Compression tester issues?

[Aggie 2012]

Evenin gents

Got a question for y'all.
I have a snap on compression tester and I am trying to figure out if the reading is correct.

(See below setup)

I have tested on a newly rebuilt MS180C (new piston, rings, and jug from NWP)
But I'm only pulling 125 psi and the saw does not run right at all.

So I checked on an ms250 that runs decent
(See below)

Now the last picture is on my MS290 that only has around 10 tanks of fuel run through it so I know for sure that this aw should read well.
155

Are these numbers about right or are my results skewed due to this schrader valve located at the gauge and not the other end?

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[burnses]

probably bout right with that gauge as you have a lot of volume to fill with the hose..you can ck the Schrader valve in the spark plug end to make sure the spring is soft and not just a tire valve Schrader valve....good luck

 

[LonestarStihl]

The schrader should be at the end of the adapters on the snapon. Looks like my same one. Your compression will increase as you break in a saw some. Don't know much about where each should be but the 290 seems fair. I'm sure you'll have better folks than I here soon to provide intelligent advice

 

[jmssaws]

Most of the time a compression tester just causes problems.
If the saw runs and idles fine then compression is fine.

 

[TimTaylor]

Have you checked it on a car engine? Or maybe against a tire pressure gauge. I have a snap on gauge as well and it reads depressingly low on chainsaws but I'm just using it as a comparison and to see if something changes i.e.broken rings or to see running in changes etc

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[s sidewall]

Have you tried a leak down tester, I trust those more, if you have a leakage past the rings, it will show up, plus you can pressure test the crank case and seals for leakage also.

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[drf255]

140-150 is about right for an MS250. I bet your gauge is on. Like the others said, check the Schrader valve.

 

[super3]

Are these numbers about right or are my results skewed due to this schrader valve located at the gauge and not the other end?

You are confusing the pressure release button for the schrader valve.

 

[Aggie 2012]

Ah ha! Thanks Super 3. That makes perfect sense now.
I am what they call a "newbie" at all of this so I appreciate you steering me in the right direction here.

 

[Okie]

I have 3 different compression testers and no 3 will read the same when testing a chainsaw. They all read the same on a auto engine or larger displacement engine or on a air compressor tank at any pressure. I've changed shrader valves around and done most all and some of the gauges are just lazy.
Of the 3 one is for chainsaws or small cc engines and it does not give the very high reading that I would like too see even on almost new saws and I have some chainsaws including Sithl that only read 80-100psi and the saws run fine with lots of power. I take compression reading with a grain of salt. I've seen saws with real good compression and pull the muffler and wow, severe cancer scratches and gouges of the piston and cylinder indicating that the saw has cancer and dying.

If I were you I would do a crankcase vac/pressure test to make sure no air leaks and if the saw is still erratic with a good sealed crankcase I would probably install a good spark plug and then look at the carb/fuel system and if that one has the rubber boot on the intake they are know for cracking and if has spark arrestor screen in the muffler will cause issues if clogged.

You have to know if the crankcase is sealed properly first or you are whipping on a dead horse. (and some of them OEM carbs for stihls can be had for less than $30. I cannot recommend a China clone carb for everyday use, but I've bought them from flea bay for $15 for Stihls just to get a test run and keep the flea bay clones around labeled as test carbs. (store them with ATF inside as a preservative.

I'm just going from memory but I think some of the MS180C do not have a adjustable carb, maybe just a hidden idle adjust and some of the non-adjustable stihl carbs are not worth trying to kit, unless it's your own saw and you want to give it a shot. I think I paid about 1/2 price of a new stihl carb for a carb kit. I think maybe it was MS 180's I had in awhile back and both of them had bad carbs even when I tried swapping carbs to get a run I found both carbs were bad and I bought a $15 china clone and got a good run, then I bought the OEM carb for one and finally repaired one of the two bad carbs after several attempts and runs thru a Ultrasonic cleaner. Took lots of time trying to get a good used carb re-built as good.

Bottom line: I think your saw should run ok at even 100 psi and will get little more psi as the rings seat. Hold the throttle open and pull the rope until compression gauge rising, may take 6-10 pulls.
You need to look elsewhere for the saws problem other than compression IMHO.

 

[LoveStihlQuality]

I think its junk. I'll pm you my address to recycle it.

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[Okie]

I think its junk. I'll pm you my address to recycle it.

