why is my saw running?

[boutselis]

Ok; I am learning about proper compression.

I have a gauge, with a schreider (I know I didn't spell it right) valve just like the site recomends.
I tested it on a compressor and it reads the same as the regulator gauge on the compressor wether it is 60 lbs or 120lbs (which is the max on this little compressor)

I put the gauge on my ms290 first because i am most familiar with it and just used it to drop some 35' palms. It runs fine and really doesn't have to many hours on it. even cuts through the thickest part of the stump which is wider than my 20" bar ( I know its only palm and not oak but its still a test of sorts)

the compression is only 65lbs cold. So it shouldn't be running from what I understand.

I also tested the 019 I just got. I used this saw to cut up some good size limbs and it runs fantastic but it also only has about 65 lbs

the saw will idle, revs up nice and when i let go of the throttle it doesn't drop in rpms to fast.

what am I missing?

 

[oscar4883]

Even though you checked the gauge, it does sound like it is a little off. But, if you are happy with the way the saws run, who cares what a gauge says.

 

[Stihlofadeal64]

I know this is not rocket science but I suspect the guage especially with the results you're getting cutting. I don't suspect the larger saw would cut at all with that kind of compression reading. Perhaps you can rent one from the local auto parts store, I'm told in our area some can do it. If not, you may be able to get a dealer to check it for a nominal fee (if that is possible).

 

[boutselis]

Even though you checked the gauge, it does sound like it is a little off. But, if you are happy with the way the saws run, who cares what a gauge says.

Thanks for the reply. My interest in the compression is to tell me wether or not my saws need to be rebuilt before any real damage is done.

I was figuring if the piston and rings were a little loose, causing compression to be a little low, then it would start to ruin the cylinder.

I don't know if thats correct thinking though.

 

[boostnut]

Describe the method you're using to check the compression. It doesn't sound like a gauge problem, more of a procedure issue.

 

[oscar4883]

Describe the method you're using to check the compression. It doesn't sound like a gauge problem, more of a procedure issue.

Good thinking. I just pull until the gauge reaches its max reading.

 

[boutselis]

when I did the check I screwed the gauge in as far is it could go without the piston hitting it, it was far enough so that there wouldn't have been any leak,
put the saw on te ground and pulled, probably 10 times.

I then tried the same thing with the throttle held in but the reading was the same.

 

[cgarman]

I'm trying to figure out the compression thing too. I just bought an 076AV, and the guy I bought it from tested it at 145 psi. When I got it, I tested it at 120 psi. I retested later, thinking maybe I hit the de-comp port, but I got 120. Then, I tried the trick of locking the throttle. Still 120. The thing is, the guy I bought it from seems to know what he's talking about, and I trust his measurement more than what I see with my own eyes. That's because I have no idea what I'm doing.

The only time I ever saw 145 on the gauge was when it bounced up there momentarily as the starter was pulled. I never got it to settle above 120 though.

So, I'd like to add my two questions to the forum:

1.) If I saw 145 for a split second, but it only settled at 120, then what's the real measurement?

2.) Should I even care if it's "only" 120? Is this normal for older saws?

 

[cgarman]

Oh yeah, and my crappy Poulan Pro 295 46cc saw hit 145 easy. Go figure.

 

[boutselis]

I'm trying to figure out the compression thing too. I just bought an 076AV, and the guy I bought it from tested it at 145 psi. When I got it, I tested it at 120 psi. I retested later, thinking maybe I hit the de-comp port, but I got 120. Then, I tried the trick of locking the throttle. Still 120. The thing is, the guy I bought it from seems to know what he's talking about, and I trust his measurement more than what I see with my own eyes. That's because I have no idea what I'm doing.

The only time I ever saw 145 on the gauge was when it bounced up there momentarily as the starter was pulled. I never got it to settle above 120 though.

So, I'd like to add my two questions to the forum:

1.) If I saw 145 for a split second, but it only settled at 120, then what's the real measurement?

2.) Should I even care if it's "only" 120? Is this normal for older saws?

I hear 120 is fine for worn in saws. if my understanding is correct its when you dip below 100 that problems begin. I know I must be doing something wrong because My saw should barely be able to start at 70 lbs but it not only starts it cuts too.

But the gauge is accurate unless my compressor and the compression gauge have plotted against me in some way.

 

[Taxmantoo]

when I did the check I screwed the gauge in as far is it could go without the piston hitting it, it was far enough so that there wouldn't have been any leak,

Far enough to seal would be far enough to pinch the o-ring on the gauge adapter against the engine. Most gauges only have 4-5 threads on them, do you have an unusually long one?

 

[boutselis]

Far enough to seal would be far enough to pinch the o-ring on the gauge adapter against the engine. Most gauges only have 4-5 threads on them, do you have an unusually long one?

Maybe this is my problem. Mine is to long to go all the way in. I believe it is automotive use. I didn't think that would make a difference.

