I ran both engines with good flow on the hose before going to the lake. Hoses burnt right at the top of the riser tails. The outer hoses are a little swollen. I don't get why inside exhaust hotter on separate engines. I could understand if same side of separate engines. Going to check the new impellers and flow water from the thermostat housing hoses and see if blockage. Just put in new merc 143 degree stats. Would it help to move the stainless 2" offset from the tips to the riser end to handle heat and pressure better? Can it be exhaust pressure building because of 2" offset at the tip and blowing the exhaust hose?

Photo of what I'm trying to solve. Opinions needed.

maybe thermostats

What was your water temp while running? Do you run crossovers?

water temp was 180-190 on a 98 degree day. i have the standard thermostat housings where water comes into housing from seapump then thermostat decides if water goes directly to manifolds or circulating pump.

My fresh air intake is on the sides directly in front of the outside risers. Think the fresh air kept the outer risers a little cooler and thats why the outer hoses did not melt, and the inner hoses did melt? it be that simple?

That might be your problem. You shouldnt be that hot, im at 140-150. I wouldnt think you would have a 180 degree thermostat. I think those are bad

Doesn't look like the offset is enough to be a consern. Could be better. Possibly welding extension to the forward end of the adapters to close up the gap, but that isn't addressing the issue. Besides, boats using silent choice or non thru hull have alot more tighter exhaust bends than yours with no problems. What do your spark plugs look like? I agree, the 40psi doesn't mean you have flow, just more in than out, (kinda). I'd be getting that down to mid 20's for safety. 10psi at idle is high, not bad, just higher than normal. Common for almost zero psi at idle. The stock Merc 2 hose t-stat housing design creates high presure in itself. The older 4 hose design with the brass tee was issue free. A relief valve will cure high pressure, but you gotta make sure you are getting enough volume out the risors first. Look closer at your plumbing. Don't see the closeness of the motors being a cause. Lack of water flow around the outer dia of your risors outlet seems likely. What was the t-stat rating when you where seeing the 180-190*?

Used brand new Merc 143 Degree in both engines. Uggh i hate to pull the plugs on the centers, but will after I check the impellers and water flow thru the risers.

Im really hoping that is all it is, that maybe I don't have good water flow around the riser tail and exhaust tubing connection.

I had the exact same problem. It's like you said above, the hose was not centered so no water was getting to it.

I changed the hose got it centered and it worked fine after that.

Have you considered putting a set of wedges between the manifolds and the risers to kick the pipes downward a little bit? That, and some of the high temp corrogated exhaust hose might solve the issue.