# Is a backfire common when moving to slow idle?



## Tseg (Jan 29, 2021)

My GX200 on my new HSS724AWD will put out a small backfire if/when I move the throttle knob from fast to slow quickly. My machine has done this every time from Day 1. Like the joke, Patient: "Doctor, it hurts my head every time I bang it like this." Doctor: "Then don't bang your head like that."... I now don't slam my throttle knob from fast to slow quickly. If I move the knob slower there is no issue.

Is this backfire a symptom of anything or to be expected?


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## JLawrence08648 (Jan 15, 2017)

Not uncommon, I can say very common with Tecumseh snowblower engines, but also my larger B&S mower engines did that. It's because you are dumping a bunch of unburned gas that is being compressed by the rising of the piston and it suddenly explodes. I slowly bring the RPMs down because I don't want excess gas washing oil off the cylinder walls.


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## orangputeh (Nov 24, 2016)

this q comes up a lot. reading up on the google machine says it is common. assuming everything else is okay , correct fuel, timing , valve adjustments and other variables the backfire is unburnt gas that explodes out the exhaust.

the consensus is to go from fast to slow more slow.


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## tabora (Mar 1, 2017)

Other thoughts on backfire/afterfire from the B&S web site:

*What causes a small engine backfire & how do I fix it?*
Backfire typically occurs when the engine is decelerated rapidly.
*Common Causes of Backfire:*

*Lowering engine speed too fast*
Gasoline which contains higher blends of alcohol
Carburetor adjustment set too lean
Muffler construction can induce backfire
Higher than normal engine temperatures
Some carburetors can induce backfire due to the sensitivity of internal transitional passages
(This condition cannot be corrected).
*Possible Fixes for Backfiring:*

Lower engine speed slowly
Follow small engine fuel recommendations and/or switch to brands with low or no alcohol
Adjust carburetor for optimum performance
Inquire with equipment manufacturer about increasing air volume to decrease engine temperature

*What causes a small engine afterfire & how do I fix it?*
Afterfire occurs after the engine has been shut off.
*Common Causes of Afterfire:*

Shutting off the engine at high RPM, causing fuel to pump through the engine for ignition
Gasoline that contains alcohol has a tendency to ignite easier, which can cause afterfire
Small engine muffler type and manufacture
Carburetor adjustment may not be properly set for correct engine performance
Anti-afterfire solenoid may not be working properly
*Possible Fixes for Afterfire:*

Allow the engine to cool by idling the engine down with proper speed (15-30 seconds)
Change to a different non-alcohol or alcohol brand fuel
Ensure proper carburetor adjustment for optimal engine performance
Contact the equipment manufacturer for updated designs in air control baffling, mufflers, etc.
Check the anti-afterfire solenoid for proper operation
*Note*: If equipped with an anti-afterfire solenoid, shut engine off at full throttle


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## Tseg (Jan 29, 2021)

Thanks for the replies. Sounds like not a manufacturing defect. It may be the first reason so far for me choosing to go ahead and install a bigger Main Jet, to see if it further reduces the issue.

Kind of like the small choke issue I had (but this a much less significant operating impact) leading to a hard start if I did not apply pressure to the throttle handle while starting that required a 3 minute very slight throttle cable adjustment... if there was a quick fix to reduce the chance of backfire, I might consider acting on it.


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## LenD (Nov 17, 2020)

Good to know. Answered a question I didn't know to ask --- but should have as it happened to me exactly as described by OP


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## ST1100A (Feb 7, 2015)

That is perfectly normal for it to backfire when going from full speed to idle suddenly with the throttle.
You are going from a richer mixture to a sudden 'lean' mixture when you close the throttle suddenly down to idle position.
Idle it down slower to prevent that problem.
The crank is spinning fast, pulling in a lot of air and fuel, then suddenly the fuel gets cut down but it is still pulling in a lot of air but less fuel because of the throttle plate closing to draw fuel from the idle passage until the spinning crank speed slows down to draw less air in.
You know it is running good and clean when it does that.
The old saying, 'Lean is Mean'.


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## TV8270 (Feb 15, 2021)

ST1100A said:


> That is perfectly normal for it to backfire when going from full speed to idle suddenly with the throttle.
> You are going from a richer mixture to a sudden 'lean' mixture when you close the throttle suddenly down to idle position.
> Idle it down slower to prevent that problem.
> The crank is spinning fast, pulling in a lot of air and fuel, then suddenly the fuel gets cut down but it is still pulling in a lot of air but less fuel because of the throttle plate closing to draw fuel from the idle passage until the spinning crank speed slows down to draw less air in.
> ...


Exactly as I've controlled it on the 8hp Tecumseh. I used to jokingly yell "fore ! " to the neighborhood expecting the backfire blast when shutting it off by quickly closing the throttle. I just gradually reduce engine speed over 30 seconds or so, no problem. Do suspect carbon build up in the combustion chamber/piston top should be looked at...(@ ST1100, deferred V4 carb work ahead, catch ya' on another site as well).


