Heat riser, timing


Paul Dwyer

DAVIS,BRIAN R <brian_davis@hp.com> writes:

> Does anyone know how that "heat riser tube" (1-05-13) works?

Not new to engines but new to Amphis, so taking a chance here, but ...

On non-Amphis at least, a heat riser tube carries heat (not exhaust
gases or vacuum) from the manifold (usually exhaust) to a bitmetal choke
coil on the carburetor. As the coil warms up, it expands (like a
mechanical thermometer, which is basically what it is), changing the
angle of the choke plate. This is how the carb knows when to lean out as
the engine warms up.
Note: Manual choke-equipped cars generally do not use a choke coil or
heat tube.
The tubes often rust out at the manifold. If this happens, you can
usually run a new copper tube to the exhaust pipe where it attaches to
the manifold (kits, possibly still available, used to include an
aluminium shroud to facilitate this). In such a case, it might take a
little longer to carry the heat to the carb, making the engine run rich
a little longer than normal.
If there is an exhaust or vacuum (depending on which manifold holds the
tube) leak, you will have to plug the hole or holes--on AMC sixes the
tube runs straight through the manifold, top to bottom, so there are two
places to leak.
SAFETY NOTE: Back before the danger of asbestos was appreciated, these
tubes were commonly coated in a woven asbestos sleeve. Handle with care!

> The manual says that ignition timing should be set at 15 degrees BTDC (8:1
> compression ratio), but there's no marks on the crank pulley to indicate
> that point! Does this mean that you just have to guess as to where that is?
> I turned the adjustment on the vacuum advance, but the timing mark didn't
> move. (Using a strobe light). Does this mean that I have to do it the 'old
> fashioned' way by loosening the distributor bolt and rotating it?

David C. says the pulleys lacked timing marks other than TDC, as
strobe-type timing lights were somewhat new back when these engines were
designed. Presumably the TDC mark is simply for alignment during
cam/timing chain assembly. I plan to set the timing by warming the
engine; reducing to lowest practical speed (to minimize the influence of
mechanical advance); capping the vacuum advance line; and adjusting by
rpm and vacuum.
Change timing by rotating the distributor (not that old-fashioned--done
at least into the '80s), not via the vacuum advance mechanism.

Hoping those more knowledgeable will correct any errors,

Paul Dwyer
1968 Rambler American
1967 Amphi (Phylise's)
1966 Hagstrom hollow-body electric