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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

could be a possibility that the compression release lobe on the camshaft is stuck allowing it hold the intake valve open slightly. If run for awhile it may come out of it, but I have also seen mowers caught on fire due to backfire through the carb.

@ilengine...i dont think so, the valves close all the way when i set the lash, you turn the engine 1/4 inch past tdc to compensate for the pressure release, or thats how i read the instructions, i have ran the engine for about an hour total, even added a little marvel mystery oil to the gas at first, everything looks great in the head, and the valves had no burnt spots, they open and close fine, and when i lapped them the pattern on the valve face was nice and even, thats what has me stumped, every thing looks great, the carb is so clean you could eat out of it,i have checked for vacuum leaks also, cant find any, it starts so easy, runs great other than the pop

Did you ever figure out the problem? I have a 17.5 hp briggs and is acting the same way.

For an engine to backfire through the intake, the intake valve must be open at the time the air/fuel mixture ignites. You have gone to great lengths to eliminate as much as possible but something is still out of time or the intake valve is sticking open or being held open for this problem to occur.

I used to work on cars back in the 80's & 90's and sometimes a flat lobe on a camshaft would cause a similar condition. You may have to open the engine and use a micrometer to check the camshaft lobes for wear. I wouldn't start there but I also wouldn't rule it out.

Here's a crazy thought: could the crankshaft be twisted without being bent at an angle? Hmmmmm.

That backfiring through the carb could be an intake valve intermittently sticking. First thing that came to mind was a partially sheared flywheel key, causing the engine to be a little out of time. On the older engines with points I would see a condenser going out, or be loose on the connections and cause backfiring; but you don't see those old points and condenser units much any more. I occasionally run into one now and then. Hope this helps

Bump...

First post... Has anyone figured this out yet? Have the same problem on a 15 horse Briggs.


Thanks

Eric

I may as well add my 22Hp V Twin Intek B&S to the list. It is doing the same thing. My right side coil needed to be replaced which I did and now it is backfiring. I too have set valves, checked flywheel key, and cleaned carbs. What next?????:confused2:

Eric_05 said:

Bump...

First post... Has anyone figured this out yet? Have the same problem on a 15 horse Briggs.


Thanks

Eric

Click to expand...

Tried Seafoam Intake cleaner, seemed to help some.
Drove the mower around for a half hour or so after.
Only popped back twice. the can is big enough for 2-3 applications
smoked like crazy for a minute while the carbon was burning off. seemed to smooth out
some.

Will report back in a few days, after spraying it again.



E

Bump, kind of curious what the outcome was

Eric_05 said:

Tried Seafoam Intake cleaner, seemed to help some.
Drove the mower around for a half hour or so after.
Only popped back twice. the can is big enough for 2-3 applications
smoked like crazy for a minute while the carbon was burning off. seemed to smooth out
some.

Will report back in a few days, after spraying it ag

E

Click to expand...

Don't have coil on upside down do U?

Xxtjtheboater95xx said:

Bump, kind of curious what the outcome was

Click to expand...

Me too. The last one I had do this had a crack in the cylinder wall right at the bottom past the stop point of the rings. It back fired through the carb so much it melted the intake on the right side. I bought it used, and the guy had tried to rebuild it after it threw a rod. It would run but the busted place, and the crack caused it to blow the compression backwards back out through the carb. I ended up getting another engine to rob the top half off of. I put it on the sump pan of this motor, and it runs like a brand new one now. Its the mower in my avatar on the up hill side of the picture dys4500.

hit it With a BFH that will fix it

vandredi said:

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,

Click to expand...

Yeah, you should backtrack to the beginning....

Can you put up a pic of this "toggle switch"??? Have you tried anything simple like a new plug?????

The problem you are describing I would say camshaft.

reynoldston said:

The problem you are describing I would say camshaft.

Click to expand...

Yep worn lobes, or missing teeth.

wjjones said:

Yep worn lobes, or missing teeth.

Click to expand...

Hey, you knew my first wife????

Fish said:

Hey, you knew my first wife????

Click to expand...

:laughing:..:laughing:.. And mine.:laughing:

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.

I have the same problem 19 hp but I would almost say exhausting through the carb. it runs but putting some exhaust out the carb..what should the compression be ? its around 75 psi seems out of time maybe? any ideas?

Remove the valve cover and have a look, put up the engine's numbers, which are likely on or near the valve cover.

You could have a sticking valve, loose seat, sheared flywheel key, but don't keep running it, or you could have a pile of rubble....

Thank you,
The numbers are Model 3p877, type 0110 E 1, code 06042oze. I took the valve cover off as advised, one of the valve studs was backed out of the head at least a turn and a half, this allowed the cap on the top of the valve to fall off, I put the cap on and tightened the stud with a wrench on the stud, thinking that the nut would turn on the stud before the stud was tight and throw the valve out of adjustment. I put the valve cover back on .It started right away and seems fine now. I think I'm going to take it apart again and put some lock tight on the stud.

thanks for the advice.

Get a set of feeler gauges and adjust the valves too! Do it right, and it won't keep being a problem. Yeah, Loctite is a good idea.

sounds Good I Have feeler gauges, what should they be set at? do you know? and do you set them running? or is there another way?
thanks again.

No, the engine must be cold, off, engine at TDC.

Briggs and Stratton Valve Clearances

wjjones said:

Yep worn lobes, or missing teeth.

Click to expand...

Even with worn lobes, wouldn't resetting the valve lash fix the problem since the valve should still open correctly at the proper time?

I had this problem on a 15.5 that had ran over a trailer anchor and stopped running. The cam shaft was messed up.

donens said:

Even with worn lobes, wouldn't resetting the valve lash fix the problem since the valve should still open correctly at the proper time?

Click to expand...

No because you adjust the valves when they are closed not open.(not touching lobes)

vandredi said:

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

Click to expand...

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.

wjjones said:

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.

Click to expand...

So, figured out the problem by splitting the sump? What did you find that figured out the problem?

briggs said:

hit it With a BFH that will fix it

Click to expand...

Turns out, apparently, that's very true.

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

Might be the spark plug, that was the case on my Briggs 450 e

Anyone think about a loose valve seat maybe?

SmallEngineHead160 said:

Anyone think about a loose valve seat maybe?

Click to expand...

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

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.