If you take the oil cap off and put the palm of your hand firmly over the hole, can you feel it pumping against your hand? This is the diesel blow by test, you can also just sit the loose cap in the hole and rev it a bit to see if the cap shuffles about but your hand gives you a better idea what is happening. If the carb to teardrop pipe is the only one you have, wash it and the PCV out. Valve should "rattle" when you shake it. If the oil filler cap is a "nice" chrome one, it might be from a B21/23 which, I've read, isn't vented. The normal one can get blocked but comes apart easily for cleaning.