question on compression

[tomdcoker]

Hello everyone, and thanks for all the past help. My question is when checking compression, to get a true reading how many times do you pull the the starter rope and do you do this with the motor hot or cold. Thanks Tom

 

[TimberPig]

You normally pull until the gauge stops rising with the throttle wide open.

 

[tomdcoker]

Thanks for the answer and quick response. Tom

 

[Ax-man]

About 4-5pulls is all it takes to get a compression reading, this is also about the same numbers of pulls it takes on average from a dead cold start for a saw to pull fuel from the tank to start and run.

Compression readings are best done on a warm saw, the difference between hot and cold readings will be about 20 , 25 lbs. JMO opinion here, I don't see a big difference in the numbers with having the throttle plate wide open or closed.

Larry

 

[Gearhead1]

Five pulls is the normal for a compression check on any OPE machine. You can also look at the gauge to see how fast the pressure builds as an additional sign.

 

[TimberPig]

Five pulls is the normal for a compression check on any OPE machine. You can also look at the gauge to see how fast the pressure builds as an additional sign.

How fast pressure builds is also a function of how fast the engine is cranked over, so it will depend upon that as much if not more than the ring sealing.

Normally it tops out at 5 or 6 pulls anyhow, so that won't significantly lower it.

 

[tomdcoker]

Thanks again for the help. Tom

 

[StihlBoy440]

whats good compression on say a 028 Stihl just for example, stock of course?

 

[timberwolf]

New stihl saws seem to be around 160.

 

[StihlBoy440]

so would 130 be sufficient for a 16 year old to run descent? if so what are some symptoms of a carb needing a kit this saw seems to run minute then not the other and it is often hard to start, its like it loads up sometimes like it is being "choked" or something.

 

[timberwolf]

should run at 130, I might be inclined to think of leaky bearing seals or a leak in the carb boot. if it is getting air in you might get the mixture by over richening the carb, buth then if the leak is pinched off as the saw warms up or is moved, it would go too rich.

After 16 years a carb job would not be a waste but should be checked for air leaks too. Cover off the other basics, plug, fuel filter, air filter also.

 

[RES]

Make sure your compression tester has a Schrader valve, where the gauge screws into the cylinder or your reading will be effected by the volume of air in the gauge. Do a search on compression testing to get a better explanation on why this happens.

 

[timberwolf]

Good point, but if it is by chance a tester without a valve, 130 psi on the gauge would equate to more than 130 psi in reality. This due to the extra air space in the hose that must also be compressed, with a shrader valve it just means more pulls to compress the volume of air in the hose, without a valve the hose becomes head space and is very inaccurate on a small engine.

I am working on a saw with 2.6 cc of head volume, the difference of space between the volume of the plug and end of the compression tester sets the reading off by 20 lbs. as the compression tester takes up a couple tenths of a cc less head space.

 

[StihlBoy440]

I have noticed that a oily residue comes out below the bottom of the muffler what could that mean other than a 16 year old saw that I found at the manned inconvenience center aka:the trash dump

 

[TimberPig]

Probably just a bad exhaust gasket, and set slightly rich.

 

[MotoBoyMatt]

I worked on an old 031 once that had a bad crankcase gasket and leaked bar oil just below the muffler.