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Thread: Back Firing Mower , Will it hurt if I ??

  1. #1
    Boolit Master

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    Back Firing Mower , Will it hurt if I ??

    I have a older Walker Zero Turn mower

    It backfires when you shut if off , if you don't idle it down all the way and let it run a min

    Well tonight I mowed the grass .... only 90 or so degrees this afternoon

    So as I sat there a bit with it idled down to turn off , Cooking my few brain cells in the sun
    I remembered I needed to cut some of the grass at dads Saturday

    So I drove it the 100 yards to his house
    Of course I had 10 other things on my mind and instead of waiting any time with it idled down
    I just went to turn it off
    But my hand bumped the choke lever to on as I turned it off

    OK my bad , but no backfire

    So I am sitting here soaking up the AC , playing a mind numbing computer game to kill time
    As I wait for the headacke meds to start working ( just another normal every day Migraine ... and I have meds for them )
    I was wondering if I choke the mower as I turn it off , to stop the backfire when I am in a hurry
    Will I hurt the motor ?

    Thanks
    John
    Yea, thou I walk through the valley of the shadow of death, I will fear no evil; for thou art with me; Thy rod and thy staff, they comfort me.
    And I carry a SIG

  2. #2
    Boolit Master
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    It can wash the cylinder walls down with the extra gas in the combustion chamber and take the oil film off. If minor, shouldn't be too much of an issue. The real problem could be either timing is off, or the motor could be running lean, neither one of which is a good thing. Chevy v-8's are a good example of this. If the issue isn't too severe, it's probably OK. If it's bad, it probably should be fixed. This can be thanked in large part to the lean-burn engines demanded by our pollution police.

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    It’s a fuel mixture issue. If it’s an adjustable carb, enriched your low speed setting a bit.

  4. #4
    Boolit Master
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    Those Walker ZTR's are beautiful mowers. I used to work on them. It would be worth having fixed.
    In general it is not wise to choke a mower to shut off. Basically what you are doing is flooding it with so much gas that it stops. You can do it a few times without major consequence but if you do it regularly you will hurt the motor. Best to find out why it is running after being turned off.
    Most often that condition in a lawn mower is a matter of timing. Check the timing or have some do so. It could be other things too though.
    It could be the wrong spark plug. It could be that the motor is very carboned up (it can get that way from choking it off all the time) It could be even a bad muffler causing the engine to get too hot. Many things... but as I said those are very nice mowers and worth treating well. Get it fixed and let us know what turned out to be the cause. We are all curious now.
    AKA hans.pcguy

  5. #5
    Boolit Master

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    OK
    I can change a Spark Plug and even know enough to put the proper plug back in

    But most of the other stuff is not something I do

    So Next week I will have it looked at

    As for choking it as I turn it off
    That was the first time for that to happen

    Well the first time I did it , but I bought it used 2 years back at a estate sale
    So I have no idea what was done before

    But thanks

    John
    Yea, thou I walk through the valley of the shadow of death, I will fear no evil; for thou art with me; Thy rod and thy staff, they comfort me.
    And I carry a SIG

  6. #6
    Boolit Master
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    You could have a lot of carbon stuck to the combustion chamber in the head. Letting the mower idle down an cool off lets the carbon quit glowing.

    If you never run your mower at full throttle you might try it. If you have a lot of carbon it will often blow sparks out the exhaust.
    EDG

  7. #7
    Boolit Master






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    your running to lean. that's cause most back fires. That's why when you killed it with your choke it didn't back fire. Either you need some carb tuning or carb cleaning or possibly the timing is to far advanced. Whats happening is your piston is getting so hot that its igniting the fuel without a spark from the plug. Its detonation. Same thing as when you hear spark knock. Don't let it run like that. You could very easily melt a piston and ruin your motor. I know spending money sucks but if you don't know how to fix it yourself a small engine mechanic should be able to cure it for 50 bucks. A lot better then having to buy a new motor. What happens many times to those old motors that were designed to run gas without alcohol in it is plastic parts inside the carburetor actually break down and plug up jet orfices. Once you get it up and running id suggest you find a station that sells alcohol free gasoline. that usually means 91 octane premium. Most regular grade gas and even 93 octane premium has alcohol in them.
    Soldier of God, sixgun junky, Retired electrical lineman. My office was a 100 feet in the air, closer to God the better

