Sudden loss of compression with Stihl

[BenchWorker]

I picked up an FS-80 that despite its age, looked like it just came off the shelf brand new. Took it to a job without testing it well and it wasn’t idling right - seemed lean.

I richened up the mixture and used it for a couple hours. As soon as it cooled down, it wouldn’t start anymore. After checking spark and fuel, I tested the compression and it’s completely toast.

What would cause a complete loss of compression on an otherwise perfect looking (but aged) motor?

 

[ElevatorGuy]

Perfect looking piston?

 

[lone wolf]

Perfect looking piston?

Prob scored,stuck ring or worn out. Maybe he will get lucky and a ring is gummed up.

 

[Mad Professor]

Sounds like you ran it with a vacuum/air leak. Lean seize.

Take a look at piston.......

 

[grizz55chev]

I picked up an FS-80 that despite its age, looked like it just came off the shelf brand new. Took it to a job without testing it well and it wasn’t idling right - seemed lean.

I richened up the mixture and used it for a couple hours. As soon as it cooled down, it wouldn’t start anymore. After checking spark and fuel, I tested the compression and it’s completely toast.

What would cause a complete loss of compression on an otherwise perfect looking (but aged) motor?

Straight gas or wrong mix probably toasted the piston.

 

[dboyd351]

Wouldn't have run a couple of hours on straight gas!

 

[pioneerguy600]

It`s only guess work until the OP comes back and does some dismantling, like pull the muffler off and take a look at the piston condition.

 

[flatsfisher]

"seemed lean. I richened up the mixture and used it for a couple hours."

maybe masked an air leak.

 

[lone wolf]

"seemed lean. I richened up the mixture and used it for a couple hours."

maybe masked an air leak.

Was it running good or running quirky after that?

 

[ironman_gq]

In my experience they run awesome when they're lean, just a little too awesome then they don't run at all. My guess would be a dry hardened crank seal that got worse after he richened it up.

 

[flatsfisher]

Was it running good or running quirky after that?

not my thread, I was just making an informed guess.

 

[BenchWorker]

Sounds like you ran it with a vacuum/air leak. Lean seize.

Take a look at piston.......

I actually think you’re right about the vacuum/air leak… but visibly, the piston looks perfect.

 

[huskihl]

"seemed lean. I richened up the mixture and used it for a couple hours."

maybe masked an air leak.

Which would be fine as long as he added enough fuel to compensate for it. A 2 stroke doesn’t care where the air comes from

 

[SteveSr]

I actually think you’re right about the vacuum/air leak… but visibly, the piston looks perfect.

That is a non-sequitur. Can't have both. Pull the muffler and post photos of the piston.

 

[BenchWorker]

I’ll post a pic of the piston. I’m really leaning towards an air leak more than ever. I actually got it to run for 10-15 seconds with the mixture set WAY rich. Going to see if I can get a good look at the crank seal also.

 

[BenchWorker]

I'll post pics of the piston. I do believe it has an air leak. I was actually able to get it running again with the mixture screws set WAY rich - but I only ran in for 10 seconds. I'm going to see if I can get a good look at the crank seal also.

 

[bwalker]

I

I'll post pics of the piston. I do believe it has an air leak. I was actually able to get it running again with the mixture screws set WAY rich - but I only ran in for 10 seconds. I'm going to see if I can get a good look at the crank seal also.

I would check tge PTO side crank seal. It can be replaced withouy splitting the cases.
I had a FS85 with the same symptoms and a crank seal as the issue.
That FS80/85 motor is nearly indestructible.