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454 mercruiser intake overheating

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zackfactor

Regular Contributor
I am working on a 1981 454 mercruiser which is having a cooling issue causing me to pull out my hair. the engine has new risers, manifolds, and heads. after checking and rechecking virtually every aspect of the cooling system, i have gotten the entire engine to cool with exception to the intake directly under the carburator. the heads, manifolds riders and both the fron and back of the intake are cool, however directly in the middle of the engine on both sides of the carb, the temp jumps up to almost 200 degrees within a minute of starting the enigine. any ideas on what could cause this, i have never seen anything like it an quite frankly i am lost. i have already removed the intake and cleaned it very thoughly and there was no blockage. this engine was converted to freshwater cooled with exception to the risers. could the freshwater pump be weak?
 
The area you refer to is SUPOSED to be HOT.

This is a noncooled area and only allows heat from the exhaust (both middle cylinder exhaust valves are side by side) factory intake gaskets should have had a stamped metal block off that covered the whole. This heat passage is used to heat the choke chimney bimetal spring.

Hope this answers your question.......
 
the stamped block off pieces may not be there??

You say you removed the intake, what was there at those ports when you did?

I believe this area gets very hot!! Exhaust temps exceed 1300 degrees at the valves..........200 degrees is not very hot for that area......

Also I do not believe ther eis any coolant flowing there I believe it is only heated air..........

What is the engine running temp when fully warmed up?
 
Water flows around the head and flows across to the other head at the front and back of the manifold only if I am not mistaken, I know it does not in the middle due to the carb and manifold opening for the fuel and air to reach the heads............
 
there are holes in each head as well as a port on the intake, and also holes in the gaskets. i will look at it again when i get to the shop. it does make sense what you are saying though. the entire engine is running about 130 degrees give or take a few. and like i said the intake manifold even stays cool, its only the area next to the carb.the heads are cool in that area as well but from where the intake meets the head over to where it meets the carb is smoking hot, and the area is only as wide as the carb. thankyou again for your help
 
Many if not all carbed engines back then (and almost all the WEBER carbs that merc went to in the 90's) used a chimney type choke bimetal spring arraingment. The bimetal spring needed heat to expand/contract which ever it was......this moved the choke link down or up and opened the choke butterfly.

Some had electric chokes where the bimetal spring was heated with a resistance (heater) inside the choke housing.

Same as with auto's for years............until EFI/TBI

If you have been around motors for a while you would have noticed that the area you speak of ALWAYS looks burnt, no paint, in other words toasted..........this is why.............
 
ive been doing this for 12 years however 99.9% outboards and diesel inboards, very rarely do i even touch a gas inboard but again what you are saying makes sense and i really apprecite the input.
 
zackfactor said:
ive been doing this for 12 years however 99.9% outboards and diesel inboards, very rarely do i even touch a gas inboard but again what you are saying makes sense and i really apprecite the input.
Click to expand...

Ayuh,.... That's the exhaust cross-over in the intake,.. All SBCs had 'em, up to the Vortec heads in '96...


Btw,... 130° is way to Cold,... what thermostat ya usin',..??
 
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