[engine work] Have Q's on benchmarking performance, and proper use of a compression gauge ;D

[Cerberus]

My new 590, barely 2-3 tanks through it (~40:1 using HP Ultra and Stabil in eth free 92 oct, fresh fuel) Even after muff mod & airbox mods, the carb didn't need adjustment for Idle or Low (hadn't set H yet, wanted to get my baseline compression benchmark, then was gonna go set H while spending half an hour getting a better feel for the unit)

Problem is I'd never checked engine-compression, I went and got a $30 comp gauge from the auto-store (which I was told was OK), I insert it to the spark-plug-hole only to find it goes in too-deep & blocks the piston...I back it out 1/2 or full turn, and now it works, each time I pull the starter the pressure builds, seemed real nice&linear, until ~91psi where it just stopped!

Would be very eager to hear you guys' thoughts on this....I was in love w/ this saw, and had/have my most ambitious saw-mod plan for it yet, but if "it's a lemon" then I'd sooner sell it and buy another (and just get the 620p, now that I know I can buy powerheads-only from one of the good saw-sites!)

This was a floor model, so people surely pulled that cord while the engine was dry IE no oily-fuel but when I did the MM I, naturally, was checking out the P&C and (at least on the exhaust side), they looked flawless...

Can't tell if my reading is sure to be off (and if so, whether it's the unit or the operator/myself), or if my new 590 is problematic :/ Would greatly appreciate any&all insight on this one!!


Thanks a ton!!


PS-- benchmarking...I don't have good "video + stopwatch" setup, and - to me - the best benchmark is cutting-speed, so to that end my plan for benchmarking - which I intend to do a ton of while modding this saw - is to simply film myself doing 2 or 3 cuts then using a stopwatch when watching the video....my thinking is that since it's 3 cuts, it should help give better #'s (IE account for variance-per-cut, and for inaccuracies of my timings) Just took home a log yesterday that's a good 1' wide, nearly 8' long nice and homogeneous, will be the 590's benchmarking-log!

 

[Ronie]

The compression gauge has to be screwed all the way in so that it creates a seal.

 

[Ronie]

I think you shouldn't worry so much about that saw, it doesn't matter that it's a floor model, I doubt that enough people pulled it over to do any damage, just tune it and go out and enjoy it.

 

[Maintenance Chief]

Depending on the piston ring manufacturer it could take 10 tanks for them break in which would greatly affect compression.
And yeah have to have the guage screwed in all the way.

 

[Lee H]

A 30 dollar compression gauge is never going to give you accurate readings.
The gauge has to be fully seated in the plug hole to eliminate leaks. You also
need to hold the throttle wide open to get all the air in it can breath. Pull the
rope until the gauge stops moving. You now have your readings.

 

[drf256]

You should invest in a compression gauge that’s meant for 2 strokes. I have an OMT and it comes with various adapters and works great.

The most important thing is that the part that screws into the plug hole is that same length as the spark plug thread and that there is a shrader valve at the tip of the part that gets screwed into the plug hole. Anything before that valve (ie an adapter) will add the volume of that part to the combustion chamber volume and cause a falsely low reading. Too long of an adapter, if it clears the piston at TDC, will falsely elevate your reading.

The screw in part must be seated and screwed all the way in to create a seal. There should be an O-ring there.

Not to start an argument, but holding the throttle open won’t change your readings in a 2 stroke. Compression starts when the ex roof is closed by the top ring and keeping the intake port at WOT at the rpm you can achieve pulling a saw over to start will have no effect on the swept volume.

That being said, don’t put too much emphasis on compression numbers. I rarely use my gauge, and when I do it’s just a reference point. If the piston is in good shape and the saw runs well, there is little reason to check it on a stock saw. Too much of a good thing can be bad. If you’re running too much compression it can make a saw run too hot and also limit rpm.

I sincerely doubt that your saw has only 90# of compression. You generally need 100-120 psi to have a saw start.

 

[CR888]

I've found holding the throttle open can increase your compression number on some saws others not so much. That's just from my experience not something I read. Not saying the Doc is wrong, he is smart and knows stuff about stuff & stuff.

