Chainsaw Repair

macbobaust.com

macbobaust.com

  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!

I have the Mityvac 8500 which is pressure and vac. But they are around $67.

http://www.ebay.com/itm/Mityvac-8500-Silverline-Elite-Hand-Vacuum-PumpTest-Kit-MV8500-Mity-Vac-/231018477698?pt=Motors_Automotive_Tools&hash=item35c9c63882&vxp=mtr

I also have a Mityvac II which is plastic and like the one at Harbor Freight.

http://www.harborfreight.com/mityvac-vacuum-pump-39522.html $36.99

Study the pictures very carefully so you can see the differences in the gauge of the combination models with the little twist valve on the left side where you change it from vacuum to pressure. The cheap vacuum only one doesn't have a change-over valve and the gauge only reads vacuum. But there is a pressure port that sticks up out the top where you can plug a pressure line onto the vacuum only pump but you will need a gauge for it.

Most of these are intended for brake bleeding and the vacuum sensors on a automobile engine. You don't need all this for a chain saw. You may get by cheaper by buying just the pump itself.

Most people proclaim the HF one only does vacuum. I have played around with it and it'll do up to about 15 lbs pressure easy enough but you can not read the pressure. You will need a "TEE" fitting and a pressure gauge to make it work. The pressure gauge may set you back $15 at the hard ware? The tee fitting can be found at the auto parts but might be at the hardware too. Extra clear plastic line can be had at the hardware and they have black rubber hose at the auto parts.

When checking POP-OFF on carburetors don't bother with the actual pop-off pressure, just run 10 lbs in the fuel inlet and see if it holds for 10 seconds. Putting 30 lbs pressure in a carb inlet only serves to ruin the pump diaphragm.

On the crankcase you are using 1/2 atmosphere pressure and vacuum. That is +/- 50 kPa or 15 in vacuum or 7 lbs pressure. It is another 10 second test.

On the crankcase, the vacuum test is a pass-fail but with the pressure, you can soap the seals and other areas for leak location.