Open the decomp lever, turn it over a couple of times. Then same, with the starter. Then close the decomp.
Now grab the wheel
and try to turn it over.
You cannot, good compression.
You can, bad compression.
MD7 will start way beyond compression is BAD. With bad comp, it will smoke: a lot when cold, less when hot. Even when hot, it will smear the cooling water
They have no jackets. Fixing compression problem can be lengthy and may cost $$$.
PS If you have bad ring(s) you may find it next to impossible to buy new ones. If you can, they will be outrageously expensive. TIP they are said to be Nissan
Do not ask me how I know ;-)