Poorman's guide to Vacuum Testing

Four Paws

Four Paws

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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.

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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.

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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.

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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.

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

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

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Go have a beer :cheers: 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!
 

KMB

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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
 
kd460

kd460

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Yes, very nice work and very nice pics.

This one will be saved in my "chainsaw" files.

Very timely as I am working on an 028 project saw, this will be a big help, thanks for taking the time to do this. KD
 
Lakeside53

Lakeside53

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I know some of you are thinking "why rotate the crank 5 times..."


Many seals can be fine in one crank postion, but not another. I rotate them left and right a few times, and sometimes gently poke the seal when under vac. Don't exceed 0.4-0.6 bar though... you may get fake results.
 
Four Paws

Four Paws

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Thanks for the compliments guys! Just trying to help out.

Andy, thanks for mentioning why I rotated the crank - I guess I took the reason for doing that for granted. For those that aren't good with conversions, .4 -.6 Bar = 11.8 -17.7 in-Hg.
 
Four Paws

Four Paws

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Hey Four Paws,

Did you pressure check as well???

Not this particular saw. It was just something I had apart on the bench and figured I would do a quick "how-to" write-up. The saw ran sweet, and there was no reason to suspect an air leak. Pressure testing is kind of like a security blanket, as 9 times out of 10, the saw will leak under vacuum before it leaks under pressure. "Poorman's guide to pressure testing" - Sounds like a good idea for a future write-up, LOL.
 
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