 

[Czed]

I have 2 different gauges
That produce 2 vastly different readings
The inexpensive automotive unit with the 8" hose
Is as much as 80psi lower on a chainsaw than my snapon one.
You may want to invest in a quality unit

 

[Sawrain]

Not to start an argument, but holding the throttle open won’t change your readings in a 2 stroke. Compression starts when the ex roof is closed by the top ring and keeping the intake port at WOT at the rpm you can achieve pulling a saw over to start will have no effect on the swept volume.

How can swept volume change?

I figured that while primary/case compression will be lower with a closed throttle that secondary/cylinder compression will be the same as air (at atmospheric pressure) can back flow in from the exhaust port before it is closed off by the piston, so it doesn’t really matter how much air comes through the transfers, cranking compression starts with a cylinder pressure close to atmospheric, regardless of throttle position, so maximum compression pressure should be mostly unchanged.

 

[drf256]

How can swept volume change?

I figured that while primary/case compression will be lower with a closed throttle that secondary/cylinder compression will be the same as air (at atmospheric pressure) can back flow in from the exhaust port before it is closed off by the piston, so it doesn’t really matter how much air comes through the transfers, cranking compression starts with a cylinder pressure close to atmospheric, regardless of throttle position, so maximum compression pressure should be mostly unchanged.

It can’t. At certain rpm, dynamic compression will change with throttle position, but at starting rpm it should have little effect.

 

[CR888]

I just did a compression test on a new to me little echo 350wes and thought I'd do my Ms261 shteeel sitting next to it. With throttle closed, I pulled & pulled till needle wouldn't go no higher which was 175psi. Now holding throttle wide open is tricky so I wrapped tape around trigger and it got to 181psi.

 

[Wonkydonkey]

I would have thought the way a two stroke works…. When checking compression the exhaust would be open long enough to reach equilibrium with the surrounding atmosphere and you’d be just compressing air.
Any little difference in readings with WOT would be with fuel/oil being compressed in the same space.

that said, the only real thing I’d have thought about checking compression is that it starts easy and don’t have to much so it either rips your digits or starter pulley & later destroys your main bearings.

so if you have a known good comp tester, I guess if a saw don’t start easily and runs poorly. This would show up on the compression and aid in diagnosing the problem without taking the saw apart.
And don’t forget about adding some extra oil helps seal the rings, thus helping in the diagnosing . Edit: dry & wet readings

 

[Cerberus]

Quick Q I forgot to ask-- This is tested on COLD saws, right? Because on a hot unit, of course, the #'s would be higher (I would imagine!)


Going to reply to "1st half" since there were - lucky for me - more than expected Thanks guys, am relieved reading this as I love this saw and actually intend it to be my 'first victim' for port-work (have always modded my saws, have done multiple restoration-builds including vintage and a polesaw actually lol but never port-work, I am good w/ my grinders though and have 20+ hours of "tinman" youtube video, specific to porting the 590, behind me already...just hoping time allows this in the coming month or two, not quater-or-two Have an echo string trimmer powerhead that I'm going to use for practice-grinding before touching the 590 (where I'll just be copying Tinman's port-work, mostly)

I think you shouldn't worry so much about that saw, it doesn't matter that it's a floor model, I doubt that enough people pulled it over to do any damage, just tune it and go out and enjoy it.

This is great to hear esp from you thank you!! I have begun taking it with me more often, used it to fell & buck a lil tree yesterday actually, but it's still on tank #3 -- I'd love hearing thoughts/advice on break-in, if you have any (I know there's a "just use-as-directed from the start" mindset...I'm "taking it easier" on it, doing heat-cycles, and doing 40:1 oil in my usual eth-free, HP Ultra fuel)

Depending on the piston ring manufacturer it could take 10 tanks for them break in which would greatly affect compression.
And yeah have to have the guage screwed in all the way.

