S21ZPN - Crank seal

M

Milanomike

Member
I haven't been here much as my Lawn Boy at around 37 years old continues to run well, that is, until the last week or 2 that is. Seriously, this is only like the 3rd issue I've had with this mower in all these years.

I noticed it was becoming harder to start, taking 6 - 8 - 10 pulls instead of the normal 2. Checked coil and it didn't spark, so installed a spare coil, got spark; however, it still took many pulls to get it going.

At this time, I also installed a new fuel line from the shut off valve to the carb as mine was severely cracked, checked the screen and made sure choke was closing etc. All seemed good.

Then I noticed the lower crack seal fell out of the case, hmmmm, guess they only last 37 years. I was able to push it back into position with my fingers but thinking it ran with this thing partially to fully out of position.

Today it fired on 2 pulls and ran well, but towards the end of the mow, it seemed to be surging a bit.

I just ordered 2 seals for this F engine, seems like replacing the bottom one is definitely needed (probably do the top also). I'm thinking I don't have to remove engine from the deck here (remove blade, collar, muffler and hopefully have access). On top, remove fan and it should be right there.

Certainly not a 2 stroke engine expert, but leaking seals allow extra air in or out I think.

Anything else I should look at while I'm there?

Edit, meant to add, should I lube the seal lip with some oil when installing?

Thanks.

Mike
 
Rivets, thanks for the manal, lots of good info here. I am not sure this F engine was only produced for 1 year, but whatever, she runs and mows well.
 
Update on this one. I got 2 crank seals for my mower. Yesterday I did the bottom seal as that was an obvious issue. Had to remove engine from deck along with mounting plate (my term) on engine.

The seal was loose so easy to pop it out, greased up new one after cleaning the shaft and things up a bit and tapped it home with my lawn boy seal installation tool (a piece of white plumbing tubing).

Needed to hit back yard before rain comes today, but she ran great, no surging, no hiccups, just smooth 2 stroke power.

I have not done the top seal under the flywheel/fan yet as I ran out of time yesterday.

Kind of wondering if I need to tear into it as the mower is running great.

I'm guessing the bottom seal is more exposed and takes most of the beating, so I'm inclined to hold off on the top seal. Any thoughts?
 
The bottom seal is the most common to pop out with age and vibration. If it is running good leave well enough alone.

It might be wise to have another spare lower seal in your toolbox for any future issues.

I don't recall if the upper and lower seals are the same on this engine, many were.

Back in the early 80's I started out working for a small CO-OP servicing and repairing small engines. I seldom ever had to deal with any Lawn Boy 2-stroke models unless they were run with straight gas and no oil mix.
 
Auto-Doc's, thanks for the input.

Yeah, both seals are the same, I've got 2 so 1 will be a spare. Also got an exhaust gasket coming as the existing one tore, a bit of high temp RTV seems to have done the trick, it ran great earlier today.
 
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