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## LenD (Nov 17, 2020)

If you just close off the fuel supply and let it starve, won't that be the same as throttling back? As you want to get the fuel out of the system anyways when you're done............

What I now do is shut the fuel off at the bottom of the driveway, walk her back and by the time I'm in the garage, she's put put putting and ... goes to sleep 😉


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## tabora (Mar 1, 2017)

TV8270 said:


> I used to jokingly yell "fore ! " to the neighborhood


If I turn off my Toro Zeroturn (Briggs V-twin) from idle, about 2-3 seconds later there's a 30-06 equivalent afterfire. That engine MUST be turned off at full throttle so the anti-afterfire solenoid can do its thing.


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## Toro-8-2-4 (Dec 28, 2013)

tabora said:


> If I turn off my Toro Zeroturn (Briggs V-twin) from idle, about 2-3 seconds later there's a 30-06 equivalent afterfire. That engine MUST be turned off at full throttle so the anti-afterfire solenoid can do its thing.


I know this is a bit off topic but Just for my own understanding, Why do you think the solonoid won't be effective at stoping a back fire at low speed but will at high? The Solonoid is normally closed. Right? So when you cut power from the relay to stop the engine the solonoid closes off the fuel to the main jet the same regardless of engine speed. Am I thinking this through correctly? Is it becasue at high speed there is more vacuum that may help the solonoid valve close a bit faster? IDK. But curious.


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## tabora (Mar 1, 2017)

Toro-8-2-4 said:


> Why do you think the solonoid won't be effective at stoping a back fire at low speed but will at high?


Actual experience... It only appears to work at full throttle. At full throttle shutoff, the engine pumps a lot of air through the muffler to cool it, but no fuel because the anti-afterfire solenoid blocked it. At idle shutoff (immediately after running the machine at full speed mowing), the muffler apparently does not get cooled down enough and the afterfire occurs.

From the manual:​Stopping the Engine​1. Move the throttle lever to Fast (Fig. 6).​2. Move the PTO to Off (Fig. 5).​3. Turn the ignition key to Off (Fig. 7).​
From the Theory behind the “Anti-Afterfire Solenoid”:​​It does exactly that, prevents “afterfiring” when the engine is cut off. This is not to be confused with backfiring, which is a totally different thing. Afterfiring is that loud bang you hear after cutting off a tractor engine. This condition is more prevalent in Briggs and Stratton engines than any other engine. The loud bang is totally harmless to the engine, but can be scary or annoying to the operator.​​What causes the “afterfiring” is raw fuel touching the hot internal parts of the muffler. How this works is when you cut off the engine after mowing the grass, the engine “coasts” to a stop (keeps spinning until it slows down and stops). When the engine is coasting, the fuel is still being drawn into the cylinder on the intake stroke, compressed on the compression stroke, and exhausted on the exhaust stroke. The key here is the fact that the fuel is not combusted under the compression stroke (as it would be when the engine is on). The fuel takes a ride through the engine and ends up in the muffler. When this raw, unburned fuel hits the muffler, it explodes (like it would during the compression stroke). The fuel continually “rides” through the engine until it stops coasting. By the time the engine stops, there may be four or five compression stroke’s worth of fuel in the muffler, ready to blow up.​​Now the job of the solenoid is to prevent this from happening. It is a very simple, yet expensive part. It works with electromagnetism and a spring. The spring constantly keeps the metal pintle (finger) in the extended position until the ignition switch is turned on. When the switch is on, 12 volts is supplied to the solenoid, which makes the electromagnet pull down on the pintle. When the switch is off, the spring automatically returns the pintle to the extended position. If you have seen an IAC valve on a car, you will have an idea of what I am talking about.​​When you have the solenoid installed on the bottom of the carburetor and turn off the engine, the spring pushes up on the pintle. This pintle actually blocks the main nozzle, preventing fuel from being sucked up through it when the engine is coasting to a stop, thus preventing the loud bang noise when the engine stops.​​If you don’t have this solenoid, you can prevent the afterfire noise by slowing the engine down to idle and let it cool off for about 30 seconds. This will give the muffler a chance to cool.​​It is possible for the engine to still afterfire if you _*do*_ have the solenoid. To correct this problem, cut the engine off in fast. _*DO NOT IDLE DOWN.*_​​​


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## ST1100A (Feb 7, 2015)

tabora said:


