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Questions about Compression testing a Stihl fs130

#1

S

SHOESNSOCKS

I've got a Stihl FS130 that I wanted to take a compression test on becasue I'm going to be parting it out. It starts and runs fine, but when I take out the spark plug and put in the compression tester, it feels like it has almost no compression when I pull the cord. Also, the compression is only about 75 psi. I never work on Stihl so I don't know if I'm doing something wrong.


#2

Boudreaux In Eunice La.

Boudreaux In Eunice La.

SHOESNSOCKS said:
I've got a Stihl FS130 that I wanted to take a compression test on becasue I'm going to be parting it out. It starts and runs fine, but when I take out the spark plug and put in the compression tester, it feels like it has almost no compression when I pull the cord. Also, the compression is only about 75 psi. I never work on Stihl so I don't know if I'm doing something wrong.
Click to expand...

If it starts and runs good why are you parting it out ????

Do you put a lil bit of oil on your O Ring on the tester when screw it in ????? ....If it leaks at the seal then it shows low ...

75 PSI is good and it depends on which brand of tester you have .........

Let us know Mon Ami ~!~!


#3

S

SHOESNSOCKS

Boudreaux In Eunice La. said:
If it starts and runs good why are you parting it out ????

Do you put a lil bit of oil on your O Ring on the tester when screw it in ????? ....If it leaks at the seal then it shows low ...

75 PSI is good and it depends on which brand of tester you have .........

Let us know Mon Ami ~!~!
Click to expand...

The reason I'm parting it out is because It has a broken Bike handle. Also, I scored it for free at the dump, so I figured it might be a fun project and since I wanted to sell it anyways maybe I could make a little more off of it. I feel you though, If anyone else thinks this is a bad idea please don't hesitate to try and change my mind.

I didn't try the oil. I will have to give it a try tomorrow thank you. I will also share the brand of tester.

Also, I've heard of different methods of compression testing, but the method I used is to just pull it 5 times or until it stops increasing in pressure. Not sure of what kind of difference this makes.


#4

B

bertsmobile1

If you can get it in a vice of some sort then hold the throttle wide open and pull till the reading stops going up.
Wide open will use less pulls.
Because the cylinders are so small you need a end on the compression tester that takes the same volume as the spark plug
I would consider 75 psi to be borderline on a FS 130 as it has a 2 ring piston


#5

S

SHOESNSOCKS

bertsmobile1 said:
If you can get it in a vice of some sort then hold the throttle wide open and pull till the reading stops going up.
Wide open will use less pulls.
Because the cylinders are so small you need a end on the compression tester that takes the same volume as the spark plug
I would consider 75 psi to be borderline on a FS 130 as it has a 2 ring piston
Click to expand...

Thank you Bertsmobile! Never considered the size of the compression tester.


#6

S

SHOESNSOCKS

So I just gave it another try while holding down the throttle. I also added a little oil to the O-ring on the compression tester. I might have pulled it 6-7 times before it stoped at probably 88-89 psi.

I also took a picture of the tester plug next to the spark plug. If the O-ring was not on the tester, they look like they would be them same length. I'm not sure how much a difference that would make.

spark.jpg

I also looked up the cost on ebay for a use bike handle, and after shipping it would cost me about $33. Also, I would need to get a new strap, and a new blade that cuts the line on the deflector sheild. I definitely don't need to keep it so I want to sell it, but I'm just kind of wondering if I should just get the parts and sell it whole or pull it apart and piece it out.


#7

B

bertsmobile1

It all depends upon the volume of the head.
Basically the volume of the combined cylinder & head is compressed into the head volume.
So you divide a big number ( combined volume ) by a small number ( head volume ) then multiply that by atmospheric pressure at your altitude.
So changing the denominator by a very small amount has a big effect on the outcome.
When the apprentices do compression reading they get presented with a dozen different meters , all correctly calibrated and they give a dozen different readings.
Then the apprentices use their own testers then the cylinder is taken apart, measured and the compression pressure is calculated.
The whole aim of this is to drive home that the number is not absolute.
The next prac session uses the same cylinder being fed air at a variety of different temperatures & pressures.
Again they get forced to do the calculations.

After which none of them ever bother with compression testing numbers again.
Just high, OK or poor.

The importance with compression testing on single cylinder engines is the variation over time.
Unfortunately no one bothers to test the engine, brand new out of the box then each season.
However doing this will allow you to plot engine wear over time and when the graph starts to drop let you know when to change the rings before excessive cylinder wear happens.


#8

S

SHOESNSOCKS

bertsmobile1 said:
It all depends upon the volume of the head.
Basically the volume of the combined cylinder & head is compressed into the head volume.
So you divide a big number ( combined volume ) by a small number ( head volume ) then multiply that by atmospheric pressure at your altitude.
So changing the denominator by a very small amount has a big effect on the outcome.
When the apprentices do compression reading they get presented with a dozen different meters , all correctly calibrated and they give a dozen different readings.
Then the apprentices use their own testers then the cylinder is taken apart, measured and the compression pressure is calculated.
The whole aim of this is to drive home that the number is not absolute.
The next prac session uses the same cylinder being fed air at a variety of different temperatures & pressures.
Again they get forced to do the calculations.

After which none of them ever bother with compression testing numbers again.
Just high, OK or poor.

The importance with compression testing on single cylinder engines is the variation over time.
Unfortunately no one bothers to test the engine, brand new out of the box then each season.
However doing this will allow you to plot engine wear over time and when the graph starts to drop let you know when to change the rings before excessive cylinder wear happens.
Click to expand...

I had no idea that there could be this many variables. Thank you bertsmobile.


#9

S

SHOESNSOCKS

bertsmobile1 said:
It all depends upon the volume of the head.
Basically the volume of the combined cylinder & head is compressed into the head volume.
So you divide a big number ( combined volume ) by a small number ( head volume ) then multiply that by atmospheric pressure at your altitude.
So changing the denominator by a very small amount has a big effect on the outcome.
When the apprentices do compression reading they get presented with a dozen different meters , all correctly calibrated and they give a dozen different readings.
Then the apprentices use their own testers then the cylinder is taken apart, measured and the compression pressure is calculated.
The whole aim of this is to drive home that the number is not absolute.
The next prac session uses the same cylinder being fed air at a variety of different temperatures & pressures.
Again they get forced to do the calculations.

After which none of them ever bother with compression testing numbers again.
Just high, OK or poor.

The importance with compression testing on single cylinder engines is the variation over time.
Unfortunately no one bothers to test the engine, brand new out of the box then each season.
However doing this will allow you to plot engine wear over time and when the graph starts to drop let you know when to change the rings before excessive cylinder wear happens.
Click to expand...

I had no idea that there could be this many variables. Thank you bertsmobile. Do you think that 89 psi is in the poor zone?


#10

B

bertsmobile1

Low is how I would describe it.
Depending upon the actual engine some where around the 60-80 psi is the lower limit for compression.


#11

I

ILENGINE

to get a rough idea what the actually compression is suppose to be, would requiring knowing the compression ratio of the engine. If the engine is a 7:1 engine your compression reading would be about right. But if it is a 10:1 engine then it is low.


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