SBC, new impeller, new manifolds. Runs at normal operating temperature until you lay down the throttle and then it instantly starts heating up. It's got a 650 demon on it with a 72 primary jet and an 80 in the secondary, I guess it's possible it's to lean but I haven't been able to pull a plug to check yet. It runs wot at about 5200 rpm. Any help would be awesome because I'm getting ready to pull it back out this week to fix an oil leak and would like to address this issue as well while it is out.

Side note, I did use the original circulating pump from the 87 model engine when I built this engine, could this be an issue? New engine is vortec

not sure if it helps, but it may need a reverse rotating circulating pump, most late model w serpentine belt run reverse pumps

Do you have a thermastat in it

The block actually came from a 97 model truck if but everything else was the original 87 merc stuff, if that makes a difference. Still using vbelts, no serpentine.

Yes it has a tstat, I was thinking about pulling it out to see if that helps but haven't yet.

ok, if your running everything off the old engine w v belt then that eliminates that, i'm assuming you built more horsepower which will build more heat, how'd the heads look? sounds like there's a slight restriction somewhere that only shows up on high rpms

I dont run a stat in my bbc and had no problems might take a few more minutes to warm up but never had condensation in the oil, but engine is not stock

Heads came from the same truck and showed no cracks or warpage. They were professionally cleaned and checked by a machine shop as well as a 3 angle valve job. I did add horsepower as well. Using stock manifolds and risers. Could they just not be flowing enough gas fast enough to cool it down? But then I know of other boats with stock manifolds and engines that turn 5000 rpm with no trouble what so ever

My compression is right at 10:1. It takes less than a minute for this thing to start heating and about the same time to cool it back down once you back off the throttle. Sorry for the random assortment of posts. I'm just trying to remember the details as they come to mind

still sounds like theres some restriction to me somewhere, you mentioned new sea pump and manifolds, how about the risers? they tend to get "gunked" up with deposits over the years causing restrictions in flow