Bad Compression Tester or Bad Me?

[jteneyck]

I bought a piston and cylinder kit from Bailey's for my Husky 55.  Seems to be nicely made.  Had a devil of a time getting the piston/ring into the cylinder, so the clearance must be very tight.  Anyway, the saw started right up and runs fine.  I also bought a compression tester from Bailey's at the same time, so that I could check the compression on this saw, as well as my other saws.  Well, didn't read more than a couple of psi, hardly anything.  So I put it on my 385XP - same thing. 

So I either don't know how to use this awfully simple looking device, or it's defective.  In order to verify the tester is bad, so I feel justified sending it back to Bailey's, I'd like to hook it up to my compressor but don't know how of any sparkplug to pipe thread adapters.  There was a recent thread related to 120 psi on a Stihl saw where the author checked his compression tester against his compressor, so it must be possible.  So, what fittings do I need?  Thanks in advance. 

By the way, I also bought a new OE Husky filter for my 385XP in this order from Bailey's, and it was defective as well.  Not all the pleats were bedded in the resin at one end, so air could bypass the filter altogether in those areas.  That was Husky's fault, not Bailey's, but I was not happy none the less.  Seems like quality continues to slide everywhere. 

John   

[ladylake]

 Isn't that the truth, companys that had good names trying to make a lot of money right now selling cheap junk.  Anyway I'd send the tester back, the last rim sprockets I bought from Bailys were so out of round I had to put them on the lathe before  I could use them.  Steve

[boobap]

make sure the compression testers "bleed off" valve isn't stuck open...that is the only thing i could think of, well other then being defective.

[terrifictimbersllc]

So, what fittings do I need?  Thanks in advance. 


John   

I didn't use a fitting, I just blew with an air nozzle (it has a tip  about 3/16 diameter, use whatever is the most pointed, smallest tip you have) right into the tester hole, trying to make a seal by holding tightly with my hands.  Did it at 2 settings 60 and 120 psi.  My tester (Sears, about20 yr old) holds the highest pressure until I release it with a button on the tester.   So it wasn't hard to "trap" the highest setting.   I put some pneumatic oil into the tester which seemed to help the function of the "trap" and release button.  If you were able to rig a fitting to it it would have to have spark plug female thread on one end and NPT air hose fittings on the other.  Don't know where you would get that.

[jteneyck]

Terrific..., thanks for the tip.  I used a soft tip on the end of my blow gun and got 70 psi downstream of my compressor which was set at 80 psi.  That's probably a reasonable number for the amount of pressure loss in a 50 foot hose.  Anyway, the gage is reasonably accurate, so I've got some other problem.  I wonder if it could be due to the length of the hose between the gage and the fitting that attaches to the cylinder.  The volume in the hose effectively increases the volume of the cylinder so the pressure won't be as high, and the smaller the displacement of the saw the greater the effect would be.  I did get a higher pressure on my 385XP than the 55, so that was consistent, but still only 10 psi.  The thing was sold by Bailey's in the chainsaw tools section, so you would think that it should work on chainsaws.  So what is it I'm missing?  Any ideas?  Thanks again.

John

[Rocky_J]

Exactly what is the process you're using to test the compression? Take us through your methods step by step.

[boobap]

pull the starter rope slowly and see if you can hear anything hissing, like an air leak would sound.

[nmurph]

are you only pulling the starter once? you should keep pulling it until the reading stops climbing.

[jteneyck]

Bad me. 

NMurph, your comment was the key to fixing my problem.  I didn't realize you had to crank the saw over repeatedly.  I wouldn't see this as needed if the gauge were attached directly to the spark plug port, but I see how it's needed to overcome the extra volume in the hose.  So I got about 130 psi on the newly installed P/C on the Husky 55, which seems pretty low, but it's only been run for a few minutes so I'll see what it is after I run it for awhile.  If it's still low I'll pull the compression release and put a plug in and check it again.  I also checked some other saws, and typically got around 150 psi.  When I tried to measure my 385XP; however, I couldn't get it above 100 psi, I think because I couldn't pull over the saw fast enough with the compression release closed.  That beast is hard to pull with the comp. release open - and nearly impossible for me when it's shut.  But I'm sure it's fine so I won't worry about it. 

So, in the end, it was operator error.  Thanks for all your comments to get me on the right track. 

John

[Al_Smith]

You have to remember as well that those are static  readings meaning not under power . Dynamic or under power will be higher .

