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This may not be a problem but it's annoying when it happens. The engine is a 18.5 hp B&S on a riding mower. The pop sounds like a hand gun being fired. It happens mostly when the engine stops for safety reasons such as when engage reverse while mowing or when I stand up while engine is running. I like to know what causes the loud pop and if there is a way to prevent that (other than defeating the safety switches).

The safety features on your machine shut down the engine when you don't follow procedures. When the engine shuts down under these conditions, as the RPM drops down, the throttle governor opens the carb throttle more in an attempt to maintain RPM. When this happens, unburned fuel passes thru the engine and into a very hot muffler where the temp is hot enough to flash the fuel causing this loud bang.
Years ago I was mowing lawns for a friend in the lawn care business who had injured himself. He asked me if I could do some of his customers lawns while he was healing and I said yes. I had finished mowing one of his customers, put my mower in my trailer, but shut it down too soon and it did the "gunshot" bang thing. As I was cleaning up this driveway with my blower, the local police came screaming up to the house, ordered me to the ground!!! The next door lady had called the police saying that she heard gun shots!!! The police took me away in cuffs, searched for the gun in my trailer, truck, the nearby sewer drain, it wasn't very funny for a while. I told them it was my hot tractor that I had shut down too soon, but they still did a gunpowder check of me and my equipment after impounding my truck, trailer, and equipment. It cost me lots of grief and time, and the town sent me a bill for $3,500 for the cost of the investigation!!! I ended up in court, didn't pay the town anything, but had to pay the court costs.
My friend thought it was hilarious, but it took me a long while to get to that point!!!!!
I did some research in local newspaper archives and found that this lady had made many calls to the police over the years for dumb reasons. I sent the police chief a request for a fireworks permit with her address on it, well, I thought it was funny, after all, they (three officers) came after me with weapons drawn and all I had was a leaf blower and hearing protection!!!! Good thing my leaf blower didn't bang when I shut it down, they would have shot me!!!!!
Mad Mackie in CT:biggrin::laughing:

Lol thats a funny story. I find it affective to put the idle all the way down let it sit for a sec then shut it off. It works on my old ranch king mower.

I forced myself to let my machines have a idling cool down from that time on. I no longer have any Onan powered Ingersoll garden tractors, they are famous for the "big bang", but still a good habit to get into.
The lady that called the police on me called them again two weeks later when I arrived to mow her neighbors lawn again. She was eventually arrested and found to be mentally unstable, a sad story actually, when I heard about it several years later I felt sorry for her, but I still had to go to court, I could write a book about this, but I won't!!!!!
Mad Mackie in CT:biggrin::laughing:

Mad Mackie said:

I forced myself to let my machines have a idling cool down from that time on. I no longer have any Onan powered Ingersoll garden tractors, they are famous for the "big bang", but still a good habit to get into.
The lady that called the police on me called them again two weeks later when I arrived to mow her neighbors lawn again. She was eventually arrested and found to be mentally unstable, a sad story actually, when I heard about it several years later I felt sorry for her, but I still had to go to court, I could write a book about this, but I won't!!!!!
Mad Mackie in CT:biggrin::laughing:

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those cops totally over reacted!....very unprofessional!

It's just the last bit of the gasoline exploding because of the high idle turn off . Turn the idle down when you stand up or turn the key off . Eventually you'll ruin the muffler .

I did some more reading on Troy Bilt web site. Actually, for my riding mower, TroyBilt recommend "full trottle" shut off. There is an electronic fuel shutoff solenoid in the carburetor to stop the fuel. So, I guess during "safety shut off", the solenoid valve is still open and that explains the continued fuel supply. It looks like a design flaw to me. Why can't they also shut off the solenoid during safety shut-off?

The Onans on my older Ingersoll GTs didn't have the fuel shutoff solenoids and a full throttle shut down only made the bang worse to the point of bursting mufflers. I repowered my newer Ingersoll with a Vanguard 18 in 2005, and it has the fuel solenoid, but I will still get a bang unless I let it idle for several minutes. The throttle/governor response on Vanguards is so quick that at shut down a fair amount of fuel still gets thru and into the muffler.
My friend whose customers lawns I was mowing wasn't able to mow for the rest of that season as he needed several surgeries on his right arm. It was mid August when I started and a very hot summer.
This arrest had been put on my record as weapon fired but not found. I was not happy with this and it took 4 years and a lawyer to get it removed. One of the officers involved was already under internal investigation and was eventually terminated. I did eventually receive a letter of apology from the police dept in that town, I returned it saying that I would only accept it with acknowledgement from the State Attorney's Office and they did this.
As I mentioned in a previous post, I could write a book about this incident!!!! My friend never did fully recover and got out of the lawn care business. He went to school for computer programing and has his own computer business.
Mad Mackie in CT:biggrin::laughing:

panabiker said:

I did some more reading on Troy Bilt web site. Actually, for my riding mower, TroyBilt recommend "full trottle" shut off. There is an electronic fuel shutoff solenoid in the carburetor to stop the fuel. So, I guess during "safety shut off", the solenoid valve is still open and that explains the continued fuel supply. It looks like a design flaw to me. Why can't they also shut off the solenoid during safety shut-off?

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This may happen on your machine, it all depends on the wiring and safety systems. Shuting down an engine involves grounding the ignition system. Closing the fuel shutoff solenoid involves removing the battery power from it. Most electrical safety systems do both, it all depends on what regulations were in effect when the machine was manufactured and if the systems are still functional.
On engines with fuel shutoff solenoids, a full throttle shut down is recommended, on engines without, a five minute cool down at idle RPM is recommended. I do the cool down thing as I can get off my machines and they continue to run as long as I have put them in the correct condition beforehand.
Mad Mackie in CT:biggrin::laughing:

If you don't mind increasing your operating cost by 3% ... you can eliminate the problem by switching to premium fuel... With enough octane, it will not heat detonate...
My Cub Cadet Kohler, use to do this when I ran less than premium fuel... :smile:KennyV

Hi KennyV,
I think by that time that I had started using 89 in my Onans, don't recall. I did eventually go to 91 for use in my Onans and two strokes. Engines with fixed high speed jets are run so lean due to emission demands that they run hotter than older engines with adjustable jets. I noticed this on my Ingersoll 448 when I repowered it in 1996 from a B series Onan to a P series Onan. I was using my infrared temp gun and noticed a dramatic increase in muffler temps. The B series Onan was developing too much blowby so I replaced it. I bought this tractor used with a nonfunctional hourmeter that stopped at 1,000 hours. Case and Ingersoll both used K series Kohlers and most of the folks that still operate them have increased the octane of the fuel they use.
All my Briggs engines run fine on 87 so that is what they get, but I still have the 91 in my trailer for the two strokes and my Stihl FourMix engines.
Mad Mackie in CT