Briggs 12.5 Self Propelled push mower misses

[AshTray900]

I've got a motor that is running but missing every now and then like 1 time every 4 or 5 seconds and the frequency is not regular. Im stumped

here's what I've done, cleaned carb, replaced plug, replaced air filter, disassembled out shell of motor, cleaned magneto coil, didn't have a feeler gauge so I used a razor to set the gap (no difference in miss before or after this procedure) checked and cleaned the magneto kill switch, it has been missing for a year or a year and a half and ive owned it for 3 years, when I first owned it I did hit a grounding stake and it stalled the motor but the problem did not start then and the blade shows no damage from the incident. I cleaned out the underbelly of the motor, checked the oil and it wasn't bad. It occasionally will backfire through exhaust once if I kill it and restart immediately within a 10 seconds or so, this may be normal as I haven't tried restarting that quick until I was testing.

what started my investigation into this is it stalled for no real reason, I wasn't in high grass and I couldn't get it to restart for awhile and it usually started right back up, plus this year the miss is worse, I have fresh gas in it only a week old or so

Ive played around with 2 plugs before I got the new one, and what I notice is black soot (dry) on the plug, unless it has stalled and I have repeatedly tried to restart it without getting it to start, then I get a wet black plug. the engine does not smoke at all except after being started after being dormant for a year and ill get a little white smoke and it goes away

Im not interested in overhauling the engine all together as my neighbor bought a Honda and his has worked flawlessly for 3 years and he hasn't changed a plug or the oil, and he didn't say but it may be running on the same air filter, all I asked was did he change the oil and he said he hasn't done anything to it, no oil not new plug. but if the resolution can be simple I would like to keep the mower running good rather than just running it until it becomes such a pain in the rear I get rid of it, or it completely dies. ive put a new plug and filter on my mower every year. and even adjusted the self propelled system so its strong again

any ideas, internal engine ring or valve wear? carberator appears to have no settings I can change and the choke is automatically controlled, there is a vaccum port in air filter cabin with a tan plastic cover to allow carb to create more vacuum in the port, the hose is not broken or disconnected from the breather to the mower. I have noticed just a very small amount of oil in the bottom of the filter cabin, but not enough to drip anywhere or even get the filter dirty. I see no way of setting timing either. easy fix or just a mower wearing out?

I also need to mention that I have weeds like crazy so the mower sometimes has to cut a lot especially the first two cuts of the season, my thinking is it maybe the auto choke, ive cleaned spring and linkage on carb side, but other than that I don't know how it works.

 

[AshTray900]

correction, its a 7.25 motor, just went in again and sanded the contacts on magnetor clean and reshimmed via the business card method someone posted. here is a video, when it misses I smell a touch of gas in exhaust

https://youtu.be/HJp21AqXx3g

<iframe width="420" height="315" src="https://www.youtube.com/embed/HJp21AqXx3g" frameborder="0" allowfullscreen></iframe>

 

[AshTray900]

Re: Briggs 7.25 Self Propelled push mower misses

 

[Rivets]

My guess is that when you hit something, you parcially sheared the flywheel key. It may not look damaged, but even a small crease can cause the problem. I would replace the key, $1.00 part, and see what happens.

 

[AshTray900]

you were correct, I assume when I hit it it damaged the key and it kept falling apart, by the time I pulled it off the outer rim had almost reached the inner rims other side by like half a mm, it was about to snap off, running good now, I know they say the key is to prevent damage to the motor or something, but it seems to me the only thing the key is concerned with is the flywheel which isn't that heavy. does it just help decrease the momentum of the motor or something? it would seem a dead strike against something would have already stressed the crankshaft.

 

[Rivets]

The key material is very soft, where the crankshaft is forged steel. As said, it is there to protect the engine. I doubt there is any damage to the crankshaft, as the key did it's job.

 

[AshTray900]

I get why its there I just don't understand its placement, its at the very top of the motor on the flywheel rather than down on the blade, I guess the flywheel must be where most of the momentum is stored, it just felt pretty light, maybe crank is even lighter, I've never pulled one

 

[ILENGINE]

The flywheel creates a lot of rotaty force and the flywheel key just sets a easy way to set the ignition timing, but has to be of a soft material to give when the engine encounters a sudden stop. Otherwise if the key didn't break you risk the flywheel shattering which would be very dangerous.