Right -No 1 - Thanks Matt for the good rep - your 372 is coming along nicely - found some interesting stuff wrong with it.......
Pressure checking, firstly absolutely NOTHING to do with compression or the performance of the top end of the engine.
A two stroke engine pulls fuel oil mix from the carb, into the crankcase and then pushes it up through transfer ports in to the compustion chamber, this action also expells the exhaust and then once the piston clears the exhaust port, we are all ready for a good bang and lots of smoke.
The air is controlled by the carb and the integrity or the seal of the engine is critical to its correct running, if the crankcase seals, impulse line or carb boot are leaking, you will pull air in to the engine in varying and unwanted amounts and this then can cause a very weak mixture. If you are lucky, the dodgy idle will warn you about the problem, but if you just adjust it out and carry on then the next thing that happens is the exhaust side of the piston melts on to the cylinder wall - a full or partial engine seizure.
So - if an engine has seized, new parts are fitted and a pressure check ensures that it wont happen again as long as this is followed by the correct adjustment of the "H" carb screw - tach tune or tune by ear!
The early signs of air leaks are poor idle, an idle that has increased unexpectedly - like when your saw runs out of fuel - that increase in speed just before it dies - you may also get the saw hitting higher revs and sounding like a demented wasp you could also get that flat spot between 3,000 - 6,000 revs - like the 345, 350, 346, 357 Huskys when they are fully cold but it will not clear after 30 seconds running - thet wohh wohh sound in the midrange!
So - a pressure check = an important bit of the saws health and not engineering sphericals - this single check has saved the 066 Magnum and 048 on my bench further damage after repair.