Boil over tube


Ken Chambers

A few months back there was some discussion about the intake manifold boil over
tube. It's described in the Amphicar parts manual
on Plate 05 as a manifold drain pipe assembly.

The purpose of the tube is to act as a fuel reservoir to collect any fuel that
may be present in the intake manifold after the
engine is shut off. A hot engine may boil some fuel out of the carburetor when
the engine is shut off. Fuel will flow to the low
point of the intake manifold and into the drain pipe. The fuel remains there in
the loop of the pipe until it either evaporates
over time or it is slowly sucked back into the engine when it is restarted.
BTW, only Amphicars have a loop in the pipe.
Non-amphibs would simply drip the fuel on the ground.

Here's what my car was doing and what I found: The engine always started hard
for maybe up to an hour after it had been running.
After it did start, it would act as if it were flooded and I'd have to rev it up
considerably to clear it out before it would run

When I had the carb off recently to change the main fuel jet, I noticed the
intake manifold was all crusted up on the inside. The
drill passage to the boil over drain pipe was completely plugged. After
chipping away the carbonized crust, running a twist drill
through the drain hole and making sure the pipe was clear, the flooded hot
starting problem was cured. The hot engine starts much
easier now with no flooded symptoms.

A few words about the intake manifold. I would think that a crusted up manifold
would hurt engine performance. If you ever have
the manifold off for any reason, it would make good sense to soak it in carb
cleaner for a few hours, or overnight, to dissolve all
that rough carbon stuff. A nice smooth manifold would certainly be better for
proper air flow to the cylinders.

And a few words about the manifold drain pipe. Someone pointed out a while back
that it seems like the engine would run lean by
sucking in air through that fairly large diameter drain pipe. That would
probably be true except that where the actual hole
through the manifold is very small. A number 56 drill is the size of the hole.
That's 0.046" diameter by about 1/2 inch long.
Big enough for fuel to run through but not big enough for much air to pass.

Thought you'd like to know.

Ken Chambers, CA
64 Amphi