  8. #8
    Boolit Master
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    Quote Originally Posted by Lloyd Smale View Post
    your running to lean. that's cause most back fires. That's why when you killed it with your choke it didn't back fire. Either you need some carb tuning or carb cleaning or possibly the timing is to far advanced. Whats happening is your piston is getting so hot that its igniting the fuel without a spark from the plug. Its detonation. Same thing as when you hear spark knock. Don't let it run like that. You could very easily melt a piston and ruin your motor. I know spending money sucks but if you don't know how to fix it yourself a small engine mechanic should be able to cure it for 50 bucks. A lot better then having to buy a new motor. What happens many times to those old motors that were designed to run gas without alcohol in it is plastic parts inside the carburetor actually break down and plug up jet orfices. Once you get it up and running id suggest you find a station that sells alcohol free gasoline. that usually means 91 octane premium. Most regular grade gas and even 93 octane premium has alcohol in them.
    /\ This

    Ethanol in gasoline does bad things to fuel systems, particularly the carburetors on small engines. Most of the non-ethanol fuel I see is actually 87 octane and that's fine in just about any low compression small engine.
    Changing to non-ethanol fuel doesn't fix anything but it stops perpetuating the damage; so fix the problem and then switch to non-ethanol fuel after you've fixed it.

    Running a spark plug that has the wrong heat range will cause the problem described and is a pretty easy fix.

    More than likely you have a fuel problem that will require a carb rebuild or a new carburetor. It probably is running lean.

    BTW, air cooled engines with governors (typically used on lawn care equipment) should be run at full throttle when under load. Running them at part throttle under load reduces the air flow over the engine and actually makes them run hotter than when operated at the maximum governed engine speed that they are designed to run at.

    So in a nutshell: Find the problem, fix the problem, switch to non-ethanol fuel, carry on.

  9. #9
    Boolit Master
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    My Simplicity riding mower has a Kohler 14 H.P. engine that loves to backfire when I shut it down. It has done this for as long as I have owner the mower, about 8-9 years. So far I have not found this to cause any problems with operation or durability. Another oddity with this mower was a real problem. I could run it for about an hour and it would begin to run slower, then slower, then shut down. The fuel filter was clear and empty of fuel. There was no gas flow from the tank to carburetor. I would have to wait about a half hour to get the motor to start again but then could get only about a half hour of work out of it before this happened again. I took the mower to a repair shop and explained the issue to a friend who is the best small engine mechanic I have ever known. He had never heard of this before. He replaced the fuel lines, fuel pump (they aren't cheap) and did a few other things like filling the fuel tank and letting the mower run continuously until it was out of fuel (full tank and the engine ate all of it, never stopped). He could not find the problem or the cure. The shut-downs came when the engine was working under the load of moving and cutting grass. Later on I was looking over the fuel lines for possible issues and bumped my hand against the side of the motor. A slab of packed, hard dirt fell off the side of the engine. As I looked at the engine I saw a cooling vane, like on the cylinder of a motorcycle. The more I looked all I could see was dirt so I started chipping away and thoroughly cleaned all of the dirt out from every cooling vane (all of these voids were full packed!). This reminded me that I used to routinely wash down the mower and engine after cool-down. I was wetting the dust on the motor and allowing it to pack the cooling vanes. That motor was overheating and vapor locking! After this cleaning the motor now works until I decide to shut it down. I was thrilled and carried this message back to my friend. I wasn't trying to embarrass him that I found a problem he could not cure, only to share the answer to the riddle in case he ever encountered this question again. He accepted the information and was grateful to have it. Air cooled engines are their own breed.

  10. #10
    Boolit Mold
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    If there is an electronic solenoid on the bottom of the carb, the engine needs to be at 3/4 or more throttle when you shut it down. The solenoid shuts the fuel off to the jet. If you shut the motor down at below 3/4 throttle, the solenoid will not fully close allowing fuel into the carb and the motor to backfire. It can also allow fuel into the cylinder when the motor is not running which will fill the crank case full of gas.

  11. #11
    Boolit Master


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    Allowing your engine to backfire can destroy the muffler, and they ain't cheap. Turning off any small engine at above min throttle allows the air/fuel mix to be pumped on through the exhaust where any carbon ember can ignite it. Find and read the manual for your mower/engine for instructions if there is a fuel retard valve. Other wise just let it idle before turning off the ignition. No need to sit at idle, just let the RPMs die.
    Information not shared. is wasted.

