Compression tester

[nmurph]

With a small cylinder like a chainsaw the air in the tube becomes a significant volume and readings will be low. Also the check valve needs to be a lightly sprung one. Old ones from the tire store don't work

I actually typed that and then deleted it. With any size of hose if you pull it enough it will get to it's terminal number. The check valve in the foot just gets you there faster.

 

[maico490]

I actually typed that and then deleted it. With any size of hose if you pull it enough it will get to it's terminal number. The check valve in the foot just gets you there faster.

Sorry but I disagree. If the check valve is at the gauge end of the hose the volume of air in it is added to the combustion chamber volume thus lowering the compression ratio. If a large capacity cylinder is being tested this is insignificant but for a small engine like a chainsaw it isn't.

 

[WV Mountaineer]

Sorry but I disagree. If the check valve is at the gauge end of the hose the volume of air in it is added to the combustion chamber volume thus lowering the compression ratio. If a large capacity cylinder is being tested this is insignificant but for a small engine like a chainsaw it isn't.

You are not measuring volume. You are measuring pressure.

Anyway, I ordered the OTC one and received it today. I was shocked at the quality of the kit. I have used them many times before on cars but never on a chainsaw before. I have a xxv micro deluxe that isn't getting fuel. I wasn't sure if it was the carb or a lack of compression. It tested at 110 psi after several pulls. I'm not sure about the correct procedure for testing on saws and if that in fact is too low to create enough impulse to pump the fuel.

Thoughts?

 

[watsonr]

You are not measuring volume. You are measuring pressure.

Anyway, I ordered the OTC one and received it today. I was shocked at the quality of the kit. I have used them many times before on cars but never on a chainsaw before. I have a xxv micro deluxe that isn't getting fuel. I wasn't sure if it was the carb or a lack of compression. It tested at 110 psi after several pulls. I'm not sure about the correct procedure for testing on saws and if that in fact is too low to create enough impulse to pump the fuel.

Thoughts?

Pull until it stops.... 110 is low... however, I've seen readings lower and still had running saws. Large displacement saws run a little lower and saws with a lot of use tend to also. But at 110 I'd pull the muffler to see either worn out rings or a scored piston.

 

[MechanicMatt]

OTC is a good one, so good its held up for the last 11 years of my abuse. I actually have two, one for at work and one for home.

 

[WV Mountaineer]

Pull until it stops.... 110 is low... however, I've seen readings lower and still had running saws. Large displacement saws run a little lower and saws with a lot of use tend to also. But at 110 I'd pull the muffler to see either worn out rings or a scored piston.

I did but I didn't see anything that looked terribly bad. Then again I have no experience looking at them to compare.

I think my dad burned it up by running it with 50:1 gas. Either that or it wasn't tuned right. It was started and put to wood just to see if it would run and ran pretty good but got hot and was leaking gas like a tall cow peeing on a flat rock. :jester: It stalled out and we just dumped the gas out of it and I ordered a tank for it. I took everything apart and cleaned it up and put it back together. It will pop and start with mix shot right into the carb and keep running by injecting more fuel but stalls out as soon as you stop.

Muffler is clear.

New plug w/good spark.

110 compression

It does have a reed valve and I have no clue how to adjust them or tell if something is wrong with it.

Any pointers would be appreciated. Just having fun with it.

 

[MechanicMatt]

I don't know if I missed it somewhere, but what is the make and model??

 

[WV Mountaineer]

I don't know if I missed it somewhere, but what is the make and model??

Poulan MICRO XXV deluxe, big pimpin baby. :msp_scared:

 

[WV Mountaineer]

I did but I didn't see anything that looked terribly bad. Then again I have no experience looking at them to compare.

I think my dad burned it up by running it with 50:1 gas. Either that or it wasn't tuned right. It was started and put to wood just to see if it would run and ran pretty good but got hot and was leaking gas like a tall cow peeing on a flat rock. :jester: It stalled out and we just dumped the gas out of it and I ordered a tank for it. I took everything apart and cleaned it up and put it back together. It will pop and start with mix shot right into the carb and keep running by injecting more fuel but stalls out as soon as you stop.

Muffler is clear.

New plug w/good spark.

110 compression

It does have a reed valve and I have no clue how to adjust them or tell if something is wrong with it.

Any pointers would be appreciated. Just having fun with it.

Shameless self bump in hopes that someone can give me some advice. I know the saw is worthless, just having some fun.

 

[half_full]

probably best to start a new thread to get your answers. Although your question is in regard to low compression, you will get much more help with a new thread.

 

[dl5205]

Does the OTC have a schrader valve at the spark plug end?

