I am having the exact same issue OZ had many moons ago.viewtopic.php?f=82&t=121396
During the restoration process, we mostly ran the M-20 in the shop, pulling it out every so often, but not really running it much. Last week we got it out of the shop and took it for a trip up and down about 1/2 mile of our dirt lane a few times to make sure the brakes, clutch, transfercase etc works.
IT was then we noticed the M-20 would start and hold at 25-30 when we first started it up, but once it hit about 150 degrees it would creep down to 12-15. When we got it back into the shop, we found it sat dangerously close to 0 at idle. We changed the oil this past Sunday after another issue ( viewtopic.php?f=82&t=206060
). While we had the oil pan off at this time, I read up on how to adjust the oil pressure and compared it to the exposed oil pump to make sure everything moved, functioned etc.
Today I installed a mechanical oil pressure gauge to the block, warmed it up then got her up and running to 1600 rpm (using a remote tach). It ran for about 10 minutes at 26 psi, then slowly started dropping off. After about 20 minutes of consistantly running at 1600 rpm it dropped down to a little less than 12. I then had my wife watch the pressure as I lowered the rpms down to idle. It crept down dangerously close to 0. I immediately shut it off so we do not inflict any damage on it.
We are running 5 1/2 quarts of SAE30 along with 1/2 qt of Marvels.
Temperature stays steady at 150 degrees.
Before we did the initial start up months ago, we turned it many times over by hand, then used the cranking motor with no juice to the coil until we had pressure built up.
Any ideas or suggestions? We do have another oil pump we could install, and have some JXD specific manuals enroute through the mail.
Thanks in advance,