RP --

My POS rustbucket bomber '82 245 (347k miles, died from rust and an injection failure) maintained about 1-1.5" of water at idle if I maintained the flame trap and associated plumbing.

Check for vacuum at the small hose (about 3/8" OD) connecting to the flame trap from the intale manifold. Pull the hose off the flame trap and feel it with your finger at idle -- it should suck. A common problem is plugging inside the brass nipple.

Next, remove the top half of the flame trap containing the actual flame arrester -- either plastic (new) or brass (old style). Start the engine and let it idle. Watch the remaining hose, which sits atop the oil separator, for vapors fumes "puffing" up from the crankcase. If you see 'em, the oil separator's not plugged. If no vapors rise, then the separator's probably blocked. You must clean it or replace it.

I found exactly this problem on my '86 when I bought it.

I measure the crankcase pressure using a modified oil cap and a sensitive gauge called a "Magnehelic" (made by Dwyer). The pictures below show how I modified the cap and how the guage measures pressure (on my '86).

If you're running with positive crankcase pressure you should expect the three front seals and the rear main seal to seep. Also, over time, the pressure may force out the big plug in the back of the head (happened to me). This can happen even after you've fixed the ventilation system.

Consider a retaining strap across the plug.