I've got a few saws torn down that will need to be vac and/or pressure tested when I get them back together -- an 034, a 460, and the Husky 350 I recently posted about. I've only done a couple of vac tests, and it seems to me that acquiring a Mityvac pump is the least of the challenges involved. What I'm wondering is, what's the best way to connect the pump to the saw engine without a lot of specialized tooling? I got most of my chainsaw tools from a small country Stihl dealership that was closing up shop last year. That tool kit included a few cast aluminum adapters to be installed in place of the carburetor. Each adapter has a pin to plug the impulse hose, and a brass hose barb to which the vac or pressure tester can be connected. That sort of hardware makes it easy, but what do people do if they aren't so fortunate to have the official Stihl adapter for their saws? Is it feasible to just make a plate to block off the intake, and connect the tester to the impulse inlet? Is there some other way, like maybe a universal adapter that replaces the spark plug? The Husky 350 presents a slightly different problem because of the way the carburetor is mounted. Unlike the Stihls I've worked on, which all had carbs that slid off of two parallel studs revealing a nice, flat surface that can be sealed off with a plate, the Husky carb is difficult to separate from the intake boot and has no such flat sealing surface. There's also no impulse hose, just a passage through the intake boot, so I can't connect a tester there either. So, what clever ways have you seen for tackling this?
I was told to cut a piece of like inner tube rubber put behind the carb like a gasket and muffler and use an impulse port to hook to. I'm no expert either. Would like to see a video as well. Also a video of seal replacement in saws husky n stihl.
I bought a Mity-vac, but have yet to use it the 360 will be the 1st saw, and I am still thinking about sending it Randy. So..... I may just sell the pump.
For most saws, yes. For something like the 350, you could block the intake boot with a hardware store rubber stopper on the cheap.
I'm guessing that when it doesn't work, it has something to do with reed valves. But I don't know anything about reed valves, except that some older saws have them. Am I getting warmer? So, what is considered passable in a vac or pressure test? How much vacuum does an engine need to hold, for how long?
P ageing MM...he has answered that question for me before on the old site , but I dont remember the answer
Reed valves or no, it wouldn't matter. It's just finding a way to completely seal the engine. Should hold about 1/2 an atmosphere (.5 BAR, roughly 7-8 psi) either way, pressure or vac, for 30 seconds to be considered acceptable.
I've used gorilla tape to seal off the intake and exhaust port......simple and it works. Just make sure the surface is very clean so the tape sticks well.
You should always do both. Also rotate the flywheel some. You can also pressure test carbs in a beaker of water. If you download a husqvarna service manual from their website it covers the procedure nicely
There are certain times when one test will pass and the other may not. Example: the MS441 uses a two-piece fuel line with a plastic fitting that plugs into the bottom half of the line under the carb box. That fitting will not pass a pressure test usually but passes a vacuum test with flying colors. Since the fuel line is really never under pressure, that is acceptable. Carbs, fuel/impulse lines, you can usually just pressure test. Vac these items is usually pointless but there are exceptions. (See example above) When checking the integrity of the "long block" you should always do both tests.
I use an old suction machine. Similar to an air brush compressor. You just need a way to move air and a vac/pressure gauge -- a mity vac is simplest but this is what I had. I added a quick connect to each side so by swapping the yellow hose I can vac and pressure test.
Just for reference if you are working on Jonsered or Husky, the OEM part numbers for block-offs are available for order at the dealer or major parts websites. Watch your psi input for pressure testing, pretty easy to blowout a seal. I keep a bottle of dawn dish soap and water close at hand to spray around seals when under the pressure side. Use some some light machine oil to place around the seals when under vacuum, when the oil gets sucked in, bingo there is your vacuum leak.
I have a mightyvac at home and 2 at work,all aluminium,and all of them have a bad diaphram. I bought a new plastic one and am going to see how it holds up. The plastic one has no prvisions to hook up for pressure though.
I've been reading various chainsaw service manuals regarding pressure/vacuum testing. The pressure/vacuum should be 1/2 atmosphere which would be 7 lbs pressure or 15" vacuum on up to .8 of a atmosphere. Where they seem to disagree is in the test analysis itself. Apparently, no chainsaw crankcase is completely air tight? So the tests vary in the parameters of the percentage of leak down within a certain time frame. Here is a typical pressure test from the 372 service manual . . . 4. Pump up the pressure to 80 kPa (0.8 bar). 5. Wait 30 seconds. 6. The pressure should not be less than 60 kPa (0.6 bar). from the crankshaft seals. Here is the same statement in the Stihl 660 Service Manual . . . . : Pump air into the crankcase with rubber bulb until the gauge (arrow) indicates a pressure of 0.5 bar. If this pressure remains constant for at least 20 seconds, the crankcase is airtight. - - - - - - - - - - - - - Carburetor? There has been some different statements regarding checking a carburetor metering valve. Some people contend that putting more than 1/2 atmosphere pressure into the carburetor fuel chambers puts undue stress on the pump diaphragm and gaskets. That you should only put 1/2 atmosphere pressure in the carb to see if the metering valve diaphragm needle and seat holds and that the gasket holds around the pump cover. Other sources want you to see what the opening pressure against the metering valve seat actually is (Pop Off Pressure) and that at reduced pressure, the needle will re-seat itself and hold that pressure? - - - - - - - - - - - - Vacuum? Stihl 009 . . . . Oil seals tend to fail when they are subjected to a vacuum. During the piston's induction stroke the sealing lip lifts off the crankshaft owing to the lack of internal counter pressure. An additional test can be performed with the vacuum pump to detect this fault. The preparations for this test are the same as described for the pressure test. Connect suction hose of vacuum pump to nipple of test flange. Then pull out pump handle until the pressure gauge indicates a vacuum of 0.5 bar. When you release the pump handle the non-return valve automatically seals the suction hose. If the vacuum reading remains constant or drops no further than 0.3 bar, the oil seals are in good condition. However, if the vacuum continues to drop (down to 0 bar), the oil seals must be replaced even if no leaks were found in the previous pressure test.
I work with vacuum processes every day. There is no such thing as airtight. Especially if rubber seals and moving parts are involved.
I have found that the might-vacs themselves can slowly leak down. I put my thumb over the nozzle and test it on both pressure and vacuum before I proceed with a test. The hoses, block offs and anything in line with the pump can cause reason to can a perfectly good seal. I spray everything down with soapy water when under pressure to rule out my own doings.
