1976 mercruiser 140 overheating

[dancewithme951]

Just completed a rebuild on a Mercruiser 140 3.0L GM engine. The engine is running great - still running out of boat on blocks - but oddly per my infrared thermometer the cylinder #1 manifold runs MUCH hotter than the other three (once that specific spot on the manifold gets to 180 or 190 I shut the engine down). I installed a new Barr manifold so it should not be the culprit. Thermostat was tested and it opened/closed as it should off the engine (I can't even run the engine long enough to see if it's opening in situ because of that #1 cylinder) and the engine water pump was removed and checked and it is fine. I know it could be a clog somewhere in the engine block water jacket but clearly I would like to find something less invasive that I can correct.

Any ideas?

 

[Bondo]

Ayuh,.... How ya feedin' it water,..??

Are ya gettin' flow,..??

 

[dancewithme951]

Feeding water via garden hose directly to thermostat housing with good flow. EVERY part of the manifold, riser is staying within normal temp range except right where the exhaust for cylinder #1 is. Since manifold is new, I'm suspicious of blockage in the block water jacket. Could there be anything cylinder specific that would cause higher temperature. I thought valve adjustment off for cylinder #1 but it would not idle/run as smooth as it does if that was the case, right?

 

[Bt Doctur]

where the manifold bolts to the head there is no water, post a clear pic of where your shooting the temp

 

[dancewithme951]

No Title

The "hot" spot is the exhaust port for # 1 cylinder where the manifold attaches to the head. I can shoot my laser at any other point on the manifold ang get temps no higher than 100 or so (keep in mind that I haven't allowed the motor to reach thermostat opening temp - 130 degrees - because of the high temp on # 1 exhaust port. I'll atempt to post a picture but have not been successful in the past.
And it's the manifold itself that is so hot not the head. And shooting temps on the opposite side of the head/block temps are fairly consistent all along that side.

 

[Bt Doctur]

there no water there and its where the exaust gas exits the head so it will be hotter.

 

[dancewithme951]

Really...200 degrees!? I guess I'll methodically pinpoint the other exhaust ports with the laser and see if they're as hot. Thanks!