstetter]

Mine still has the original muffeler on it. I'll have to take another picture of the side but it has a flame redirector, opps I mean arrestor

over the opening to help with the hot sparks and backfires. I need to clean off all the old dirt and oil before I end up with a fire. Probably should replace it, the rust is getting to the point that it will flake off and come out as hot sparks.

 

[kide]

Bruce:

Since the air/fuel mixture is heavier than air, if you were to take it out and down, it probably wouldn't bark... I believe the issue with the big Cub mufflers is that the exhaust opening is at the top of the up-slanted muffler. The muffler is flooded with an over-rich mixture when you shut 'er down, and ignites as soon as the mixture get a little oxygen flowing in from the outlet..
Pull!!!

 

[bnolte]

Ken, you have a point, though I'd need a bit more plumbing to make that happen.

 

[hcooper]

You may well have a sticking exhaust valve. Even a relatively new engine with low time can have a sticking valve from sitting and no running for a long period. You can put mouse milk in the oil and gas and hope that works, but the best thing is to pop the head, open the access to the valve keepers, and take the valve(s) out, then ream the guide with the peoper sized reamer. Even a drill bit will work in a pinch. Polish the valve stems with some fine emery, oil them uip, then reinstall. Sure, it's a little work, but you'll have done the right thing!

 

[thoffman]

HKC,
WTH is "Mouse Milk"???