  12. #12
    Boolit Master popper's Avatar
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    AC motors have a vane governor, check that it moves freely. It pulls the carb closed to limit speed.
    Whatever!

  13. #13
    Boolit Man
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    A backfire when shutting the engine off without letting it idle down is common with engines of this size and is DOES NOT indicate you have a lean fuel mixture condition .

    What's happening is very simple ... when you shut off the engine at higher speeds while it is still hot you are only cutting off the spark .
    Fuel and air is pulled threw the engine and not burnt but still heated up from compression . It then gets pushed into the exhaust which is still hot enough to ignite the unburnt pre heated fuel causing the backfire . .. a very common problem that has nothing to do with your fuel air mixture . ..

    Cayton mentions the fuel shutoff solenoid that some manufacturers added to cure the problem . If you your engine has it its obviously not working . If it doesn't try your best to remember to idle it down for 30 seconds or so to let the exhaust cool enough to not cause the backfire .

    Another cure for this is to change to a different exhause system that puts the carbon collecting muffle baffles that collect the heat and ignite the fuel causing the backfire further away from the exhaust valve 6-8 more inches of pipe between the two would provide enough cooling to stop it .
    Last edited by redneck1; 06-01-2018 at 01:43 PM.

  14. #14
    Boolit Master






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    my guess is your motor is getting hot from to much timing or to lean of a mixture and is vapor locking.
    Quote Originally Posted by Thin Man View Post
    My Simplicity riding mower has a Kohler 14 H.P. engine that loves to backfire when I shut it down. It has done this for as long as I have owner the mower, about 8-9 years. So far I have not found this to cause any problems with operation or durability. Another oddity with this mower was a real problem. I could run it for about an hour and it would begin to run slower, then slower, then shut down. The fuel filter was clear and empty of fuel. There was no gas flow from the tank to carburetor. I would have to wait about a half hour to get the motor to start again but then could get only about a half hour of work out of it before this happened again. I took the mower to a repair shop and explained the issue to a friend who is the best small engine mechanic I have ever known. He had never heard of this before. He replaced the fuel lines, fuel pump (they aren't cheap) and did a few other things like filling the fuel tank and letting the mower run continuously until it was out of fuel (full tank and the engine ate all of it, never stopped). He could not find the problem or the cure. The shut-downs came when the engine was working under the load of moving and cutting grass. Later on I was looking over the fuel lines for possible issues and bumped my hand against the side of the motor. A slab of packed, hard dirt fell off the side of the engine. As I looked at the engine I saw a cooling vane, like on the cylinder of a motorcycle. The more I looked all I could see was dirt so I started chipping away and thoroughly cleaned all of the dirt out from every cooling vane (all of these voids were full packed!). This reminded me that I used to routinely wash down the mower and engine after cool-down. I was wetting the dust on the motor and allowing it to pack the cooling vanes. That motor was overheating and vapor locking! After this cleaning the motor now works until I decide to shut it down. I was thrilled and carried this message back to my friend. I wasn't trying to embarrass him that I found a problem he could not cure, only to share the answer to the riddle in case he ever encountered this question again. He accepted the information and was grateful to have it. Air cooled engines are their own breed.
    Soldier of God, sixgun junky, Retired electrical lineman. My office was a 100 feet in the air, closer to God the better

  15. #15
    Boolit Master






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    you are partialy right. Idling down slowly does help because its going to run cooler and your relying on your low speed jets or orficies in the carb instead of the high speed jets or orphices. Something has to get that combustion chamber hot enough to detonate ( light the gas off without a spark) Again two major cause of this are a lean mixture or to much advance in the timing. I can run down the road in my truck wide open and shut the key off and it will not backfire.

    The fuel spray from your carb is what burns. Everyone knows that. Its also a major player in cooling your combustion chamber. Not enough and you get hot spots that will cause pre ignition (detonation) Advancing the timing to much cause static compression to increase. It makes the motor fire when the piston is still on its way up and at close to its highest point increasing compression. That is why high performance motors need higher octane fuel. Many think premium burns hotter and that's why you can get more power out of it. Just the opposite is true. Octane retards burning. It make your fuel harder to ignite so its less likely to ignite before its suppose to. the myth goes that premium gives better gas mileage. Its a myth. It actually will give you worse gas mileage unless you increase timing or compression (either one increases static compression) to take advantage of it.

