912 backfiring/compression loss

[Donald T]

Hod my 912 motor rebuilt in 2015 and have had no problems until recently. It spends most of its time in storage with only summer/fall usage. Took it out a couple of weeks ago and got backfiring when lifting the throttle. Did a compression test and cylinders on one side were 150 lb and half that on the other side. Any ideas on the culprit? Looking for good mechanic in NH or New England. Know of anyone?

[WFBowen]

Donald
For starters, you might retorque your case studs to see if anything came loose or even broke. If you can get a borescope into and between those cylinders you can look for signs of a cracked or warped head or signs of compression leaks between the cylinders.
Bill

[callard]

Backfiring is usually an exhaust leak; bad muffler or pipes. Lower compression on one side could be from between the cylinder and head. Best troubleshooting method is a leakdown test. Otherwise, let it idle and lay down beside the engine and listen for an exhaust noise in that area. Compare noise on both sides, if louder on the low compression side, you probably have a head leak. Broken head stud on a 912 or 356 engine is EXTREMELY rare.