I bought an 86 BRAT and pulled the engine to give it a good once over, and found something weird. There is a valve on the bottom of the metal intake box, just upstream of the filter, with a couple of hoses going to it. Someone in the past has attached a piece of black plastic dryer vent hose to this port, ran the hose down to the exhaust manifold, and attached that end to a fitting welded to the exhaust manifold. It doesn't actually open into the exhaust stream, it looks like it's designed to just pull air against the hot manifold then feed it into the intake.

 

So what's going on here? I'm not sure if this is someone's jury-rigged fix of some part that's supposed to be there or if it's some crazy thing someone invented that I can just remove. I guess my question is, where is that valve on the bottom of the intake supposed to draw air from? Down near the exhaust manifold or somewhere else?

Edited June 29, 201510 yr by musubk

That would be the carburetor preheater. This all sounds correct except "black plastic dryer vent hose". Dryer vent hose would be 4", and not likely plastic. I cant see many types plastics lasting long down at the exhaust pipe. Odds are, only the connection at the air cleaner is black rubber, or the hose its self is actually a thin metal with a black coating.

IMO, living in Alaska, you need to keep this setup and get it working if its not.

It is for warming the intake air when cold, and avoiding carburetor icing.

Hmm. Well it's some sort of ridged, flexible tube and it's got a ton of crud on the outside stuck in the ridges, which makes it hard to tell what it's made of. I thought generic flexible plastic hose when I saw it but you're right in that wouldn't last on the exhaust manifold. I guess I'll try to clean it off and leave it in place. Thanks!

As DaveT said, this hose prevents your caburettor from icing up.  It is most useful in high humidity climates when the temperature is just around freezing.  It would probably help the engine to warm up faster in the extremely cold climate of Fairbanks.  It may have more complex functions as well...

 

Is it automated, so the valve opens and closes on its own, or is there  a lever to flip from summer to winter?  If the leve, it is simple carb icing prevention. 

 

Honda Civics of this era had a complex preheating system that was absolutely necessary to the proper operation of the carb.  It was so finely tuned that it needed air of a certain temperature to give the right mixture.  The Brat may have a similar system? 

I remember habing a switch there for summer/winter position so probably he has it too. Pretty good help on wintertime.