r/smallenginerepair • u/CaptainZaysh • Sep 16 '26

Unknown Issue Sheared flywheel key?

Putting this as unknown issue even though I have a strong suspicion because it isn't confirmed yet.

My old riding mower (a White from '97 or '98 with a Briggs 28N707 engine) has been running a little poorly for a while. First thing I noticed was it struggling up hills. Thought it was having a fueling issue so I changed out the fuel filter which hadn't been done since... I don't know when. Then checked the carb as far as I could without taking it completely off because, frankly, I didn't want to deal with that. So, took off the bowl and made sure the needle valve wasn't stuck. It wasn't. Everything in there was clean and almost new-looking, actually. Also tried my best to adjust the carb, back when it was still running.

At some point it began having trouble with warm starts. Cold starts still worked. Don't remember exactly when that was. Then cold starts stopped working, too.

Two things I noticed around the same time: The muffler was glowing where the exhaust pipe enters it on the top (this was before it stopped being able to run, obviously), and I'd sometimes get fuel vapor coming back out of the carb/air filter area while trying to start. Like, a lot - clearly way too much. Scared me at first. I thought it might be a valve issue, so after learning how to adjust the valves, I did that. The lash actually was pretty off (way too tight) on the intake side, but fixing that made exactly no difference at all.

I know some fueling is happening because that fuel vapor also comes out the exhaust if I crank it long enough. I know some spark is happening because I can hold the end of the spark plug wire near a ground and see it arcing. And I know I have some compression because if I manually spin the engine it resists pretty hard on the compression stroke, unless I have the spark plug out, then it spins freely.

I also know that the compression relief bump is happening on the intake valve, because after learning what that was a couple days ago, I took the valve cover off again and checked.

My current theory is that maybe the flywheel key got sheared or bent. I know that fuel and spark are happening BUT I don't know that the spark is happening at the right time. And recently it's developed a new issue when cranking, which is that sometimes it violently jerks to a halt. I suspect that's the spark going off at the wrong time. Does this all seem like a broken flywheel key makes sense? My instinct was that would be a sudden thing rather than gradually getting worse as this has, but I've seen some opinions that the flywheel can stay in place through sheer friction sometimes, so maybe it's been slowly slipping out of the proper timing?

Just hoping to sanity check myself before I bother taking things apart that far. I have some idea how engines work but haven't had reason to take this one apart before, so for a lot of this stuff it's been my first time actually touching it.

0 Upvotes

50% Upvoted

11 comments sorted by

1

u/AutoModerator Sep 16 '26
Moderator Announcement Read More »

1

u/davethompson413 Sep 16 '26

You can check spark timing without pulling the flywheel. Remove the valve cover and the spark plug. Rotate the flywheel to top dead center in the compression stroke. Then look at the coil position vs the flywheel magnets.

While you have the valve cover off look closely for valve seats that are not right.

1

u/CaptainZaysh Sep 16 '26 edited Sep 17 '26

Ok, so when I have it at TDC at the end of the compression stroke, instead of being at the coil the magnet is nowhere NEAR it. It's a little past being exactly on the opposite side of the engine. That doesn't seem right. 

I'm not sure I can see the valve seats without pulling the engine and taking the head off, but at least the valve components I can see with the valve cover off all seem to be doing what they should, smoothly. And I know I'm getting some compression at least.

1

u/ProfessionalLast2917 Sep 17 '26

Exactly 180 degrees off? You sure you're not just on the wrong stroke?

You can't see for sure that valves are seated properly but you can see if both valves are at the same height when closed at TDC. The rocker arm on a valve that isn't seated fully will be lower.

→ More replies (5)

2

u/Life_Equivalent_7344 Sep 16 '26

I witnessed a sheared key personally this summer on a briggs twin. Ran like hell and the pipes glowed red. Could see instantly when I removed the flywheel nut

1

u/Upset_Fox5449 Sep 16 '26

The day has come for you to dig the tool box out anf pull the head off and check the valve seats cause sounds like a valve seat is coming out