 

[WV Mountaineer]

pressure and volume are related. A small cylinder such as in a chain saw has a very small combustion chamber. If the check valve is located right at the spark plug hole it will give a more accurate reading. If the valve is located at the gage the hose becomes an extension of the combustion chamber increasing its volume and reducing the pressure reading. I have a gage with the valve located at the gage. I notice that my larger saws all seem to post higher readings than my smaller ones even though my 38cc saw will just about pull your fingertips off. This is probably because the volume of the hose when added to the 38cc cylinder has a large effect as compared to the same volume being added to a 85cc cylinder.

Don't care to argue semantics or Boyle's law. Let me try to explain it to you this way.

Regardless of where the check valve is located, the entire hose becomes an extension of the combustion chamber, up to the gauge, when the check valve is open. Then it closes when the pressure on the chamber side drops. Regardless of the check valve location its job is to stop the pressure from escaping so that we can read the value on the gauge.

You are thinking of the check valve as a solid object that doesn't let air through. That simply isn't the case. If no air passed through the check valve then you would be correct.

BTW the OTC gauge is very impressive for the cost. Looks like snap on quality at a fraction of the cost. Hopefully it last.

 

[dl5205]

Does the OTC have a schrader valve at the spark plug end?

dl

 

[WV Mountaineer]

Does the OTC have a schrader valve at the spark plug end?

No, it is next to the gauge. Not sure how that would work next to the plug. Wouldn't it be hard to open and restrict spinning it into place?

Amazon.com: OTC 5606 Compression Tester Kit: Automotive

Corey

 

[maico490]

I am pretty sure most compression testers have the check valve located at the spark plug hole because they will read low if it is located at the other end of the hose. Even my cheap Harbor freight unit is built that way. The check valve isolates the hose and gage volume from the combustion chamber. It captures peak pressure and holds it to be read from the gage. If it is located at the gage end the volume of the hose will add to the volume of the combustion chamber and will result in a lower peak pressure and a lower reading. I think some people confuse the pressure release valve with the check valve. Most of the relief valves are mounted on the gage and the check valve (Schrader valve) is at the spark plug hole. I don't know how they could work otherwise.

We agree totally but some people seem to think that you can effectively increase the volume of the compression chamber and still keep the same maximum pressure.

 

[WV Mountaineer]

that is a nice looking gage. If you look at the end of the threaded spark plug hole adapter you will most likely see something that looks like a tire valve stem insert. That is the Schrader valve/check valve. The thing next to the gage is the pressure relief valve.

You are right about the schrader valve being at the bottom. I had my terminology wrong but not my physics. The reason your other gauges read low can not be due to the schrader valve being closer to the gauge. I can draw out a diagram but I'm sure if you stop to really think about it you will agree. When the valve is open, the pressure equalizes on both sides of the valve until a drop in pressure closes it. The valve's location has nothing to do with the equalization of pressure on either side of it.

There are a number of reasons for a different reading including inaccurate gauge, hose flex(increasing volume), and a faulty schrader valve.

I would suspect that the schrader valves fail over time because the otc kit included replacements and by putting them in the end they are effectively giving you several valves.

Now, if i could just figure out how to give my saw more compression....

 

[WV Mountaineer]

We agree totally but some people seem to think that you can effectively increase the volume of the compression chamber and still keep the same maximum pressure.

Longer connecting rod would increase the total volume and raise compression without any other changes. I know what you mean tho. I think the squish and port jobs are meant for volumetric efficiency and are usually missing a gasket to keep compression up.

 

[Majorpayne]

this is where the train runs off the tracks, you are right when you say " When the valve is open, the pressure equalizes on both sides of the valve until a drop in pressure closes it." If the valve is placed a good distance from the spark plug hole, say at the end of a rubber hose, the compression chamber has been enlarged due to the volume of the hose, and because the displacement of the piston/cylinder remains the same it must result in a lower pressure. By placing the valve at the spark plug hole it maximizes the point of pressure equalization due to the diminished volume on the piston side of the valve.

View attachment 296808

You are pumping pressure in the hose no matter which end the valve is on.

 

[WV Mountaineer]

this is where the train runs off the tracks, you are right when you say " When the valve is open, the pressure equalizes on both sides of the valve until a drop in pressure closes it." If the valve is placed a good distance from the spark plug hole, say at the end of a rubber hose, the compression chamber has been enlarged due to the volume of the hose, and because the displacement of the piston/cylinder remains the same it must result in a lower pressure. By placing the valve at the spark plug hole it maximizes the point of pressure equalization due to the diminished volume on the piston side of the valve.

View attachment 296808

You would be 100% correct if the valve never opened, but it does....

At what state should the valve be at 1 psi, open or closed? It would be open unless you have a bad valve.

Now with your valve at the spark plug end, what is the total volume of the test????

 

[Majorpayne]

:bang:

true, but putting the valve farther away from the piston will result in a lower reading since you are displacing the same amount of air into a larger receiver.
we are not pumping pressure, we are pushing a fixed quantity of air in the cylinder into a combustion chamber, the larger you make the chamber the lower the resulting pressure of the compressed volume will become.

:bang: