I have a carbed 502 raw water cooled, 140 F tstat, lake is 73 F,
Temp doesn't get over 100, running rich and sooty.
I have merc 330 exhaust manifolds and 330 tstat housing, so there are water hoses to the manifolds, and to the elbows. The 502 tstat housing just has hoses to the manifolds....Should I run the 502 housing, and plug the elbow inlets?

Have you verified the actual temps and that the T-stat is not stuck? Does it require the plastic checkballs in the t-stat housing assy? I would not plug anything off, you have a gremlin elsewhere. Both elbow and manifold need a water supply of some sort depending upon what type you have hoses may be different. Google Mercruiser water flow diagram and match your setup to correct diagram and verify everything is routed properly. Perhaps the parts diagrams at Bam marine may help.

-top of t stat housing stays cold to the touch,
-T stat was put in water beside a new one, both opened at 140...using new one because it does not have holes in it.
-I've found a few flow drawings, whati've found is that my setup looks like a combination of other setups, which is probably the problem. I'm using the 330 thermostat housing on the 502....and have it aligned as per the 330, the gasket with the block off part is the at top of the housing, tstat under the sleeve as per parts drawings...

My concern is that set up like this the bottom of the exhaust manifold only gets coolant after the tstat opens, the gasket and the sleeve block off flow until that happens, the elbows get water right away from the housing with the check balls.
There is a sleeve (not like mine, mine is solid) with slots in it that allow water to the manifolds with tstat closed.

I'm thinking I should just get the proper 502 housing...If I ran the proper 502 thermostat housing, there is flow to the bottoms of the manifolds and through manifolds to elbows, from the mixing chamber, and when the t stat opens there is more flow,

Well it sounds like you're smarter than the average bear and willing to be persistent! Perhaps compare your intake manifold part number and match up the thermostat housing with same. Without seeing what you are doing and parts on hand that seems it would be a sure cure? Use the part number search at BAM and find out what exploded diagram you need to look at.

Thats an idea, I don't know what the intake is from, just that it's aluminum...and has the hose from the water pump at the front...I'll run the number and see what tstat housing works with it...I thought that's how I'd find the right sleeve for it too...

lol...not persistant....just at wits end!

Post a pic of what you have. Either T stat housing should work fine.

BTW, colder engines run leaner, not richer, unless possibly you have a choke on your carb that is not opening fully.

intake is 6269318, when i ran that it came up as 454 magnum/502...the matching tstat housing is the 4 hose 502 style, not the 6 hose style i'm using...i'll get you pics...

Which if either is your setup?
mercruiser water flow diagram
different engine etc but some exhaust have 2 hoses, one to elbow and one to manifold and others only a hose on the manifold. Risers, gaskets, elbows etc. may differ.

That intake is the same as any other GM 454/502 intake, just aluminum. That is the one that came on my 502 crate Gen VI crate motor and the T-stat housing should/could look like this-
https://www.mercruiserparts.com/bam/...1804/2044/120f

and looks to be same design as this one
https://www.mercruiserparts.com/bam/...31798/2690/140
and this looks like what you are using presently?
https://www.mercruiserparts.com/sear...true&orderBy=0

OK, so know we know what parts are what but I don't know the functional differences. I am familiar with the operation of the first two I linked, not the one you are using. There are few of the 8M0061381 here and there on e-bay and amazon has a china knockoff in stainless but probably crap.

I think you have to match the T-stat housing design and operation to the type of exhaust manifold you have, two hose 4 hose?
https://www.mercruiserparts.com/bam/...31953/5725/110
vs.
https://www.mercruiserparts.com/bam/...31796/1614/190
Never really thought about it that much but did not have mix-and-match parts.

-Yes this is similar to what I'm using, the '4 hose" which was stock on the 330 454 '86-92, https://www.mercruiserparts.com/sear...true&orderBy=0,
The only difference is my top gasket does not have a hole in it. with the solid sleeve in place, and no hole, the only way the exhaust manifolds get cooling water is when the tstat opens. The old 330 had signs of overheating the heads, and I think this might have something to do with it.
-On the 454 magnum, and 502, the manifold is the two hose, which has cooling to the exhaust manifolds right away, but no direct cooling to the elbows, the elbows get water from below from the manifolds. when the tstat opens it just allows hot engine water to go to the exhaust manifolds...in addition to the water already coming from the cold side of tstat.
-the only difference from the 4 to 2 hose elbows is the 2 hose have plugs not inlets, so they get all their water from the exhaust...

It looks like the 502 two hose style was more common, used on small blocks and big blocks...

I'm thinking the 502 style 2 hose is more stable, with the manifolds getting water right away, for the price of a tsat housing the hoses and two plugs, i've the 4 hose...might just try it and use a temp gun to check temps...maybe with this style my temps will run hotter, I still can't figure out why the 4 hose is running so cold...

Do you have experience with the 4 hose? I have mine as per the parts drawing, and it's just too cold...