Looking for explanation of heat exchanger on 1964 Evinrude 40HP Big Twin

[newhamburgboating]

I have the motor running amazing now. My question is about the two ports on the side of the exhaust shield. Here is a thread where another user plugged them both because his choke wasn't working right.

http://forums.iboats.com/forum/engi...on-about-exhaust-heat-exchanger-big-twin-40hp

Ive also stolen his picture and marked it up since i forgot to take a picture of my motor. As you can see in the picture there are two ports on the side. The top one is the heat tube and goes to my choke. Working great. What is the lower hole for and should i leave it open or plug it? generally why is it there and what is it for? and exploded diagrams I find don't discuss or label it.

Thanks in advance!

Steve

 

[Chinewalker]

It's for the hot air choke. Leave it open as that is the only way air can flow through the system. Works on convection, as near as I understand it.

 

[racerone]

When the motor starts a wee amount of air is pulled into the lower hole.---That air is heated and the warm / hot air is pulled accross the bimetallic coil spring.--The bimettallic spring regulates choke tension.---On more than one occasion have I seen the bottom opening plugged.---Sure sign of a novice on motor repairs !

 

[newhamburgboating]

haha, well thats why I asked, With iboats and this great community you practically start as an apprentice.

I've left it open obviously and the motor is running amazing. I turn heads now when I launch beside all the guys with $20,000 boats and she starts in half a second and idles down nice and quiet nearly instantly.