possible compression problem
 

as the title says i think i have some sort of weird compression problem. the girlfriend got a hpi rush evo with t-15 engine. it ran ok for the frst tank or two but then when i tried to tune it a bit everything went to pot. all i done was turned the hsn down a little bit then the car runs ok for a bit then revs like mad like its going to blow up. tried tuning from factory ect but no avail until. my gf uncle came along and fiddled about with it for a while, then decided to see how it will run with the lid of the fuel tank open.. suprise suprise it runs perfect. so in order to run it i take the hose from the exhaust to the top of the tank of and it runs like a dream. i've changed the tanks over and i have no idea what else it can be. anyone have a idea??????????????????????????????

First, let start with the compression threory: pull the plug, put 2 drops of after-run oil in and turn over by hand. If it is tight at top dead center and you can turn over to BDC then your pinch is OK, if it turns over with no resistance than the pinch is loose and will cause tuning problems. Next..put the plug back in and turn over, It should be a little tighter than with out plug..If so then the compression should be ok. Now...If you turn the HSN out or counter clockwise that will richen it...and vise-versa. Ive also heard from some people that say if it will only run with the tank open then it is way to rich. Im not fimiliar with that rig so i dont know default settings so check the MANUAL....good luck