A compression check question(s).

[John from Cle Elum]

Hi
I am trying to measure compression on a Stihl HT75 pole saw. It feels right when I crank it. It runs pretty good but it has starting issues when it is hot. I did a compression check using an Lisle automotive gauge and only got about 60psi after about 10pulls. Oil in the cylinder did not help. It's about the same hot or cold. If it was actually that low it probably would not run. The gauge has about an 18" flexible tube on it. The gauge holds pressure, no leakage. The engine is only 25cc. I am wondering if the air volume in the flexible tube and whatever air volume is in the gauge itself is screwing up the reading. It seems to me the air volume in the gauge may effectively increase the combustion chamber volume and show a low reading. I have checked larger saws using the same gauge and got somewhat low readings on a known good engine but they were all a lot larger (60cc and 85cc). It works fine on a 250cc/cylinder snow mobile. Can the check valves be sticking? As a check, I pressurized the gauge from the tester using an air compressor and the tester gauge tracked the pressure on the compressor. I don't believe the tester reading and I am about to purchase a new one.

Does anyone have any ideas? I am out of ideas. Can someone recommend a good, moderately priced tester known to work well on tiny engines?

Thanks
John

 

[Justsaws]

If you are concerned about the internals pull the muffler and check for damage and or worn rings.
Hot start, restart issues are usually fuel, vent, or ignition.

 

[drf255]

OMT makes a good gauge at a reasonable price.

The shrader valve stays the tip of the tester is a must. It has to be the white O ringed low pressure type of needle. You’d be surprised how much difference it can make.

The adapter that plugs into the plug hole can make a huge difference, especially in a lower displacement motor. If it’s not the same length as the plug threads, you will get a false reading.

The hose length makes no difference as long as there is a valve at the tip of the adapter leading into the hose. This has been proven, and I also modded a hose to prove it to myself. The hose can be a mile long, it will just take more pulls to reach an equilibrium with the chamber pressure. The shorter the hose, the less pulls needed to reach a reading.

As stated above, a compression test is ok for reference, but if the exhaust side of the piston looks good and isn’t scuffed, you are probably fine regardless of the reading. Cranking compression vs dynamic compression are 2 different things. Many engines have a small notch or slot in the ex port purposely put there to decrease cranking compression and make starting easier. Many concrete saw have a “T” slot for this very purpose. The slot allows air to be vented at starting compression, but not much escapes at running RPM.

 

[Okie]

Read some of this one: It will tell you why for.
Also several threads on this site about compression testing.
I've repaired saws that had only minor starting/run issues that guys (usually Automotive type mechanics) would replace a chainsaw piston and cylinder because their automotive gauge indicated that the saw had low compression and after they installed new piston and cylinder their reading were even lower using their big fancy high dollar automotive tester. They would say my tester is reading correct when I connect it too my air compressor.
Watch out and keep a heads up, them compression testers will lie to you.
I have two, one is homemade and the 2nd one is too get a 2nd opinion if I get a low reading.
I do not use my small cc compression testers with the weaker/wimpy schrader valves for taking compression reading on Automotive or larger cc engines.
You will at times have to clean a wimpy in the tip schrader valve due to piece of debris, usually carbon hanging in the check valve.
I very seldom even do a comp test unless I see a piston that looks bad when looking through the exhaust port. I know then that cancer is attacking the little feller and even if it runs it's on it's way out and I've seen saws with good compression have a reject piston.
A pressure/vac tester is a good thing.


http://www.arboristsite.com/community/threads/compression-tester-issues.310133/

 

[lone wolf]

Check your gauge in a saw you believe to be in new like condition the compression should be close to 160 or so. Keep pulling till it stops rising dont stop at ten. If still low get another gauge.

 

[John from Cle Elum]

Thanks for all the help. The link that Okie sent had a lot of information. Apparently my issue with compression testers is not unique. The tester has a "red" schrader valve not the recommended "white" type. I gotta go find one of those.
After reading everything, I am convinced the length of flex hose is not the issue.
DRF255: I did look through the exhaust port and the piston looks pretty darn good. No scoring. I don't think that is an issue. The adaptor plug is a lot shorter than the spark plug, I will bet that is part of the problem. That friggin' piston/cylinder is TINY. The volume of combustion chamber has got to be tiny also.

 

[HarleyT]

I have a compression gauge that I never use. I got it just for a pic on an e-bay auction.

 

[lone wolf]

Thanks for all the help. The link that Okie sent had a lot of information. Apparently my issue with compression testers is not unique. The tester has a "red" schrader valve not the recommended "white" type. I gotta go find one of those.
After reading everything, I am convinced the length of flex hose is not the issue.
DRF255: I did look through the exhaust port and the piston looks pretty darn good. No scoring. I don't think that is an issue. The adaptor plug is a lot shorter than the spark plug, I will bet that is part of the problem. That friggin' piston/cylinder is TINY. The volume of combustion chamber has got to be tiny also.

Sounds like you got a good understanding of it now . Lenghth of hose only means you have to pull it over more. But that adapter could be the whole problem even with the white Schrader.

 

[Wise Guy]

I don't see anyone mentioning this but its important when testing to have the choke off and full throttle. Otherwise you are creating a vacuum in the inlet and hence a low reading. 60 lb is way too low.

 

[John from Cle Elum]

I replaced the red schrader with a white valve. Choke off, throttle full open, and lots of pulls gets it to about 120psi. I am satisfied that is a good number.

Thanks for all the help.
John

 

[Okie]

I replaced the red schrader with a white valve. Choke off, throttle full open, and lots of pulls gets it to about 120psi. I am satisfied that is a good number.

Thanks for all the help.
John

Did you notice in the link I sent where I custom made some low pressure schrader valves. I keep a spare valve handy in case one starts leaking back not holding pressure. I use a good operating valve to first get a reference then start removing coils to weaken the spring.
I'm kinda ashamed of the price they ask for just one low pressure valve.
Maybe I should be mass producing them for 1/2 price.

 

[drf255]

I replaced the red schrader with a white valve. Choke off, throttle full open, and lots of pulls gets it to about 120psi. I am satisfied that is a good number.

Thanks for all the help.
John

You don’t need to open throttle for a compression test. Not how 2 strokes work.

I bet the difference in the adapter vs plug length is dropping you more than one may think.

 

[Okie]

You don’t need to open throttle for a compression test. Not how 2 strokes work.

I bet the difference in the adapter vs plug length is dropping you more than one may think.

I wanted to say this about 2 cycles also not needing the throttle and choke wide open but knew I would get severely flogged so I just stayed quiet.

 

[lone wolf]

Did you notice in the link I sent where I custom made some low pressure schrader valves. I keep a spare valve handy in case one starts leaking back not holding pressure. I use a good operating valve to first get a reference then start removing coils to weaken the spring.
I'm kinda ashamed of the price they ask for just one low pressure valve.
Maybe I should be mass producing them for 1/2 price.

Anything you could produce that people need now may pay off nicely ,things are hard to get right now.

You don’t need to open throttle for a compression test. Not how 2 strokes work.

I bet the difference in the adapter vs plug length is dropping you more than one may think.

Plus 1 on that.

 

[Wise Guy]

I replaced the red schrader with a white valve. Choke off, throttle full open, and lots of pulls gets it to about 120psi. I am satisfied that is a good number.

Thanks for all the help.
John

 

[Wise Guy]

My apologies - I got it wrong. A 4 stroke you need to have the throttle open for a compression test. But a 2 stroke has its exhaust port open to the atmosphere every cycle. Live and learn!