Over heating Rubber Boot below riser

I have a 1980 or 81 350 merc cruiser on a 26 ft well craft. I am burning out the rubber boot below the riser on the starboard side of the engine only after about 40 hrs of use. The engine runs cool, 150 max under power. I have checked water flow seems O.K flushed engine and cooling system, check for blockage, changer thermostat. I have not changed the impeller, which is the next step- but the fact the engine runs very cool make me think it may not be the issue-- any thoughts-
Thanks

 

can you post a pic of the burn and surrounding area, please?

 

The starboard riser probably has one or two of its 3 exit gaps clogged by a piece of rusty Mercruiser. This is the narrowest part of the cooling system.

Water takes the path of least resistance, which in your case will be the port riser. The starboard one doesn't get enough to cool the whole circumference of the boot, so it starts burning.

 

If it is a straight inboard that can also be caused by excessive droop of the hose where it leaves the riser. If the hose droops excessively the exhaust gases will blow against the wall of the hose through the cooling water and overheat it.

 

No pic available threw old boot away last weekend will dbl check

 

CDK went where I was I was probably going to go. I was looking for visual confirmation. You know what he's saying, right?

 

I think I get what CDK was thinking blocked passage-Makes sense if impeller checks out-I did pull the riser and flushed the heck out of it, but there is that small opening on the gaskets between the riser and the manifold that could easily plug up ,If I had a photo you would see the rubber boot with the interior looking like charcoal, I am surprised the hose clamps don't melt into the boot but they don't. I Smell the rubber burning at low RPM in channels
( 1000 rpms or less) the as soon as I throttle up at all it stops

 

The hose clamps are in places where heat is distributed evenly by the metal underneath. The rubber boot doesn't conduct heat very well and will burn if there is insufficient water supply to wet the whole circumference.

 

I've solved this problem in the past by installing a short auto exhaust flange adapter inside the rubber boot. It fits with small end up inside the riser, wedged in place with a couple of stainless screws jammed between the casting and the exhaust adapter pipe. You can get these at auto stores for solving exhaust problems on you car. They are about 3 inches long and just perfect for protecting that rubber boot from the heat.
I've used on in my 1981 mercruiser 470 for over 6 years now with no problems and no more burned through boots.

 

Most likely the manifold is rusted and has less water flow than the other. Time to change both of them