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

[vandredi]

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

 

[ILENGINE]

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.

 

[vandredi]

@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

 

[sunofah]

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

 

[Lawnranger]

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.

 

[DaveTN]

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

 

[Eric_05]

Bump...

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


Thanks

Eric

 

[dfburns]

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]

Bump...

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


Thanks

Eric

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

 

[Xxtjtheboater95xx]

Bump, kind of curious what the outcome was