> Actual experience... It only appears to work at full throttle. At full throttle shutoff, the engine pumps a lot of air through the muffler to cool it, but no fuel because the anti-afterfire solenoid blocked it. At idle shutoff (immediately after running the machine at full speed mowing), the muffler apparently does not get cooled down enough and the afterfire occurs.
> 
> From the manual:​Stopping the Engine​1. Move the throttle lever to Fast (Fig. 6).​2. Move the PTO to Off (Fig. 5).​3. Turn the ignition key to Off (Fig. 7).​
> From the Theory behind the “Anti-Afterfire Solenoid”:​​It does exactly that, prevents “afterfiring” when the engine is cut off. This is not to be confused with backfiring, which is a totally different thing. Afterfiring is that loud bang you hear after cutting off a tractor engine. This condition is more prevalent in Briggs and Stratton engines than any other engine. The loud bang is totally harmless to the engine, but can be scary or annoying to the operator.​​What causes the “afterfiring” is raw fuel touching the hot internal parts of the muffler. How this works is when you cut off the engine after mowing the grass, the engine “coasts” to a stop (keeps spinning until it slows down and stops). When the engine is coasting, the fuel is still being drawn into the cylinder on the intake stroke, compressed on the compression stroke, and exhausted on the exhaust stroke. The key here is the fact that the fuel is not combusted under the compression stroke (as it would be when the engine is on). The fuel takes a ride through the engine and ends up in the muffler. When this raw, unburned fuel hits the muffler, it explodes (like it would during the compression stroke). The fuel continually “rides” through the engine until it stops coasting. By the time the engine stops, there may be four or five compression stroke’s worth of fuel in the muffler, ready to blow up.​​Now the job of the solenoid is to prevent this from happening. It is a very simple, yet expensive part. It works with electromagnetism and a spring. The spring constantly keeps the metal pintle (finger) in the extended position until the ignition switch is turned on. When the switch is on, 12 volts is supplied to the solenoid, which makes the electromagnet pull down on the pintle. When the switch is off, the spring automatically returns the pintle to the extended position. If you have seen an IAC valve on a car, you will have an idea of what I am talking about.​​When you have the solenoid installed on the bottom of the carburetor and turn off the engine, the spring pushes up on the pintle. This pintle actually blocks the main nozzle, preventing fuel from being sucked up through it when the engine is coasting to a stop, thus preventing the loud bang noise when the engine stops.​​If you don’t have this solenoid, you can prevent the afterfire noise by slowing the engine down to idle and let it cool off for about 30 seconds. This will give the muffler a chance to cool.​​It is possible for the engine to still afterfire if you _*do*_ have the solenoid. To correct this problem, cut the engine off in fast. _*DO NOT IDLE DOWN.*_​​​


Very good explanation of the 'Fuel Cut solenoid'/'Anti Backfire Solenoid' and its function. 
You are correct, that little solenoid is not cheap either, sometimes $70-$100.
The do go bad, and when that happens, the engine will not start because it doesn't open to let fuel in.


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## ST1100A (Feb 7, 2015)

I have seen mufflers blow apart from what Tabora described about the 'After-fire' when the key is turned off. The canister type usually blow the end caps off.


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## ST1100A (Feb 7, 2015)

The Kohler 'Magnum' engines with the big cannister type mufflers love to make that '30-06' sound when you shut them down. They don't have the 'Fuel Cut' solenoid, so you have to let them cool down, idle for 30-60 seconds before you shut off the ignition or you will be replacing the muffler prematurely, and they aren't cheap.
The bigger the muffler, the worse the problem is usually, the smaller mufflers don't do it as much, but they still can do it.


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## 140278 (Aug 27, 2020)

both my old kohler SV and command v's with electric shut offs would backfire when throttled down to fast , the sv blew the muffler apart i've even had a old K series kohler do it. it's a simple fix throttle back slowly 
tabora like always has done a great detailed write up of why it happens thanks to him and our resident OPE dealer tech ST1100A for helping explain the how and why


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## Tseg (Jan 29, 2021)

LenD said:


> If you just close off the fuel supply and let it starve, won't that be the same as throttling back? As you want to get the fuel out of the system anyways when you're done............
> 
> What I now do is shut the fuel off at the bottom of the driveway, walk her back and by the time I'm in the garage, she's put put putting and ... goes to sleep 😉


Several things, where I particularly had my backfire issue was at start up, the Honda engine would run fast under choke to warm it up for 20 or so seconds else it would surge in High or stall in Low, then I would move to High, then move to Low for the rest of the warm up process (integrated choke/throttle lever). I now move to Low slower from High (2-3 seconds) vs. slapping it into Low. I’m supposed to move to Low before I begin to warm the hydrostatic transmission oil.

I don’t think closing fuel supply is the same as throttling back... when I close fuel supply at High, the engine still stays at high RPM until it starts surging right before runout. My old Honda clone engined blower would die with a backfire with a closed fuel system at the very end, my new real Honda engine does not die with a backfire, just some surging on its last breaths. Or if I know new snow is already in the forecast I’ll just turn off the key and it pleasantly, quietly turns off.


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## Toro-8-2-4 (Dec 28, 2013)

Tabora: Kudos on that write up. It Makes perfect sense.


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