Checking compression

[Meds]

Hi all,

 

Sorry and thanks in advance for the noob question...

 

Checking compression on a saw with those screw into the spark hole kits....I understand I have to pull the saw a few times to get it up to pressure. Can I do this with no fuel mix in the saw? 

 

I just bought a second hand 261 and wanted to check it over. Piston looks good but upon hanging, the saw drops fairly easily. How does the decompression button work with this? (Was wondering if that is the reason why it drops)

Edited May 25, 2021 by Meds

[Dan Maynard]

If the decomp button is in then it will fall easier, yes, that's it doing its job. The button should pop out when the engine fires but I believe you can gently pull it out manually yourself - not done this myself though.

Does it start and run?

[Meds]

Thanks, that makes sense.

 

I've not tried it down the woods yet - don't want to advertise my new addition to the neighbours in the back garden!

[Paddy1000111]

You can pull the decomp out by hand if you want but it won't fully seal sometimes like it would  if it started. You can undo the decomp valve, push the button in and then clean it off with a brass wire brush if you want to get any carbon/rubbish off it. I found mine got a little crappy

[Meds]

Thanks for the tip.

 

With the 2 stroke mix acting as cylinder lubricant, when doing the pressure test, will I risk damaging anything internally if there's nothing going into the cylinder (empty tank) ? If I'm pumping the piston by hand on empty will it be ok?

[Paddy1000111]

26 minutes ago, Meds said:

Thanks for the tip.

 

With the 2 stroke mix acting as cylinder lubricant, when doing the pressure test, will I risk damaging anything internally if there's nothing going into the cylinder (empty tank) ? If I'm pumping the piston by hand on empty will it be ok?

Assuming you haven't just assembled it and put it together dry it will be fine. I would do a quick test "dry" and then put a couple of drips of oil in there and do it again. gives a good idea on how well it's sealing 

[Meds]

Many thanks. Makes sense to put a couple of drips in there (through the plug hole?) as I don't know how long it's been dry for

[spudulike]

Just run the saw for a few seconds, stick the tester in the plug hole, don't use the decomp and pull the saw over once, look at the reading (should be 70psi+) pull it over 5-6 pulls more and expect a reading of 150-175psi on a decent saw. Lower than 150 and it isn't too good.

On saws that haven't been used for a long time, the fuel in the bore can evaporate leaving a coating of oil that can create a better seal and more compression than the saw will normally have. 

A saw that has been apart and is dry, will drop compression from not having fuel oil mix around the rings.

A saw that is hot will make around -20psi than a cold one!

Lastly, make sure the gauge has a schrader valve in the brass union and is a very light pressure one as many cheap gauges are for cars and a small engine won't open the valve in these gauges enough.