    Alchohol does the same thing as octane additives. It makes your fuel harder to ignite and burn less efficiently and is why cars that have flex fuel systems get worse gas mileage with alcohol fuels. Its why some souped up cars are tuned with alcohol fuels. You can run much more compression and timing with a 85 octane alcohol blended fuel then you even can with 93 octane pure gas. But again if its not tuned for it you actually loose power. You did hit on one correct point though. If your motor has run rich long enough to carbon up the motor you can get hot spots on the carbon deposits that will cause pre ignition and if you get enough carbon build up you can actually raise compression because of the smaller combustion chamber enough to cause pre ignition.

    Put some good gas in it to start with. Like I said try some 93 octane fuel with no alcohol. Maybe a bit (small amount) of something like GM top end cleaner that will clean your carb and help with deposits if there not to bad but if its back firing its probably beyond that point and needs a mechanic. 85 octane with lots of alcohol will do about the same thing but your carb isn't designed for it and in the long run it will do damage. Problem you run into is the super dupper small engine mechanic around the corner. Most are parts changers. Find one that can measure your exhaust temps and your fuel/air ratios and will actually tune a carb and adjust timing correctly. that means mostly going to a big shop that has equiptment and shelling out the cost of a real mechanic instead of a local hack that cant afford the equiptment to do it.

    Most don't think twice about taking there modern car or truck to a mechanic that makes 50-100 bucks an hour but cringe when a REAL small engine mechanic wants to charge them 30 bucks an hour. Take it to the local (self proclaimed expert) with a problem like this and he will tell you you need a new carb because swapping them is easy and he can make money of the install and sale. He will probably slap on a new carb that has some generic jetting set up so it will at least make most motors run. He still wont take the time to actually tune it to your application. Or stick a pipe on the exhaust (a band aid) to hide the fact your running to lean or to advanced in timing and it might just run fine for a while until it burns a hole in your piston. Ive heard of unscrupulous ones that will do nothing but put some premium fuel in the tank and claim they did a bunch of work and send you on your way with a few less greenbacks only to have the problem pop back up again and some are even stupid enough to go back to the same guy to get fixed what should have been done the first time.
    Quote Originally Posted by redneck1 View Post
    A backfire when shutting the engine off without letting it idle down is common with engines of this size and is DOES NOT indicate you have a lean fuel mixture condition .

    What's happening is very simple ... when you shut off the engine at higher speeds while it is still hot you are only cutting off the spark .
    Fuel and air is pulled threw the engine and not burnt but still heated up from compression . It then gets pushed into the exhaust which is still hot enough to ignite the unburnt pre heated fuel causing the backfire . .. a very common problem that has nothing to do with your fuel air mixture . ..

    Cayton mentions the fuel shutoff solenoid that some manufacturers added to cure the problem . If you your engine has it its obviously not working . If it doesn't try your best to remember to idle it down for 30 seconds or so to let the exhaust cool enough to not cause the backfire .

    Another cure for this is to change to a different exhause system that puts the carbon collecting muffle baffles that collect the heat and ignite the fuel causing the backfire further away from the exhaust valve 6-8 more inches of pipe between the two would provide enough cooling to stop it .
    Soldier of God, sixgun junky, Retired electrical lineman. My office was a 100 feet in the air, closer to God the better

  16. #16
    Boolit Master






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    ill give you some good advice here. Want to find a good small engine man that knows his stuff and tunes these motors correctly. Go to your local go cart race track and see whos winning. Ask him who tunes his motor. Some of these motors are even supped up and run higher compression. They run down a straight away wide open and get out of the gas instantly and you don't here them back firing. Why? Because there carbs and timing are spot on. they go as far as even adjusting timing and fuel air mixtures for the temps of the day and humidity.
    Soldier of God, sixgun junky, Retired electrical lineman. My office was a 100 feet in the air, closer to God the better

  17. #17
    Boolit Master
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    Thereís one gas station within 50 miles of me that has alcohol free gas and itís 85 octane. Once a year or so I load up all of my cans and make the trek to buy $100 or so of gas for my mowers, chain saw, etc.