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Yep, probably junk and should be re-cycled. (China needs some Stihl crap so as to upgrade their recycled materials)

Be sure and remove any gas/oil before you mail it to LSQuality so as to not pollute the trail and post system along the way.

 

[Aggie 2012]

Yea this saw is junk and certainly not worth fixing.
But it was given to me at no cost and I am trying to use it as kind of a learning experience more than anything.
I appreciate yalls suggestions here


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[TheTone]

When you are checking compression on a 427 big block it's not going to make much difference where the Schrader valve is. But when your piston diameter is, say, 37 mm, adding the volume of the "pass through" hose before you get to the Schrader valve makes a huge difference. When checking chainsaw compression your Schrader valve MUST be at the spark plug hole. Here's my solution. This is for those who just like to make their own tools and/or are on a tight budget. As you will see, these homemade goodies will serve not one but two purposes. This seems like a lot of work, but it's not that bad - the instructions are long because they are detailed. Many of you will only need to make one of these. Materials: one sparkplug, the siize used on your saw; one short piece of copper tubing, about 1/4" diameter; one discarded tire valve; one low-pressure Schrader valve; JB Weld. Items #1 and 3 could be had free at a small engine repair shop and tire shop, respectively. When you cut away the rubber from a tire valve, you are left with a brass tube into which a Schrader valve can be placed (see first picture). Grind the small end of the valve to reduce the diameter. If needed, drill and expand one end of the copper tubing. The two pieces just need to mate up nicely, they're going to be glued anyway (second picture). Once dry, fit the Schrader valve and note where the top thread is. Remove valve and cut away to just above that point. You want the plunger to stick out above the housing. Cut the ground electrode off the sparkplug. You'll have to remove the center electrode and ceramic from the plug, and it helps to grind away the crimp just above the hexagonal nut. Use a hammer and punch to do the rest. Fit the dried assembly into the plug - you may have to drill out the plug a little. Use enough JB Weld to seat the assembly with about 1/16th inch of the valve housing above the bottom edge of the sparkplug (third picture). When dry, fill in between valve housing and plug at the bottom and fill the rest of the void at the top. Find a nice O-ring to replace the plug gasket. Now for the two uses: with the Schrader valve in place attach your compression tester; with the Schrader valve out attach to your Mityvac for crankcase P and V test.

 

[Okie]

When you are checking compression on a 427 big block it's not going to make much difference where the Schrader valve is. But when your piston diameter is, say, 37 mm, adding the volume of the "pass through" hose before you get to the Schrader valve makes a huge difference. When checking chainsaw compression your Schrader valve MUST be at the spark plug hole. Here's my solution. This is for those who just like to make their own tools and/or are on a tight budget. As you will see, these homemade goodies will serve not one but two purposes. This seems like a lot of work, but it's not that bad - the instructions are long because they are detailed. Many of you will only need to make one of these. Materials: one sparkplug, the siize used on your saw; one short piece of copper tubing, about 1/4" diameter; one discarded tire valve; one low-pressure Schrader valve; JB Weld. Items #1 and 3 could be had free at a small engine repair shop and tire shop, respectively. When you cut away the rubber from a tire valve, you are left with a brass tube into which a Schrader valve can be placed (see first picture). Grind the small end of the valve to reduce the diameter. If needed, drill and expand one end of the copper tubing. The two pieces just need to mate up nicely, they're going to be glued anyway (second picture). Once dry, fit the Schrader valve and note where the top thread is. Remove valve and cut away to just above that point. You want the plunger to stick out above the housing. Cut the ground electrode off the sparkplug. You'll have to remove the center electrode and ceramic from the plug, and it helps to grind away the crimp just above the hexagonal nut. Use a hammer and punch to do the rest. Fit the dried assembly into the plug - you may have to drill out the plug a little. Use enough JB Weld to seat the assembly with about 1/16th inch of the valve housing above the bottom edge of the sparkplug (third picture). When dry, fill in between valve housing and plug at the bottom and fill the rest of the void at the top. Find a nice O-ring to replace the plug gasket. Now for the two uses: with the Schrader valve in place attach your compression tester; with the Schrader valve out attach to your Mityvac for crankcase P and V test.

