I have an Echo SRM-2100 in the shop, and it's running strangely. The fuel tank was empty, except for a little "syrup" at the bottom, so I poured out what little I could and put fresh fuel in it. At first, I thought it was just a mis-adjusted carburetor. The low speed screw was out way too much, and the high speed screw was in too far. So I started over with both of them about 1-3/4 turns out, and I got it adjusted to where it was idling smoothly, responding well to the throttle, and running smooth at the high end. I thought it was OK, but a few minutes later I picked it up again and started it, and it idled fast, then slow, then fast, then slow, then fast, and then died. I started it again and ran it for a couple seconds at high speed, but when I released the throttle it died again. I tried playing with the carburetor adjusting screws again, but it wouldn't improve.
So then I dragged out the training materials I got from my 2-stroke engine class at the GIE+ EXPO last fall and started doing some testing. I haven't gotten to do this stuff often enough to get good at it. I spend most of my time on v-twins these days. So the first thing I do is check the compression. I got 120 lbs., which I'm assuming is good enough (I wish it were just a little higher). The next thing I do is test the fuel inlet. I pressured up the fuel line and it held 10 PSI. Good. So then I blocked off the intake and exhaust ports and gave the crankcase a pressure test. It held 7 PSI for a minute. Good. Then I pulled 14 PSI on the crankcase to see if that would hold for a minute. It wouldn't hold 14 PSI, but it did hold once it got down to 12. Did I just find my problem? Leaking crank seals? I know technically it failed the test, but I would've thought that bad crank seals wouldn't hold 12 PSI vacuum. It's awfully close, and I'm afraid to tell the customer definitively that he has leaking crank seals.
What do you think?