Wednesday, November 21, 2018

Workshop Wednesday-Pressure Testing

 One of the tools that not all two cycle engine mechanics have in their box is a pressure/vacuum tester. I don't know why because it is vital to diagnosing problems with these little machines. Because the incoming fuel and air charge passes through the crankcase of a two stroke, that crankcase must be sealed and not have any air leaks. These leaks are checked under pressure and vacuum with a tool like this one made by Mityvac.

 To test, we pump up 7 pounds of pressure and see if it holds without losing more than one pound in a minute. A well sealed engine will still be holding that pressure after returning from a lunch break. It will also hold 10 inHg or .33 bar of vacuum. You may think that if it passes one test it will automatically pass the other but that's not always the case. The design of crankshaft seals, with a lip on one side of them, means pressure from the inside of the case with just make them seal tighter and pulling vacuum from inside will allow the lip to give way and let air pass. This was the case the other day when I was working on a saw that was all jacked up with an engine failure in the form of a scored cylinder and piston.
 The first step is to seal all the external holes and ports. The muffler is removed and blocked off with a plate, or a piece of rubber sandwiched between the muffler and cylinder.

 The carb is removed and the intake is blocked off, in this case with special tools.

 The spark plug is removed and our adapter is screwed in place. The mityvac is then connected to the adapter.

 Yes its a lot of dickin around to do these tests but its the only way to do it right. If you're working for a dealer then it has to be done this way for filing warranty claims. It adds a lot of labor time to the repair so it usually only gets done when the repair can't be remedied by other means. This saw came in with a complaint that it wouldn't start. An engine needs fuel, compression and timed ignition so I always look at fuel quality first. Its easy to see if there is fresh mixed fuel without water in the tank. Next I pull the spark plug and take a looksee at the cylinder for signs of scoring. In this case the cylinder and piston were pretty bad.

  This engine wouldn't start because of low compression. The ring was stuck due to metal transfer between the piston and cylinder, causing that low compression. We determined the fuel supply was fresh and had oil in it and we knew the saw was still under warranty so the next step was a pressure test which it passed. When I tried the vacuum test it failed. As mentioned above that points to the crank seals. Pressure tests are checked with soapy water. If there is a leak then you'll see bubbles.

 To confirm a crank seal leak I like to use two cycle oil. I turn the saw on its side and coat the seal with oil before applying vacuum. You'll see the oil disappear as the vacuum gets stronger...

 The problem has been discovered and the repair can begin. Oh I know this is a big yawn fest to most of you but I enjoy it. Teams of engineers designed these engines to run decent and there's no reason an old one can't be repaired to run like new. It's like a puzzle and you just need to figure out which pieces are out of whack. I like puzzles.



  1. As the owner of a few small engines, some of which run well and some of which don't, this wasn't a yawn fest at all. But the amount of work involved in diagnosing and then repairing these small engines explains why you see so many in the dump. It's sometimes easier, and cheaper, just to buy the next made-in-China piece of crap than go through all that.

    But I too like puzzles - and taking things apart(!) - so will invest whatever it takes to get something running again.

  2. We recycle a lot of cheap equipment. Rebuilding them is fine if its on your own time but shop labor rates make most of them cheaper to replace with new. Junk pro saws are saved indoors. High end consumer stuff is stored outside. All else goes in the scrap dumpster.We sell a lot of used parts for those repairs that would otherwise not be worth the cost. More puzzles.