LM318 HELP
 

A 318 engine in a boat.
I just rebuilt the eng. Stock except bored .030 OS and a mopar P4452759 cam.
After installing engine in boat, needed about 12 degrees ignition advance due to valve overlap with the new cam to attain a 800-900 idle.
Vacuum reading at idle steady at 17"hg. with the tick due to the valve overlap.
Engine ran fine during the cam break in period.
After about 17 hours of operation, I had surging at idle, would vary between 600-1000 RPM and stall. started right up and idle surging.
Unable to load engine.
Vacuum reading at idle now fluctuating. 10-15"hg
Closing choke plate restores engine idle to 17"hg and smooths idle.
(hot engine).
I was thinking intake manifold leak and tried carb cleaner on the parting surfaces / no help.
Replaced edelbrock 1409 carb with rebuilt carter AFB and no difference.
Total Ignition system is new. not a fuel quality or delivery problem as pump and filters are new. No water in oil.
I keep thiking intake manifold leak. Is it possible to have a vacuum leak on the wet side of the manifold?
any thoughts would be appreciated.
Joe

yes a crack may have developed in the manifold [does the oil smell like gas]. Another option might be a clogged passage in the idle system of the carb [air bleed or idle tube]. How is the vacuum at higher rpm's, did it change too?

Just reaching, but with all that we have read on the subject of zinc no longer in oil, what would be the symptoms of a wiped out cam lobe? It is a new cam, and did run well for a while. Feel like doing a compression or better yet, a leakdown test? Got a laser I.R. thermometer to check the exhaust temps, see if one cyl is out of whack? I'm broke now and cant afford gaskets, let alone swapping parts, so I will try any test to keep from opening it up. Keep us posted.

Lm318help
 

Thanks for replys,
Some additional info:
idle mixture screws needed four or more turns out (rich) since rebuild.
vacuum reading at 2000 rpm is steady 19" hg.
Joe