View Full Version : Briggs & Stratton backfire

03-11-2005, 12:10 PM
After cleaning carbarator and checking all parts my B&S will just cough a low backkfire each time I pull the starter cord. It sound like it might be too rich, but will not even fire if I don't push the primer button several times.
Any help would be much appreciated. It's a lawnmower.
Thanks, jim

03-11-2005, 02:11 PM
sounds like it might be but without the HP rating i won't know really how to help, but it either sounds like its rich or its lean, if its a 3.5hp now it will have a diaprahm and that will need replacing. its either that or a flywheel key. i would like to know the HP and its age.

03-11-2005, 03:18 PM
Thanks for the reply bugman.
The HP is probably the smallest they make... can't find it on the engine, but it is for a small rotarymower. Maybe between 1/2 and 1 HP. It's few years old and lives outside under a tarp. It ran last year.
Where is the diaprahm?

03-11-2005, 04:48 PM
If it lives outside under a tarp, I will be willing to bet $20 that the carb/fuel tank is full of water. Even if it was protected from the rain, leaving it under a tarp will cause lots of condensation in the tank. You probably need to empty the gas tank and clean the carb out, then fill it with fresh gas and see if it will fire up.

03-11-2005, 05:19 PM
yeah may have water in it, plus it should have a new diaprahm if its original. the diaprahm is on the carb. it may be a 2hp engine, does it have the model #'s etc. because you could go to brigg's website to find out the HP rating. oh and tarps are to be used to lightly cover em. you maybe should find a place to just put it under rather then wraping it up. most use under their houses but that can be worse.

03-14-2005, 03:20 PM
Have taken apart the carb several times and cleaned every part I could find and then cleaned again. Tried to find a diaphram by completely taking apart the carb... Looked on the B&S site at the diagrams of my model 12H800 and couldn't find anything resembling a diaprahm.
Squrted some gas in the cylinder and still only puffs of smoke out the carb and nothing near running.
Could it have to do with timing? Thanks.

03-14-2005, 06:06 PM
Your model number is 12H800, but what is your type number? The model number tells us what engine block it uses, but doesn't tell us anything about the carb. If you can give us the type number so we can determine what type of carb you have, we can probably help you out. There are little tricks and problems that are specific to different carbs so knowing which carb you have will really help.

03-14-2005, 08:18 PM
yes give us the whole #'s. all of em to be sure.

03-14-2005, 08:19 PM
i'll bet 20 that a valve is stuck open , or a sheared fly wheel key, or a fouled plug will pop like that too

03-14-2005, 08:21 PM
maybe, just can't tell unless we get rid of the carb as a problem but it may be a valve or key.

03-14-2005, 08:21 PM
i would rather have the first 2 digits of the code, does it feel like it has the same ammount of compression ? i will just about bet its the valve stuck open

03-15-2005, 11:38 AM
#s are : 12H802-2675-E1 99031056
The plug is not fouled. The compression seems good as before.
How do I check for a stuck valve? Do I need to take the head off?
Thanks... having fun now.

03-15-2005, 12:23 PM
yea pop the head off and turn it over and watch the valves make sure they close all the way , thats a 1999 engine so that rules out points , have you ever hit anything while mowing ? that will shear a flywheel key , but i would s till check out the valves if i was working on it ,having" fun now." thats what its all about ,

03-15-2005, 02:11 PM
having fun now probablly means he's having fun fixing it i guess. yeah just pop the head off and look for a sticking valve, did you check for gas and spark from the engine, does it generate spark while the plugs hooked up but hanging out, and does it get gas. if it has no spark you could have a corroded connection, sheared key, or bad coil. # of things can be it. but if it gets little to no spark, try and check the flywheel key. that may be sheared if you hit something.

03-17-2005, 03:31 PM
Popped the head and sure enough, the upper valve is stuck. Since one of the bolts has sheared off and I can't quite get a grab on the nipple, I'll bite the bullet and get a new mower.

If I can learn how to better take care of the next machine from my experiences with this, it'll have been time well spent. Have no idea why the valve stuck.

Thanks again.

03-17-2005, 07:17 PM
you can always get the valve unstuck very easily without much effort.

03-17-2005, 07:36 PM
dont trash it just yett ,,,,, do you have a welder ? you can get it out easy

03-17-2005, 07:40 PM
"Have no idea why the valve stuck" have seen engines do this that were ran on stale , old gas , ran low on oil , moisture is probally the biggest cause , that and stale fuel

03-17-2005, 07:49 PM
yeah. that marvel mystery oil really keeps em from ever doing it too.

03-18-2005, 12:02 AM
I've got a gas welder sitting a couple of feet away from the B&S with the sheared off bolt in the cylinder. If I'm careful, maybe I can heat up the head and extract the bolt with well... maybe vice grips? The bolts had a hard white coating that didn't seem right.
Maybe I can pull this thing out of the fire yet!

03-18-2005, 12:12 AM
that white stuff is what keeps the bolts from seizing to the alluminum. if you can get it out which i hope you do the valve is a quick fix oh and use some marvel mystery oil when you do get it right

03-18-2005, 08:53 AM
hold a nut on the bolt thats broke and weld it to the bolt down inside the nut might take a couple tries but it works