This...this is fantastic and what my mind kept going to (well, is it because the compression is building, at least primarily, via oil buildup or oil+fine-metal-shaving buildup of the fresh saw?) Good to hear, was expecting as much but couldn't stop worrying my exhaust mod may've been hurting this as well was going to plug it and re-attempt (I hate taking these readings, I have the worst luck breaking plug-boots!!)

Re screwing-in fully.....I cannot! The gauge, if bottomed-out, wouldn't allow the piston full travel! Does this mean I need to try finding the packaging & seeing if Auto Zone will refund my $30? Any tricks to get-and this? Teflon/pfte tape? Or an "electrical-tape-washer" on the gauge's plug-threading that'd effectively act as the 'stop' to create seal?

If this type of generic unit cannot be used (its gauge-tip looks the same as all these 'generic comp kits') then it's not usable....which I was told it was, so that sucks lol, so if there's no way aruond "it's never gonna work / get your $ back", I'd be curious for model-recommendations that are affordable!

A 30 dollar compression gauge is never going to give you accurate readings.
The gauge has to be fully seated in the plug hole to eliminate leaks. You also
need to hold the throttle wide open to get all the air in it can breath. Pull the
rope until the gauge stops moving. You now have your readings.

Are there any tricks for seating it properly if the gauge-tip's threading is simply too-long for the hole? If fully inserted it doesn't allow piston full reach to TDC so I have to have it backed-out a lil....surely I can still create an effective seal, for instance a tapered PTFE-tape 'washer' on the threading (that I can further reinforce after the thing is seated) If I have to return(or throw-out) this thing and buy anew I probably will 2nd-guess it as I'm 2nd-guessing how important it is to me to even know the comp-PSI...I have my 1.25yr old 355t that has higher compression and will surely be a "good benchmark" as the 590 approaches (and hopefully passes) the 355's compression (which feels like 50% stronger right now!)

You should invest in a compression gauge that’s meant for 2 strokes. I have an OMT and it comes with various adapters and works great.

Various adapters...I thought all spark plugs had the same 'seat' threading??? That's kinda "mind blown" if I was mistaken on that, was quite sure! The "right diameter" gauge from my kit seemed to have matching threading to the spark plug hole, thankfully, but its tip was too-long so I couldn't get an effective seal...

 

[Ronie]

I'd love hearing thoughts/advice on break-in

I just run it like I use it from day one.

 

[Cerberus]

Oh Re crappy compression gauges....IF I achieve a decent seal (tapered PTFE 'washer', maybe even just a permatex 'plug washer' made in-place on the threading..), if that's the case & I still expect the accuracy is off, is it likely it's still reliable for changes in PSI? IE, sometimes a bodyweight scale is off of true-weight by 5lbs or something, but it's still able to faithfully show up/down movements.

 

[Cerberus]

I just run it like I use it from day one.

AWESOME thanks am gonna stop pussy-footing with it and just bring it always, will be "tapering to 50:1" over the coming tankfuls though, and will not be testing the limits-of-lean for High until at least a few more tankfuls through

Testing is done on a cold engine right? Glad I caught the "Hold the throttle WOT" as I was not doing that, obviously that'll make a difference!!

 

[Ronie]

Testing is done on a cold engine right? Glad I caught the "Hold the throttle WOT"

I quit doing compression test awhile ago unless it's an old saw but when I do It's cold and I don't hold the throttle wide open, keep in mind that I'm not a saw mechanic and could be completely wrong.

 

[Ronie]

and some friendly advice coming from someone who's messed up his fair share of cylinders, get some old or cheap AM cylinders and practice grinding on them before grinding on an expensive OEM cylinder. When you think you have a handle on it, post some pics of your work and ask some people with experience to look at it and tell you what they think.

 

[Bjorn]

Here is a good quality tester. https://www.ebay.com/itm/223545177133

 

[Nutball]

I like testing mine hot because the engine runs when hot. Cold testing is good for repair diagnostic. I tend to get lower compression when hot, and I think it is because the oil is thinner and isn't pooled around the edge of the piston as it does when sitting for a while. Also the hot crankcase may expand the air keeping less volume from getting in the cylinder.

Echos read on the low side anyway compared to other brands.