Poorman's guide to Vacuum Testing

[PogoInTheWoods]

I use a dish soap and water spray mix to look for bubbles. I have dunked a whole case in a tank of water before to identify a particularly mysterious and pesky leak. (Not an uncommon practice, btw.) With the carb removed, block everything off and apply low pressure before immersion to minimize water getting in anywhere. I found the leak right away, though a little water did manage to get into the oil tank through the oil passage (which I forgot to block). The saw happened to be a 55 Rancher with the notorious intake issues, which is indeed where the leak was. My fault, actually, as I managed to pinch the impulse tube putting the intake assembly into position on the cylinder. Fixed that, blew out the cylinder and case with compressed air afterward for good measure, and headed on down the road with another leak-free (and very clean!) saw.

 

[heimannm]

Post #99 - if the problem is a vacuum leak you may want to avoid dunking it...perhaps a drop of oil on the seals one at a time and see if that does it, at least then you will know where to investigate.

Mark

 

[Moparmyway]

Post #99 - if the problem is a vacuum leak you may want to avoid dunking it...perhaps a drop of oil on the seals one at a time and see if that does it, at least then you will know where to investigate.

Mark

I agree 100% and have never dunked anything. Oil will get sucked in to a leak and is a great way to find a pesky leak under vacuum. If the oil disappears or the vacuum holds, you have found your leak.

 

[PogoInTheWoods]

Not trying to be difficult or contentious here, but...

I agree with the oil (or grease) approach for a vac leak as well -- depending on the convenience/practicality of applying the method to a given suspected area. Seals are obviously easy as are most cylinder bases and case gasket areas. Intakes on some saws can sometimes be a bit more difficult to navigate due to visual restriction with even the soapy water method for a pressure test, let alone the oil/grease method for a vac test. Realizing that a one-way leak is a bit more tricky as it actually represents a 'valve' in its function, on a saw, a vac leak will usually translate into a pressure leak under the right circumstances with enough pressure -- which doesn't necessarily mean crank up the compressor until the seals pop. Rotate the crank, flex the intake assembly, twist the handle/tank against the case mounts, etc. You'll usually find a pressure leak if there's a vac leak and usually with the soapy water method.

Also make sure your vac and pressure levels are appropriate and not way into overkill land....., and ensure that any home brew testing rig itself isn't the problem and providing erroneous results. Personally, I use a MityVac 8500. Paid around 50 bux for it. Does vac or pressure with the flip of a knob.

I'm just saying if all else fails for me, I'll dunk a case any day to find a leak fully realizing the risks and considerations and not look back. The key is already having pressure in the case when doing so and ensuring the integrity of the case and cylinder afterward relative to moisture/water infiltration. It's a last resort method for sure, but one that is effective if carefully implemented and been successful for me. Mileage may vary for others.

But in this case, I'd be suspecting the decomp anyway since it is indeed a valve by design and enough pressure would tend to 'close' it and enough vac would tend to 'open' it....or dislocate enough carbon or other small debris in either direction in the valve to simulate same.

Good luck 'tnt'. Be interested to learn what you end up doing and what you discover.

Just tryin' to help.

 

[heimannm]

vacuum leaks suck

Mark

 

[PogoInTheWoods]

And pressure leaks blow.....

And all too often we overlook the simplest and most obvious causes and solutions.

 

[tntblaster14]

Yea i'm a little bit slow on the repairs right now, been spending pretty much everyday outside w/ the cattle, or equipment has been breaking down in this wicked cold weather (spent 6 hours outside straight bare handed today fixing tractor).. or w/e is busted in this cold weeather. It never goes down in nice 40F days no... on the 4F days haha...

so the chainsaw repairs are like.. back burner right now since I have mild tingling in most of my fingers all night right now lol. I picked up a 1991 and 1999 346xp really cheap and am practicing taking them apart.. slowly ! One has a scored piston so obviously an air leak, and I have the Mityvac 8500.. and it is pretty sweet device. Can even bleed brake lines easily

 

[JWG]

Well, I have learned a thing or two from this site, so I figured I would give something back. We all talk about vacuum testing crankcases to determine if a saw has an air leak. Seems as if the "how do I vacuum test my saw" question gets asked quite often here, so I put together a step-by-step pictorial guide.

The first thing you need is a vacuum bleeder. Since we are doing this on the cheap, you probably don't want to fork out the money for a Mity-Vac, so this model from Harbor Freight works just fine, and is currently on sale for $17, normally $25.

Now, you will need to determine how you are going to hook the vacuum bleeder up to your saw. I made this handy adapter to hook into my impulse hose on the saw I am working on. Cost me $2.50 - $1.25 for each nipple, and I had the connector laying around in my box of parts.

Okay, now you need to block the intake and exhaust ports. This step requires some disassembly of the saw, but I am assuming that if you are vacuum testing your crankcase, you feel comfortable taking your saw apart. I used some rubber from a rim strip - covers the spoke nipples on the inside of a dirtbike wheel - you can also use an old bicycle inner-tube, gasket paper, etc. Cut the pieces to length, use your exhaust and intake gaskets as a template to transfer the bolt holes to the rubber, and use a punch or leather working tool (like I used) to punch holes in the rubber.

