B&S 6.75 Sputter/Misfire Diagnosis

[activelife92]

New to the forum and pretty much confused about my mower. I have a craftsman mower with a B&S 6.75 L-Head engine on it. Over the last year or two it's developed a sort of sputtering problem. It isn't losing much power, but there's a noticeable miss every now and then. Sometimes a few in a row.

So far I've: Replaced and gapped the plug to .030, changed the oil, pulled the carb off and rebuilt it. I should note that there wasn't any junk in the carb but I did replace a broken gasket between it and the air filter housing. I replaced the bowl gasket, bottom nut gasket, and o-ring where the carb attaches to the cylinder. I also replaced the float pin, needle and seat. I ran a pin through all the jets and hit them with quite a lot of carb cleaner. When I put it back together there wasn't a bit of improvement. I've also added seafoam to the fuel tank with other additives over time.

I'm beginning to wonder if it's a valve problem or timing issue that's affecting the spark. I'm thinking of pulling the flywheel to inspect the key. The key was replaced several years ago after an unfortunate incident with a tree stump. It ran fine after the replacement however. The mower doesn't use any oil. I pulled the head and the valves appeared to be functioning fine, but that was just a visual inspection. I also re-torqued the head bolts to ensure I wasn't losing any compression.

I'm at a loss now, any help would be appreciated. I'm thinking of pulling the flywheel this afternoon for an inspection. I'll report back with what I find.

 

[lugnut1]

does it seem to start sputtering when engine is warmed up?

may want check coil for constant spark
(if you have spark testing tool that plugs into spark plug wire and clips to spark plug)

run motor till it starts sputtering then use the tester to see if its a sparking issue.

 

[activelife92]

It seems to be doing it cold and warm the same. I bought a gap tool and set the magneto to .010 today. When I started it I think it was slightly worse. Im going to double check the gap again. I also cleaned the edge of the flywheel and magnetos with brake cleaner. All connections look good as well including the ground.

 

[activelife92]

Update: I tore into the motor again, this time taking the flywheel nut off. I found that the key was not lined up with its grove on the crankshaft. It doesn't appear to have sheared but moved. I'm going to remove the flywheel tomorrow and replace the key. Hopefully this will solve my sputtering issues.

 

[lugnut1]

It seems to be doing it cold and warm the same. I bought a gap tool and set the magneto to .010 today. When I started it I think it was slightly worse. Im going to double check the gap again. I also cleaned the edge of the flywheel and magnetos with brake cleaner. All connections look good as well including the ground.

.010 is usually the maximum setting, may want to close it up a couple or so.
keep us posted.

 

[activelife92]

I just replaced the flywheel key as well as re-gapped the ignition. As far as I could tell from a B&S PDF with the gap listed, .010 should work. It referenced a range between .006 and .010. The mower fired right up and is 99% better. It still has a miss or two every few minutes at most. I am attributing this to either the gap being a bit too big or how loose the key felt in its slot. Either way the mower is vastly improved and *almost* perfect.

Interesting how the key wasn't as "snug" as i'd have expected in its slot. Since the crankshaft appears to be semi-soft metal, I'm wondering if the tree stump hit years ago resulted in a slight deformation of the key slot. Any thoughts on this?

Edit: Just occurred to me, the flywheel key had notches on one end. Does anyone know if top/bottom orientation matters with these? I put it in with the notches down, but just watched a guy on youtube do it the opposite way.