Craftsman Push Mower Misfire

[Merkava_4]

Craftsman Push Mower
Model: 907.388920
Serial: 112905M023952
Engine:10M902 0489 E2 05101754

Mower starts up on the first pull. Runs perfectly except for a series of muffled pops out the exhaust. The engine has never had this issue until this year.

This is the super simple Briggs & Stratton vertical shaft push mower engine with the Pulsa-Prime (tank mounted) plastic carburetor.

The first thing I did is change out the fuel pump diaphragm and matching gasket. The second thing I did is change out the primer bulb. That's usually all these carburetors need. After that I tried a different carburetor, different ignition coil, and different spark plug. No change in the misfire issue.

Right now I'm at a total loss. I don't know what to try next. I will try a brand new carburetor if you guys think that will solve the issue.

 

[bogieboy]

These are getting to the point where the carb bodies can start to warp. And you may need to double gasket the carb to the tank. Also verify theres no water in the fuel...if it aint fresh, dump it and get fresh, non ethanol recommended, but not required

 

[Merkava_4]

bogieboy, thanks for your reply. I ran a straight edge on both the tank and the carb body. Both are dead flat, but what I could do is try to poke a .002" feeler gauge in between the carb body and the tank for added insurance. That's an old trick when checking for carb body warpage.

I may try some different fuel, but that seems too easy of a fix. It's never that easy.

 

[Moparmyway]

bogieboy, thanks for your reply. I ran a straight edge on both the tank and the carb body. Both are dead flat, but what I could do is try to poke a .002" feeler gauge in between the carb body and the tank for added insurance. That's an old trick when checking for carb body warpage.

I may try some different fuel, but that seems too easy of a fix. It's never that easy.

Just spray WD-40 around the carb as shes idling ……. You’ll know right away if you have a leak, as rpm will change quickly if any WD-40 gets sucked into a leaking area

 

[bogieboy]

bogieboy, thanks for your reply. I ran a straight edge on both the tank and the carb body. Both are dead flat, but what I could do is try to poke a .002" feeler gauge in between the carb body and the tank for added insurance. That's an old trick when checking for carb body warpage.

I may try some different fuel, but that seems too easy of a fix. It's never that easy.

If it is flat, and the gas is fresh, try what @Moparmyway said, wd40 works, so does non chlorinated brake cleaner.. if that doesnt make any changes, pull the recoil off, remove the flywheel nut, and check if the flywheel key has sheared. If its even partly sheared, like half a key off it can cause misfires and poor running, but still start like normal...

Easy trick with the checking the gas for water, either pour or siphon some out into a clear glass jar, swirl it around a bit, if it clouds up, the fuel has absorbed moisture and is dead, if it stays looking that clearish yellow and smells like fresh gas its probably fine...

 

[Mammyrat]

Make sure the blade is tight.

 

[Matts Mower and Saw]

Check for a partially sheared flywheel key.

 

[EFSM]

Check for a partially sheared flywheel key.

I absolutely second this. Although it's a little weird it would show up after sitting all winter.
The keys are aluminum on these so it can shear without damaging anything. Do NOT replace it with a steel one.

 

[Moparmyway]

If the keyway is normal, it’s time to pull the head and look at the exhaust valve and its seat. You might have a loose or sunken seat, maybe there’s a little bit of carbon stopping it from fully seating ? Remove & check the valve spring, file the valve stem where the retainer sits and remove the valve to inspect it and the valve seat.

 

[jasper89]

I’m not sure this is the model you have, but some BS engines had plastic camshafts that wore kind of easily and cause timing issues.

 

[Merkava_4]

I think I may have found the problem. With the carburetor mounted (without a gasket) and the screws tightened, I can slide a .006'' feeler gauge in between the mating surfaces on the intake manifold side. That is the side opposite of the primer bulb. Same check with the gasket installed, I can slide a .0025'' feeler gauge in between the mating surfaces on the intake manifold side. The other sides of the carburetor mounting surface are not quite as bad. The other side with the primer bulb is almost completely flat. Being that the pickup tube and the emulsion tube tower are in the way, I don't think I'll be able to file the surfaces flat. I'm probably looking at a new carburetor. The service manual specifies 15 inch-lbs for the mounting screws. I'm thinking about switching those screws over to hex heads so that I can use an inch-lb torque wrench with a little 1/4 drive extension and socket.

 

[Arcticmiller]

If the keyway is normal, it’s time to pull the head and look at the exhaust valve and its seat. You might have a loose or sunken seat, maybe there’s a little bit of carbon stopping it from fully seating ? Remove & check the valve spring, file the valve stem where the retainer sits and remove the valve to inspect it and the valve seat.

I have had to repair these several times on various Briggs motors. What I did to fix the last loose seat was take the head off, clean everything up, rotate the engine to where the exhaust valve was closed completely and use a center punch around the perimeter of the seat to displace some of the aluminum and “tighten” the seat back up I’d punch in a star pattern like you tighten the lugs on a car. I did the fix and I’m pretty sure it ran well for another 10 years on a farm with little to no routine maintenance. It was on a DR trimmer brand brush mower.

 

[FergusonTO35]

I think I may have found the problem. With the carburetor mounted (without a gasket) and the screws tightened, I can slide a .006'' feeler gauge in between the mating surfaces on the intake manifold side. That is the side opposite of the primer bulb. Same check with the gasket installed, I can slide a .0025'' feeler gauge in between the mating surfaces on the intake manifold side. The other sides of the carburetor mounting surface are not quite as bad. The other side with the primer bulb is almost completely flat. Being that the pickup tube and the emulsion tube tower are in the way, I don't think I'll be able to file the surfaces flat. I'm probably looking at a new carburetor. The service manual specifies 15 inch-lbs for the mounting screws. I'm thinking about switching those screws over to hex heads so that I can use an inch-lb torque wrench with a little 1/4 drive extension and socket.

Used to be those plastic tank mount carbs were less than 20 bux, maybe still are. I always found they were good for 5-10 years before they were too warped/leaky to work properly. Are you a Cornwell dealer? I've got a bunch of Cornwell tools and a big box from them.