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Intake manifold hot in middle, both sides

Posted: Mon Jun 06, 2016 8:01 pm
by mmoody435
The boat – 1959 CC Continental, 21’ Hard top with original 283-H engine, 235 hours. I bought the boat in July, 2015.

The issue - I recently put the boat in the water for the first time and have put about 5 hours on it. The engine was freshly painted when I bought it. I have noticed the paint burning off near the center of the intake manifold, on both sides (photos attached). I took some temperature readings after the engine had been running at high RPMs for about 30 minutes. On the center part of the manifold, where the paint is gone, the temp was around 150 to 190 F on both sides. Moving outward from there, towards the front and back, the temps were around 120 to 130 F. As far as I know, this is an original intake manifold.

Is this an issue?

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 4:06 am
by charlesquimby
Most likely an automotive manifold with a feature that picks up exhaust heat to operate the choke. You will have to remove the manifold and block those passages to eliminate the burn. There are s/s blockoff tabs made specifically for that purpose, but you will have to remove the manifold to install them along with new manifold gaskets. With the tabs installed, your choke will not heat up, but you will probably not need a choke in Summer weather anyway. If you need a choke it is simple to adapt a mechanically operated one. CQ

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 6:00 am
by boat_art
Charles, is there any problem with leaving these passages open? I have it on an AMC engine and it appears it was always like that.
Tom

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 7:05 am
by Bilge Rat
My 327 Graymarine with a 2 barrel Rochester has the exhaust passages in the intake manifold to pre-heat the air/fuel mix and provide heat to the climatic choke. When I restored her, I used a high temperature exhaust manifold paint there. Looked good for about 2 seasons and now looks like yours. I was having the usual difficulties with the climatic choke, so I converted it to electric and starting/warm up is much more reliable. I left the exhaust ports in the manifold as-is.

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 3:39 pm
by evansjw44
The burned paint is around what I will call the "hot well". There is a "well" (5 sided box) cast into the manifold that exhaust gas heats that is used for the automatic choke sensor. On many GM engines there is a spring inserted in this cavity that changes tension as it warms to control the choke on the carb. If you look, there is a tube extending down into this area which is picking up warm air to warm the choke spring in the choke box (the black cylinderical thing) on the carb. The burned paint is normal for an engine that has been run.

Unless--- Most CC early V8s didn't use the "hot well in the intake manifold. CC intake manifold gaskets blocked it off. Instead CC cast a "hot well" into the exhaust manifold and a heat riser tube (as seen in your pic) to the exhaust manifold "hot well" instead.

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 5:18 pm
by mmoody435
Thanks all for the explanation and comments.

I suppose the channel on top of the wedge (see photo) is somewhat in reference to the same thing?

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 5:58 pm
by boat_art
Im still wondering if there is any harm in leaving the passages open.

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 7:40 pm
by mfine
The passages were left open on our OMC marine conversion. The biggest downside is the extra heat can make it more prone to vapor lock. I certainly would not pull the manifold just to fix it, but next time the manifold is off I would block them off.

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 9:41 pm
by boat_art
Thanks!
Tom

Re: Intake manifold hot in middle, both sides

Posted: Tue Jun 07, 2016 10:34 pm
by jim g
The passage is suppose to be open. If you block it off your choke will not open up. It operates off the heat that passes through there.

Re: Intake manifold hot in middle, both sides

Posted: Mon May 20, 2019 8:24 pm
by NOT Firewood
So I picked up a hand held digital read out thermometer it showed me today that I have the same issues as this post which gives me a bit of comfort this hot spot is to happen. My hot spot is on the choke intake is 308f and on the other side is 205f. This seems real excessive for temp so what is to be normal? Do is have something else going on?

Re: Intake manifold hot in middle, both sides

Posted: Thu May 23, 2019 8:06 am
by NOT Firewood
Hey guys any thoughts on this? is there a issue with the high temp of 305 F that I am getting?