blow by cured.

[michael3006]

I believe there was something preventing a valve from closing fully.
I tow started the 720 again to get it to the mechanic's for a compression test.
Ran it for a while then opened the oil fill cap and there was lots less air pressure in the valve/rocker cover.
Had the compression test done and all 4 cylinders are 400psi. Not bad for a 1985 model.
Turns out that when I ordered the new glow plugs recently I was given ones for the SD23 in a Nissan Urvan not the 720.
Different voltage they were not heating up enough.
And one had failed already missing the very tip.
It now starts and runs perfectly.

[plenzen]

I suspect that the missing glow plug tip may have got itself lodged under one of the valves and caused your blow-by issue

Hopefully it was the exhaust valve that it was lodged under and has gone out the tail pipe now.

You have a 50/50 chance that it was

P

[dn29626]

Glad it is resolved.