Now, this piece of rubber will be insterted, as a gasket would, on your intake/exhaust ports. The only difference is the rubber covers the ports. Now, install your muffler and intake boot and sandwich the rubber gasket between the ports and the muffler/intake boot. You can see the rubber gaskets I made, installed as I described, in the pictures below. If you have a spigot mount carb, you can use a piece of innertube and a hose clamp to seal the intake.

Now, make sure your spark plug is installed and hook up your vacuum bleeder to the impulse hose.

Looks like you are ready to go to work. Pump the vacuum bleeder until you reach your desired vacuum.

Go have a beer then come back in 5 minutes or so, and check to see if your vacuum gauge is reading the same as when you left it...hopefully it is. Now, rotate the crank slowly by hand. Do this 5 times or so...is your vacuum still holding? If it is, GREAT! Now, port that big bore kit you bought, get it installed, file your chain and go make some chips!

Thank you and good job. Nice clear pics, easy instructions, and done on a poor man's budget . I've added the link to my favorites and will be referring back to it to do my 044 once I get all the parts and get it back together.

Kevin

 

[president]

I spray the suspected leak areas with windex and look for bubbles.

you must have watched that great big fat greek wedding movie
and dad with his windex fetish

 

[gomoto69]

here's everything i used for my first vac/pressure test performed recently on a jred 2171. My vac pump was leaky, so i tee'd in the mac vac/pressure guage, and would pinch off the line to the vac pump after pumping it up to avoid losing vacuum thru the pump. I already had an air line brazed to a spark plug bidy i had used years ago ti replace valvesprings on a 340 mopar i have without removing the heads. Used innertube to block carb, and innertube plus a blockoff plate for exhaust. First prrssure test i couldn't find a leak, but i think i was expecting a much bigger, louder sound of escaping air. Would only hol vac with crankshaft in one position, bled off quickly when turned. Only then did i remove clutch and flywheel, and next pressure test showed obvious leakage (bubbles) around both seals. Replaced seals, good to go now!

 

[PeterM]

I can not see the images, but the only one I need to see the "handy adapter to hook into my impulse hose"
I see other images on the website, but these all have generic icon .
Anyway, I need to check the pressure and vacuum on my saw as I have replaced piston and still can not start the saw. On the first pull, it sounds like it wants to start, but then nothing. I could pull the cord until I am blue in the face.
Thanks!

 

[Four Paws]

All of my original images were lost. The post above yours shows all that you need.

Figure out a way to hook into your impulse hose and connect to your vacuum pump. I use a brass barbed fitting.

 

[PeterM]

Thanks for the reply, I hope to stop at hf on the way home tonight.
If you could explain what the impulse hose actually does, I would be very grateful. They call it a suction hose and valve. I noticed it when I was replacing the piston and cylinder, and could not find any information on it, nor could technical support be any help. I really appreciate your feedback and this website is great!

 

[Four Paws]

What saw?

Impulse hose is open to the crankcase and transmits a positive or negative pressure to the carburetor to pump fuel.

 

[PeterM]

it is a Blue Max, 18". The suction hose itself (it is black) appears intact, there is a asbestos sleeve around it that has been singed in a couple of spots. This hose connects to a brass valve one end and on the other end, to the carburetor. I did try to see if it were clogged, and I blew into it, an also was able to draw air by sucking.
I called Blue Max earlier this week, and they eventually said I should replace the valve, even though the engineer at NATI Tools there wasn't any way to test it.
What do you think?
I also put my thumb on the end of the hose and gave the saw a couple of pulls and could feel nothing, so then I checked here to see if I could learn a way to fix it.
thanks for your help!

 

[xPosTech]

Where will i find the four paws vac test pdf? Thanks

I too would like to find that PDF. It should have all of the images of the OP.

This was one post I was happy to see brought back from the past.

Thanks Four Paws for a great how to.

Ted

 

[JonCraig]

Sorry to bump this, but any chance it could be updated with pics? Or does anyone have this pdf with the original pics?

I've never been satisfied with my vac/pressure setup and would like to see how it's done properly. Thank you!

--JC

 

[PogoInTheWoods]

MityVac 8500 is the way to go. Various block-off plates and setups are up to you for your particular application, but a spark plug adapter for testing from the top down is very handy to have as an alternate approach in any number of situations, especially if you only want to check your case and seals. Gorilla Tape at the intake and exhaust on clean surfaces provides more than an adequate seal for both vac and pressure tests of a case.

 

[JonCraig]

Guerilla tape! Genius! Many thanks!

I have a mighty vac 8000 (the one that only does vacuum for brakes). Might have to upgrade to make the pressure test easy.

When using the spark plug adaptor, the piston would need to be at the bottom of its stroke, right? That way the transfer ports are exposed/open?

 

[briantutt]

I used Gorilla tape last time I blocked an intake. Works good.

Brian