SBC getting hot at high rpm
 

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

Risers aren't new but they had very little crud in them if any at all. It seems to flow plenty of water. It's just got me baffled. Is it possible the circulating pump is failing or weak and can't keep up with higher rpm?

its possible, though usually they leak out the weep hole on top when they go

It's an alpha drive and it's been run on the hose a good bit. Would the impeller even tho it's new cause this kind of problem or would a bad impeller be obvious at other rpm as well

Will it heat up if you're running on plane around 3000-3500 or only wot.... You could try to change out the hoses with clear ones to check water flow

It's fine at cruise speed around 3500 rpm but as soon as throttle is opened it starts to heat up with in 5 to 10 seconds and after about 45 seconds at wot I have to throttle back and it cools right back down

Are you sure the gauge is accurate

If you're sure that the impeller and water pump housing are in good shape, you might backflush and clean out everything carefully - amazing what some old pieces of a prevous impeller can do if lodged say in the neck on the transom assembly... restrict enough water that when you turn up the heat it can't keep it cool...

In addition, alpha water pumps are a little touchy. If the water pump housing has grooves in it, etc... it can significantly reduce pump flow.. so can overtightening the nuts holding the water pump on... or a kink in the line fromt eh drive to the transom assembly.. I know it's a pita from experience. Hope you figure it out soon!

When we took the old impeller out it was intact but was just brittle and not keeping cool at idle last year but was fine at wot, but that was the old engine that only turned 4100-4200 rpm, maybe the housing is worn out but didn't show itself until it got something with some ass behind it. I will recheck all of it again when we pull the drive back off

99% sure, but I would be willing to check it, is there a certain test for the gauge other than replacement?

Ok I just had a thought and I want to see what you guys think, when we first started the motor we set the float levels down a good bit because it was trying to flood when we were priming the fuel pump, now I'm wondering if maybe it's a fuel problem but not because the jet but because it's sucking the rear bowl dry after being opened up. It's a standard bore 350 with a 650 demon carb. At idle the fuel level in the sight glasses is between 1/4 and 1/2. I'm thinking that maybe in fact it's running the bowl out of fuel and just misting enough to keep it running but not cool.

hmmm... if it'll suck the bowl dry when you open it up now, it'll still do it when the level is a little higher, just take a little longer, eh?

are you getting a lean surge when running at higher rpms? what do the plugs look like? surely if it were leaning out, you'de know it?

I'm gonna raise the floats some and see what that does, I'm gonna check the plugs Sunday and see what they tell me and go from there, I'm also gonna try to get out early and play with trim some at wot and see if by chance I'm trimming it to far. I'm only gonna try one fix at a time so I can see what the problem actually is once it's fixed. Its got to be something simple that I'm overlooking