Low compression, why?

[samaritan]

After new piston, cylinder, rings and seals, compression is only 115, and it wont start, its a Stihl ms 251c . The piston and cylinder look fine, and so do the rings. I added oil to the cylinder and compression went to 125, still wont start.

 

[Patrick62]

that should be enough, it is something simple.
Spark?
Fuel?
We can assume it has air...

 

[Clint Eatswood]

Had a similar issue recently and some member reminded me to check piston ring end gap. That was my problem.

 

[thompsoncustom]

Should start with that compression I'd check for spark first and if it passes that spray some fuel down the carb.

 

[muddy42]

As above, test the spark and fuel. Also why was the piston and cylinder replaced? Have you performed a pressure test? Has the original issue been fixed?

 

[samaritan]

Had a similar issue recently and some member reminded me to check piston ring end gap. That was my problem.

my ring end gap is .014 for a 44mm piston ring. Was yours too wide?

 

[Clint Eatswood]

Mine was 0.020". I believe for that size saw you're looking for .006-.010". Somebody can correct me if I'm wrong.

 

[samaritan]

according to my google search, .006 is correct for min. gap Another source said .007-.010"

 

[sean donato]

115psi is pretty normal for a fresh not run in cylinder. I highly doubt it has to do with your ring end gap. Besides that there's no way to tighten it up without either replacing the cylinder or hoping a new ring is tighter.
Pressure and vac test the saw, check spark, fuel etc. It should run with the compression you have.

 

[grizz55chev]

I see no mention of your squish #, might have something to do with it?

 

[stihltech]

That Compression is not causing a no start. Recheck the basics.

 

[Bob Hedgecutter]

Did you ascertain why the saw needed a new top end in the first place?
Find and fix that reason and I am willing to bet the saw will fire up just fine on the compression you have.
New rings need to "learn" the new cylinder- sometimes this can take several tanks of fuel before they truly get to know each other and reach full compression values.

 

[Okie]

It should give a promise pop with that compression if you give it a light/slight prime.
Do not over prime or it will flood easily.

 

[ballisticdoughnut]

After new piston, cylinder, rings and seals, compression is only 115, and it wont start, its a Stihl ms 251c . The piston and cylinder look fine, and so do the rings. I added oil to the cylinder and compression went to 125, still wont start.

Are you certain your compression tester is accurate? 115 PSI is low but not low enough to prevent the saw from running. Compression test a known good running saw and compare results.

 

[Okie]

Are you certain your compression tester is accurate? 115 PSI is low but not low enough to prevent the saw from running. Compression test a known good running saw and compare results.

Are you certain your compression tester is accurate?
Compression test a known good running saw and compare results.

Good question:

I've had guys bring me their chainsaws because they replaced the cylinder and piston due to low compression and compression was lower afterwards and saw STILL would not run.
Their automotive compression gauge was reading really low when testing small CC engines.

They got mislead at first due to their Automotive compression gauge not reading correctly.
Most generally their saws had fuel issues.