When I start my "96" 850 turbo, the engine cranks and sometimes will start right off, but, sometimes it cranks for almost a minute then it almost feels like there is too much compression and then like no compression at all, but doesn't start.
It then repeats the same feeling (but has short periods of what seems like air leaking and then stops, as if you quickly let air out of a balloon, but no engine crank) then after 4 or 5 tries it will again crank and may or may not start. This is a real strange one. I also just replaced the plugs a couple of months ago and it ran fine for two months. Any ideas greatly appreciated.

I may not be understanding your description, but it sounds similar to the time when my starter was dieing. It would crank with difficulty, but then stop and you could hear the pressure in the cylinders wheezing out.

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'96 850T and '83 244 DL both at about 200K, '97 850T5; gone but not forgotten '81 245 and '64 Amazon

That 'leak' is a vacuum line leak. Your no compression sounds like the cylinder walls are being washed with gasoline, enough so that the rings no longer have oil on them.

Klaus
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Always willing to listen, just not able to take direction.