HELP..briggs & stratton popping or backfiring through the carburetor

Curtisun

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  • / HELP..briggs & stratton popping or backfiring through the carburetor
i have a craftsman lawn tractor with a B&S 15.5 hp OHV engine, engine model=28N707..type=0173-01.. it had been setting for several years due to a coil problem, i repaired the problem a few weeks ago, the kill wire had grounded out killing the fire, so i put it on a toggle switch, the mower will crank and run, no problem, but it popps or backfires thru the carb, you can see fire in the carb, i have tried many things to correct the problem to no avail, here is what i have tried, 1;checked the key way=no shear..2;changed the spark plug= no change..3;gaped the coil at .010=no change..4;removed and cleaned out gas tank=no change..5';replaced fuel line and filter=no change...6; cleaned out carburetor with a guitar string and cleaner, all ports clear, solenoid working correctly= no change...7; removed head and inspected,& lapped the valves cleaned off carbon deposits, replaced head gasket ,push rods are fine...8; put head back on & adjusted valve lash to (in) .004...(ex) .006 = still popping, i also checked all gaskets to intake and carb,..it will smooth out at one place on the throttle, almost full throttle, but if you try to idle it down any the popping starts, the choke is working properly and all linkage is correct, i checked it with images from the B&S web site, this thing has me stumped, i dont want to use it backfiring like it is, and im sure it must be fixable, the mower has had light use over the years and is in great condition with no alterations other than the kill wire is now on a toggle switch, if any more information is needed please ask, i tried to be as detailed as possible in my description...thanks for your time
I figured this out though there are many other things that can cause this problem. The two main things are the fly wheel having magnetism around the fly wheel other then where the magnet is, and incorrect fuel air mixture.
First let's deal with the flywheel. I was having problems with backfire and actual flame coming out of carb. I had put new carb, fuel lines and fuel pump on machine due to fuel lines and pump being completely stopped up. The carb needle valve would stick closed so had to replace card. I tried all the normal stuff to stop it and nothing helped. Then I decided it had to be firing at the wrong time. So I started thinking now how could it fire at wrong time? There is only two reasons it can fire wrong they are key is damaged or magnetic problem with fly wheel. I took a screw driver that was not magnetic and check around fly wheel and found it was magnetic all way around just stronger at the magnet. This can cause coils to fire intermittently at wrong time.
Now to demagnetise the fly wheel there is only three ways take it to generator shop and have them demagnetise it, use heat to 700 degrees or use a hammer to strike around it. I used a hammer and extension and hit it from top all way around but not the magnet. This decreased the number of the actual backfire but not the sputtering. Now I also found even with the new carb the fuel air mixture was off. I could pull choke out about 1/4 way and adjust idle screw to proper idle, then sputtering and back fire stopped completely at all speeds. This tells me the mixture (which on this carb has no adjustment) is wrong.
Back fire can be caused by the engine not buring all the fuel and if the fuel air mixture is wrong it will not.
 

Dadmech

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  • / HELP..briggs & stratton popping or backfiring through the carburetor
A good idea was mentioned though. How does the plug look? If you put in a new one then check it again after you have been trying to start it. See if it is gummed up with oil, carbon, etc. I finally figured out the problem with mine by splitting it from the sump to check everything inside. If you go this route you will need a new sump/ oil pan gasket to put it back together.
So, figured out the problem by splitting the sump? What did you find that figured out the problem?
 

slomo

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  • / HELP..briggs & stratton popping or backfiring through the carburetor
Improper ignition timing guys..... Plug is firing when the intake valve is open as in before TDC.

Might de-carbon the cylinder/s as the manual states yearly.

slomo
 

Kodi_latham

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  • / HELP..briggs & stratton popping or backfiring through the carburetor
Might be the spark plug, that was the case on my Briggs 450 e
 

slomo

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  • / HELP..briggs & stratton popping or backfiring through the carburetor
Anyone think about a loose valve seat maybe?
Sure possible. People never clean the engine block cooling fins with packed in grass and oil. That over heats the block causing valve seats to jump loose.

slomo
 

StarTech

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  • / HELP..briggs & stratton popping or backfiring through the carburetor
Or even a loose muffler or missing exhaust gasket can do it. I know this as I had a seat on my personal mower to lose a seat last Summer myself where the exhaust blew out and I didn't notice it.
 
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