    I mainly do it because of the negative effects ethanol has on the fuel system parts of small engines (my oldest saw is now on its third set of fuel lines) but I have also noticed a definite increase of power with the lower octane fuel. Plus, it doesnít varnish up the carb like regular old road gas does.
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  18. #18
    Boolit Man
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    The devil is in the details ... and the op gives the little detail that is the problem .

    A backfire when he forgets to idle the engine down when shutting it Off.
    Again this is a common thing to happen and is why the fuel shut off solenoid was added by a lot of manufacturers.
    Shutting the engine off at anything over 1/4 throttle means you are working off the main jet ... not the idle circuit .
    Which gives plenty of un-burned pre heated fuel to ignite inside the exhaust causing the backfire .

    Many years ago i worked for ventrac In Orrville Ohio and had to sit threw a 4 hour seminar from both kohlor and kiehen carbs On this exact problem .

    It's not a tuning issue unless it's running poorly ... the op doesn't say anything about his mower running poorly .

    It's simply a fairly large muffle that is collecting a huge amount of heat from being close to the exhaust valve and igniting un-burned fuel when the engine is shut off .

  19. #19
    Boolit Master

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    My Cub Cadet has a Kohler and did the same thing. I started running either mid grade ethanol free gas from a local gas station or reg unleaded with ethanol killer stuff/Marvel Mystery and idled down before shutting off and the backfire stopped. Could be coincidence but i don't believe in those. I also pulled the plug and gaped it correctly.
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  20. #20
    Boolit Master






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    not true. a motor with advanced timing or a bit lean will run just fine. in fact it will put out even more power right up to the time it melts a piston. if it was a build up of fuel then it would back fire every time you went from full throttle to idle fast and ive done that with lawn mowers, outboards, atvs, cars, trucks, snowmobiles, heck about anything with a motor. I raced snowmobiles and motocross where you had to have a tether that killed the motor instantly if you went flying. Never heard a single backfire. Ive seen 1000 hp motors on a dyno go from wide open to having the throttle dumped and never heard a back fire. Run on and detonation are caused but a hot spot in your motor igniting the fuel before or without a spark. No hot spot no backfire. Fuel actually cools a motor. Its why fuel injected motors can run higher compression. THe fine mist cools before it ignites. If your motor doesn't have a hot spot (as it wont if its tuned properly) worse thing that will happen chopping the throttle from wide open is flooding your motor or momentarily going rich and fouling. Rich doesn't detonate lean does. Two much gas left over or shot into your motor will just make it harder to ignite not easier. If you doubt that run your motor wide open and take a basting bulb and shoot a bunch of gas into your carburetor and see what happens. It will just shut down your motor like you choked it. If left over gas was the culprit then shooting a basting bulb of gas into your motor would blow it sky high.

    want to hear a lawn mower motor really backfire? Take the muffler right off. Why? because the muffler causes back pressure that doesn't allow as much air to flow through your motor. take it off and more air (LEAN) is allowed in and the leaning out causes hot spots on your piston and detonates the fuel without a spark. this is why John didn't get a backfire when he bumped the choke. It richened his mixture enough to cool the hot spot and is why when he idles it down first it doesn't do it. Its because he allowed the motor to run on the low speed circuit in his carb which is probably richer and again cools the combustion chamber and the hot spot goes away. If the muffler was igniting the fuel it would back fire worse when he hit the choke.
    Quote Originally Posted by redneck1 View Post
    The devil is in the details ... and the op gives the little detail that is the problem .

    A backfire when he forgets to idle the engine down when shutting it Off.
    Again this is a common thing to happen and is why the fuel shut off solenoid was added by a lot of manufacturers.
    Shutting the engine off at anything over 1/4 throttle means you are working off the main jet ... not the idle circuit .
    Which gives plenty of un-burned pre heated fuel to ignite inside the exhaust causing the backfire .

    Many years ago i worked for ventrac In Orrville Ohio and had to sit threw a 4 hour seminar from both kohlor and kiehen carbs On this exact problem .

    It's not a tuning issue unless it's running poorly ... the op doesn't say anything about his mower running poorly .

    It's simply a fairly large muffle that is collecting a huge amount of heat from being close to the exhaust valve and igniting un-burned fuel when the engine is shut off .
    Soldier of God, sixgun junky, Retired electrical lineman. My office was a 100 feet in the air, closer to God the better

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