True Compression - Facts and opinions needed

[Warped5]

Let's face it. None of us are as young as we were yesterday. This seems to be a continuing trend. Pulling over a high-compression saw 'until it stops climbing' to get a reading can be a PITA.

Recently I was going through this 'exercise' on a Mac SP81 and it stopped climbing at about 160 or so.

.... but then I got an idea. What if I got my electric drill and cranked the saw over? Would I get a different reading? If so, how much?

Whatever, the outcome, I know my elbow would thank me.

So I did this with the drill and got a reading just a bit above 180. I ran the drill at about 1/2 speed to keep the flywheel moving, yet not pop the nut loose.

Which reading was more 'correct'?

How?

Why?

 

[Fruecrue]

Subbed.
Interesting discussion is sure to follow.

 

[Simondo]

Ether maybe depending on which method you chose to use as your test .... A constant rotation "may" overcome any small leak back , constant rotation "may"compress the air faster giving a little more heat to the cylinder . In a multi cylinder engine you tend to have the luxury of a starter motor to crank it for you when a comp tester is hooked up to one of the cylinders. Maybe its just down to a saw having a manual start .... pull /recoil/pull recoil ............ just giving the reading you would normally get and the only way of doing it usually. I would think the guide PSI given in manuals will be generated from the normal way of doing it and if that test comes up good but then you attain a higher PSI the "other way" it would not matter . If you got less PSI the normal way and then using the drill you got a better PSI reading....well that may just be a different situation . If you have a high PSI saw thats a grunt to test the normal way and you are already way above the factory standard and in no danger of having low PSI...It then just becomes a matter of "interest" rather than a test finding just what the highest PSI is i would think.

 

[rocketnorton]

static vs. dynamic, for lack of better words. port or cam timing havin so much overlap, feels like no/low comp @ crankin speeds.

 

[EvilRoySlade]

Truth is, I think we as a chainsaw loving community put too much emphasis on compression numbers. We all know it helps power up to a point. We all know it adds stress on the bottom end above certain psi. We all know temp, humidity and altitude have an effect.

Do I enjoy higher compression, yes. Does my father, no.

Now, the difference between pull cord and drill, I believe should be none if it was duplicatable. I think the drill would get slightly higher just based on piston speed, you will have a slightly higher ring pressure from the compressing gas pushing the rings out against the wall. Small but still present. I would still believe both methods. But like porting a saw, it's not about numbers, it's about you measuring the same way to get those numbers every time.

 

[jake wells]

a heavier flywheel would take some of the bite off when starting it.

 

[Warped5]

Simondo & Roy - Good replies!

Jake - As would a slightly larger recoil spool

 

[jake wells]

Simondo & Roy - Good relies!

Jake - As would a slightly larger recoil spool

use a kick starter the longer the better lol.

 

[Homemade]

.... but then I got an idea. What if I got my electric drill and cranked the saw over?

Used this method several times when diagnosing ignition problems and points adjustment on 4 strokes. Or any engine that "hasn't ran in a few years". Wayyyyy better then pulling your ass off getting a dry carb to suck up fuel and so on.



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[fearofpavement]

Most saws that pass through my hands I never check the compression on. It's a useful tool for diagnosis but in my opinion is considered more important than it actually is. As far as the actual numbers, I think the most value is in relative numbers. ie, I have 4 Stihl 026 saws. They should all have the same compression or close to it. If 3 have 154 and one has 135, then something's amiss. If I check some random saw, say a McCulloch 10-10, and get a number of 140. Is that good or bad? I don't know. So without some comparisons, compression numbers in and of themselves don't mean that much.

 

[Terry Syd]

As would a slightly larger recoil spool

If you can't get a larger recoil spool, you could work with the rope size and length. Sometimes a slightly larger diameter rope can be used to bring the outside diameter of the spool plus rope as far out as you can get it. The larger diameter rope works best for this as when you pull it, the diameter of the spool decreases faster than a thin rope, so it spins the engine faster as it reaches the end of the pull.

 

[Jimmy in NC]

Most saws that pass through my hands I never check the compression on. It's a useful tool for diagnosis but in my opinion is considered more important than it actually is. As far as the actual numbers, I think the most value is in relative numbers. ie, I have 4 Stihl 026 saws. They should all have the same compression or close to it. If 3 have 154 and one has 135, then something's amiss. If I check some random saw, say a McCulloch 10-10, and get a number of 140. Is that good or bad? I don't know. So without some comparisons, compression numbers in and of themselves don't mean that much.

Best ansewer?

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[Warped5]

Best ansewer?

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Thus far, probably

 

[Mastermind]

Interesting......

 

[Jimmy in NC]

Thus far, probably

Well I have always felt the same... Don't own a compression gauge.

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[67L36Driver]

Used this method several times when diagnosing ignition problems and points adjustment on 4 strokes. Or any engine that "hasn't ran in a few years". Wayyyyy better then pulling your ass off getting a dry carb to suck up fuel and so on.



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At my age, pulling and pulling to get fuel up in the carb is a PIT elbow and shoulder.

Recently solved the problem with a ketchup bottle from Walmart.

Just give it a squeeze.

And you can run it hooked up that way.

 

[Al Smith]

A prime works on most saws that have sat for a while but there are some exceptions to the rule .For example I have a bone stock( I think ) Husqvarna 2100 CD that will flood with a prime .It takes one pull with the choke and usually two after that no choke .This is one saw you don't want to flood because it pulls over about like rope starting a Harley .A combination of high compression,large carb,small starter drum .

 

[Basher]

Back when we would hold friendly lil get to gathers to cut up firewood for fun I have a heavily modified 044 that has no choke plate in the carb. I always wanted my saw to start first pull and get to cutting right away, bit of friendly competition was always present at these events. To ensure my saw started right off it rode in the cab of my truck with me and I had a small diameter Tygon line affixed to the choke shaft hole in the carb that lead out to the exterior above the carb tuning screw holes. A small shot of mix from a syringe needle into that line ensured it started pronto.

 

[Wood Doctor]

A prime works on most saws that have sat for a while but there are some exceptions to the rule. For example I have a bone stock (I think) Husqvarna 2100 CD that will flood with a prime. It takes one pull with the choke and usually two after that no choke. This is one saw you don't want to flood because it pulls over about like rope starting a Harley. A combination of high compression, large carb, small starter drum.

Hard to believe, I ran into this "easy flood" situation with my Stihl MS 251. When cold, anymore than two choke pulls will flood it every time and sometimes only two will. I've tried about every carb setting there is, and it still floods easier than any 2-cycle engine I have ever owned. I doubt there is 50 hours on this engine.

So, I do the same as Al -- "one pull with the choke and usually two after that no choke." Some things you just learn to live with.