I thought it was the threads that kept the compression in more than the plug being seated to the cylinder head?

I'm knew at this.

 

[cgarman]

I hear 120 is fine for worn in saws. if my understanding is correct its when you dip below 100 that problems begin. I know I must be doing something wrong because My saw should barely be able to start at 70 lbs but it not only starts it cuts too.

But the gauge is accurate unless my compressor and the compression gauge have plotted against me in some way.

The saw may be worn a little, but that wasn't where I was going. I've heard people say that 20 years ago, saw were DESIGNED to have lower compression, and had larger bores with shorter strokes. The design was for torque, not speed. As engines evolved, it was easier to make an engine go faster than it was to make it have more torque. Since power = torque * speed, either path results in a more powerful saw.

I'm wondering if anybody can back this up. Maybe a 20 year old saw with 120 psi is comparable to a newer design with 150. I don't know, though.

 

[oscar4883]

If your gauge is not tightened against the cylineder you may be losing the comp. you are looking for out the plug hole. Just snug the gauge up and pull til it won't go any higher. I don't think throttle position is important either. Compression takes place at the top of the cylinder. You're not really keeping any air in by locking the throttle. Plus, there is a big hole thru the muffler too LOL. Keep us posted.

 

[stevohut]

If your gauge is not tightened against the cylineder you may be losing the comp. you are looking for out the plug hole. Just snug the gauge up and pull til it won't go any higher. I don't think throttle position is important either. Compression takes place at the top of the cylinder. You're not really keeping any air in by locking the throttle. Plus, there is a big hole thru the muffler too LOL. Keep us posted.

You should hold your throttle wide open when doing a comp test because you want the max air volume you can get in the cylinder to get an accurate pressure reading. With the throttle closed you won't get a full charge in the cylinder and your going to suck in atomized fuel into the mix of air more then if the throttle is wide open. Just my opinion. This can be proven by the fact when you flood an engine you hold the throttle wide open to reduce the fuel being mixed with the charge. It is based on the principle of how venturies and low pressure areas work to draw the fuel through the carb.

 

[stevohut]

The saw may be worn a little, but that wasn't where I was going. I've heard people say that 20 years ago, saw were DESIGNED to have lower compression, and had larger bores with shorter strokes. The design was for torque, not speed. As engines evolved, it was easier to make an engine go faster than it was to make it have more torque. Since power = torque * speed, either path results in a more powerful saw.

I'm wondering if anybody can back this up. Maybe a 20 year old saw with 120 psi is comparable to a newer design with 150. I don't know, though.

Along with your comp test which yielded low numbers I would perform a leak down test to make sure the engine will hold the comp it is trying to generate.

 

[boutselis]

The saw may be worn a little, but that wasn't where I was going. I've heard people say that 20 years ago, saw were DESIGNED to have lower compression, and had larger bores with shorter strokes. The design was for torque, not speed. As engines evolved, it was easier to make an engine go faster than it was to make it have more torque. Since power = torque * speed, either path results in a more powerful saw.

I'm wondering if anybody can back this up. Maybe a 20 year old saw with 120 psi is comparable to a newer design with 150. I don't know, though.

I think you are right. If memory serves from my dirt bike days A high compression 4 stroke motorcycle engine could reach the same hp as a lower compression 2 stroke of the same size. the 4 stroke ran at a lower rpm at higher compression and the 2 stroke ran at higher rpm and lower compression

 

[MostShady1]

I'm just curious, but where exactly is the check valve located on your comp tester? It you see 145 on the gauge at any time, it should lock it there unless it see a higher pressure, or you hit the pressure release.

Maybe a pic of your comp tester would help. We all Love pics around here!

 

[treemandan]

To answer the original question: because you started it?

I had an HO pull out a practically new 250 Stihl that he said hadn't be run in 3 years. I pumped the primper bulb and fuel filled it then choked and pulled it. Fired right up. HE was going to give to or something like that but I said I would kill it in a week. I told him it was worth more sitting around til he needed it.

 

[boutselis]

To answer the original question: because you started it?

I had an HO pull out a practically new 250 Stihl that he said hadn't be run in 3 years. I pumped the primper bulb and fuel filled it then choked and pulled it. Fired right up. HE was going to give to or something like that but I said I would kill it in a week. I told him it was worth more sitting around til he needed it.

I was wondering how long it would take for someone to say that.

I guess your story about the 250 centers around "why would someone rebuild a ms290" and I kinda agree but My interest in compression centers around my ms460. It fired right up and seemed to run great but had really low compression so i tried to test my ms290 which I had just used and knew should have proper compression because of the way it ran and cut

but it had really low numbers also so I am trying to figure out what I'm doing wrong.

Any way, I bet you couldn't ruin a ms290 in a week even if it was running a 50 hour work week I bet it would last 5 or 6 weeks at least.