Very good:
I'll try this in a few days. I have fooled around with my automotive compression testers on chainsaws, trying small hoses and re-locating Schrader valves, etc, and never could get a automotive type gauge to respond as well as the 2 cycle compression tester that I bought that was designed for small 2 cycle engines. (but I never placed the Schrader on the end of the plug inside the cylinder)

Couple questions

Where is a source for low pressure Schrader valves? (or can a low pressure one made using a standard Schrader valve core)
and
Why do you want the plunger of the Schrader valve to stick out of the housing below the spark plug and into the cylinder?(if you have a compression release plunger on the output side towards the gauge))

 

[lone wolf]

Try another o ring on the area of the tester where it screws in. Also plug your compression tester into an air compressor air line and check to see if the pressure matches the gauge on your compressor.

 

[Khntr85]

Well I remember a time when I just had to have a compression tester....I asked a very knowledgeable saw friend of mine which one to get, he told me and I bought it....it works very good,(can't remember the brand)....

I used it I think on 2 or 3 of my personal saws, and have not touched it sense....I have rebuilt and sold probably 20-30 saws since I got it, and I haven't used it on any of them....I could see a saw porter using one to show a customer the reading, but for the clamshells and few pro saws I rebuild I don't ever list the compression...

As far the reading you got, it may be right....OEM jug and slugs always hold more than compression than AM ones, (atleast in all the saws I have rebuilt)....

I always make sure a saw I am selling is 100% ready to work....I know that if I used the compression tester and it gave me a low reading, it may make me question my work, and "over-think" it, when in reality the saw will run fine even if the compression is 125....

Last thing, make sure your jug and slug are not dry when you are testing the compression!!!

 

[Okie]

Try another o ring on the area of the tester where it screws in. Also plug your compression tester into an air compressor air line and check to see if the pressure matches the gauge on your compressor.

Gauges will usually read the same in your reference on a air compressor tank but you will get a surprise if you try to use a automotive compression tester on a little chainsaw or weedeater 2 cycle engine. If you believe what the standard type automotive compression tester indicates on a small chainsaw or weedeater 2 cycle engine you will usually trash a good engine. The automotive compression tester is lying to you.
Do some on-line searching and reading about such or try it yourself and you will be surprised at the results.

A guy (good shadtree mechanic) brought me HIS OWN Stihl MS290 awhile back said he replaced the jug and rings due to low compression and it still has low compression and it still will not won't run. It had a bad carb. He was using his automotive compression tester to get a reading unaware that the gauge would not read correctly on a chainsaw. Tough lesson learned the hard way by him.
When I showed him my compression tester reading on his saw he was slack jawed.

 

[Okie]

Well I remember a time when I just had to have a compression tester....I asked a very knowledgeable saw friend of mine which one to get, he told me and I bought it....it works very good,(can't remember the brand)....

I used it I think on 2 or 3 of my personal saws, and have not touched it sense....I have rebuilt and sold probably 20-30 saws since I got it, and I haven't used it on any of them....I could see a saw porter using one to show a customer the reading, but for the clamshells and few pro saws I rebuild I don't ever list the compression...

As far the reading you got, it may be right....OEM jug and slugs always hold more than compression than AM ones, (atleast in all the saws I have rebuilt)....

I always make sure a saw I am selling is 100% ready to work....I know that if I used the compression tester and it gave me a low reading, it may make me question my work, and "over-think" it, when in reality the saw will run fine even if the compression is 125....

Last thing, make sure your jug and slug are not dry when you are testing the compression!!!

Very good:

Agree: I very seldom reach for the compression gauge. I do usually pull the muffler and look at the piston and cylinder carefully. I've seen chainsaws with good compression and running good and strong with cylinder or pistons going away due to severe scrapes and scarring. (appears to be bad cancer and not going to cure itself only get worse probably due to the saw previously run lean or running lean now)

 

[Khntr85]

Very good:

Agree: I very seldom reach for the compression gauge. I do usually pull the muffler and look at the piston and cylinder carefully. I've seen chainsaws with good compression and running good and strong with cylinder or pistons going away due to severe scrapes and scarring. (appears to be bad cancer and not going to cure itself only get worse probably due to the saw previously run lean or running lean now)

Yes sir, I do ALWAYS looks at piston and cylinders to "assess" the saws health...the piston can tell you a lot about the saw...

I will say that I am by NO means saying you shouldn't use a compression tester, I am just saying sometimes all is well even if you get a low reading....

Good luck and let us know how the saw runs...