IML: 68 No-start when hot

["Geoff Fors" <wb6nvh@xxxxxxxx>]

It sounds like the fuel is boiling and/or vapor-locking in the carburetor
bowls.  I presume that once the engine is running, it doesn't stall when
hot, it just won't re-start when hot?  That points to a fuel system problem.
I also presume this is running a Carter AFB series carburetor.

Sounds like the heat riser valve is frozen in position, and its thermal
clock spring may be rusted away or missing as well.  The spring keeps it
closed until the engine heats up, then it holds it open after that.  The
exhaust force will also push it open when the engine is brought above idle.
It's possible to wire the heat riser open, especially if you live in a warm
climate.  However, the heat riser assists in rapid warm-up and reduces the
time that excess gasoline enters the combustion chambers during warm-up.
You may have a valve which is rusted closed, even though it appears to be
open from an exterior view.  That would certainly account for this problem.
Sometimes the paddle moves on its shaft.  The only way to tell is to drop
the exhaust pipe from the manifold and look, rather a pain.

It's not unheard of for the heat riser paddle to come off the shaft and fall
down the pipe, jamming at some point, plugging that side.  I haven't seen
this on Imperials but on 1960's Buicks it will totally plug the exhaust
system!

The gasoline sold today tends to vaporize far more quickly than 1968
gasoline and thus older cars without fuel injection can vapor-lock easily.
This is a perennial subject in other make car clubs at the moment.

The paint always cooks off on the heat riser path section of the intake
manifold after awhile.  That's normal.  It shouldn't come off the rest of
the manifold, however!

If we were to take a survey, I think you will find that there are quite a
few of us running stock '67-'68 setups with working heat riser valves and
original carbs and gaskets, and we don't have this hot-start issue.  Thus I
think something is wrong.  It wouldn't hurt to make sure the choke is fully
pulled-off when the engine is warmed up.  The setup worked fine originally,
so if it isn't doing so now, something is not right.

I don't see any point in changing the intake manifold.  The heat comes from
the riser tube, not the manifold design.  If you really want to eliminate
the heat, use a gasket that blocks the riser tube holes, and you can retain
the stock manifold. And, it wouldn't hurt to use a thicker gasket under the
carb.  But, it should run fine as a stock setup, and not get too hot.

Geoff
Monterey CA


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