How to diagnoise air leaks

[Cannon51]

Last fall I bought a Craftsman/Poulan That had good compression,View attachment 313202 would crank, run wide open and could not be chocked down. Yesterday I bought a Ryobi Trimmer that will run on choke but die when you pull the trigger. While running on choke I can completely cover the carb intake with my thumb and it makes very little difference. I took the carb off and sprayed out all the passages, no change. With the carb off I put my finger over the intake and pulled it through, I couldn't feel any vacuum (should I). Both engines have about 120 lb. compression. I searched "air leak" but didn't find and discussions on how to proceed in these situations. Should I continue looking into the carb or move to the crankcase?

 

[AKDoug]

The proper way to test for an air leak is to vacuum test it. You will need to seal off the exhaust port and intake port and apply vacuum through the impulse line. There are several threads on it. Try searching for vacuum testing.

Here is one good thread... http://www.arboristsite.com/chainsaw/42768.htm

 

[w8ye]

It would be easier to find the actual leak with a pressure test.

Same procedure as the vacuum test but to use 7lb pressure instead.

The pass fail part of the test is that it holds at least 5 of the 7 lbs for 30 sec.

The advantage of the pressure test is that it is easier to find the leak with "Leak Lock" or soapy water or even hear the leak.

 

[Cannon51]

That was a very good thread, thanks for the link. I downloaded the PDF to my computer. As far as the pressure test, would a regulator like the small one on my gravity feed paint gun work?
I was able to turn the low speed jet out about a 1/4 turn and the high about a third. It will run now but it still seems to be screaming a little at WOT but that's all I can turn the Pacman screws with my wife's tweezers. I think I will get a female disconnect that will fit my compression tester and a barb fitting this afternoon and see what happens. Thanks
Cannon

 

[w8ye]

Some of the illustrations in the "linked-to" thread show using a regulator and connectors to use your air compressor for the pressure test.

I have the MityVacs but the factory Husqvarna tester uses a squeeze bulb like is on a blood pressure cuff to make the pressure. The adapted a gauge to it.

You don't want to use much pressure or vacuum or you will ruin the seals or the carb boot on newer chainsaws.

The accepted pressure or vacuum is 1/2 atmosphere or +/- 50 kPa or +7 psi or - 15" Hg

 

[Cannon51]

I prowled through my junk and found enough to make a vacuum tester, unless someone sees a flaw in it. It's the hose off my compression tester, a new female disconnect, a 5/16 hose barb. It will hold 15in HG with my finger over the plug end. I have to take my wife to town shortly so I won't get to take the carb and muffler off today.View attachment 313214
Cannon

 

[turtle561]

"I have the MityVacs but the factory Husqvarna tester uses a squeeze bulb like is on a blood pressure cuff to make the pressure. They adapted a gauge to it."-wye8

i used a ball inflater gauge from a sports shop-$8- with the blood press bulb and some auto vac line fittings to rig my tester up.

they even make a digital one.

 

[w8ye]

I prowled through my junk and found enough to make a vacuum tester, unless someone sees a flaw in it. It's the hose off my compression tester, a new female disconnect, a 5/16 hose barb. It will hold 15in HG with my finger over the plug end. I have to take my wife to town shortly so I won't get to take the carb and muffler off today.View attachment 313214
Cannon

That Mityvac will also make pressure but there is no gauge for pressure. Some vinyl hose, tee, and a 15 psi gauge from the auto parts will make it work. The pressure tap on that Mityvac is right in front of the vacuum gauge. There is a cap on it. Take the cap off and plug your hose onto the barb. But you will need the 0-15 psi gauge. The return spring in the Mityvac will make the 10 psi pressure.

Just get some rubber hose from the auto parts or clear vinyl hose from the hardware and hose barbs. You don't need your compression tester hose

 

[turtle561]

finally got a camera to the bench.
my version of pressure tester, quick and dirty.

 

[AKDoug]

It would be easier to find the actual leak with a pressure test.

Same procedure as the vacuum test but to use 7lb pressure instead.

The pass fail part of the test is that it holds at least 5 of the 7 lbs for 30 sec.

The advantage of the pressure test is that it is easier to find the leak with "Leak Lock" or soapy water or even hear the leak.

I like to test with vacuum first. My theory is that if you apply pressure first you might actually pop a seal back into a sealing position. When you apply vacuum it might seal up just fine, but fail once the engine is running. Leaking under vacuum is what draws in air and leans out your mixture, so I test for vacuum first.

 

[w8ye]

A couple things to consider is that if you use too much pressure or vacuum, you could ruin the seals. Do not use more than 1/2 atmosphere. (15" vac or 7 lbs press) (.5 kg/cm[SUP]2[/SUP])

Another thought is that all saws leak a little, some more than others. It is usually though the seals . . .

You are looking for the saw to not leak more than 1/4 of your vacuum or pressure in 30 seconds. This is the pass - fail test!

If you go somewhere else and drink a couple beers, smoke a cigarette, and talk to the XYL some, most saws will have little of the test pressure or vacuum left.

 

[w8ye]

I forgot to say here and this goes along with the post above mine is that if your chainsaw passes the vacuum test, there may not be a need for the pressure test?

Many people have the capability to perform the pressure test but not the vacuum test.

 

[angelo c]

That Mityvac will also make pressure but there is no gauge for pressure. Some vinyl hose, tee, and a 15 psi gauge from the auto parts will make it work. The pressure tap on that Mityvac is right in front of the vacuum gauge. There is a cap on it. Take the cap off and plug your hose onto the barb. But you will need the 0-15 psi gauge. The return spring in the Mityvac will make the 10 psi pressure.

Just get some rubber hose from the auto parts or clear vinyl hose from the hardware and hose barbs. You don't need your compression tester hose

My mity vac (8500 ?) has pressure and vacuum. Gauge goes both ways....+\ -

 

[redunshee]

I like to test with vacuum first. My theory is that if you apply pressure first you might actually pop a seal back into a sealing position. When you apply vacuum it might seal up just fine, but fail once the engine is running. Leaking under vacuum is what draws in air and leans out your mixture, so I test for vacuum first.

That's why when testing for vacuum I'll slowly pull recoil so vacuum drops. It should then return to the original number. This allows the test to be performed while the crank is turning.