With few exceptions it takes several tanks of fuel ran through a saw before the rings seat also which will raise the comp .This could vary depending on the cylinder finish, piston, rings etc .It took me the better part of one summer before the rings seated on an 038 Mag only cutting short little 10 minutes stints .When it finally hit home it was very evident and now has a D-handle on it to save my poor old fingers .

Converesly the last saw I rebuit was an 034 Stihl with an aftermarket piston and a used 036 cylinder .That cylinder came in on two tanks of fuel .

[Cut4fun]

Bad me. 

When I tried to measure my 385XP; however, I couldn't get it above 100 psi, I think because I couldn't pull over the saw fast enough with the compression release closed.  That beast is hard to pull with the comp. release open - and nearly impossible for me when it's shut.  But I'm sure it's fine so I won't worry about it. 

So, in the end, it was operator error.  Thanks for all your comments to get me on the right track. 

John

You could do it if you have the will power.
I have a Dolmar 166 gas saw that pulls 190psi without using de-comp. Yes I have snapped a stock 166 starter handle in to before doing so. Just a matter of how bad you want to know your psi.

[jteneyck]

Will power I've got - it's horsepower I'm lacking!  If I really want to know I'll have to ask my much larger, much stronger neighbor to give it go. 

A further item on this matter.  I went to the hardware store to get a 10 mm machine screw to plug the decomp. valve on the Husky 55.  I wanted to know if the decomp. valve might be leaking and the reason for the lower measured psi than I expected.  The decomp. valve turned out to be fine, although I found that it was a little loose when I went to remove it.  So I tightened it and measured 135 psi, then I took it out and put in the plug and got 135 psi again, so I concluded that the decomp. valve must be sealing. 

When I was talking to the shop owner he asked me if had checked the ring gap when I installed the piston.   I said no and didn't even know how, but that the piston, ring, and cylinder came in one set so they must be matched.  He said that wasn't necessarily true, and that if the ring gap was too small the piston could score the cylinder when it got hot and ultimately sieze.   I could see how this could be true, but find it hard to believe the parts in a complete kit wouldn't be matched.  I remember reading in a Husky manual how to measure the ring gap where the ends of the rings meet, but that was in the context of measuring for ring wear.  Anyone have any insights/experience to share on this subject?  Thanks.

John 

[Cut4fun]

I dont know if this is right. But is how I do it.

Put ring in cylinder by itself, then put piston in pushing on ring to square up the ring in the bore, measure the ring gap.

[boobap]

as far as i know...that is the easiest way to do it, maybe the only way. just be careful about putting pieces back together to make sure the "clock stud" on piston and notch on rings are lined up...you will be PEEVED if you break a ring because of this.

[Al_Smith]

You just shove the the ring in on the end of a piston and check the gap with a feeler gauge .

You're making a mountain out of a mole hill though .If you have 135 PSI and it runs okay just put it together and run it . For some reason someone came up with these magical numbers of over 150 and less than 190 that everybody thinks is Gospel .Nothing could farther away from the truth .

[jteneyck]

Thanks for the good intel guys.  So it seems you guys do measure and adjust the ring gap, if necessary.  I assume you position the ring so that the ends end up in one of the ports so that you can reach in with the feeler gage, correct? 

But I'm taking your advice, Al.  It's running fine, and I really don't want to take it apart again, so I will leave it alone and monitor the compression every few tanks of fuel to see how it's doing.

Thanks again everyone.  Always good advice, and much appreciated.  In a few more years I'll start to know what I'm doing. 

John   

[Rocky_J]

Quit loving it to death before you break something. In 25 years I've never once had to check compression on a saw, and I currently own and use about ten of them. Any sawyer who can run a saw should be able to tell if the saw is running well without having to hook up gauges. Use the gauge between your ears to tell you if the saw is running well or not running well.

Do you go out in the garage and pull the heads off your car motor once a month to see how your car engine is doing?

[boobap]

just be careful not to over torq the spark plug if you do remove it so many times...then your problems will go away because you will be ordering a new jug and slug anyway 

[jteneyck]

Well Rocky, some guys can tell what's going on by ear, and some of us prefer data.  I guess it's just my background where, in one example, I did testing where a pass/fail was often determined by a difference of 3 microns in machine control.  And since I no longer have a lab to play in since I retired, chainsaws and a few other things have filled some of that need to tinker. 

[Al_Smith]

For the record I've never taken the time to check ring ends on a chainsaw engine .I have however on antique four cyles wirh cast iron rings.

I've done so many two cycles that I can tell by rolling them over if something is amiss .Besides that ring gap has very little to do with how they run for what that's worth .