It has been frequently mentioned herein to plug the two "hot slot" holes in intake manifolds used on Q-Jet cars with two 7/16 steel freeze plugs, as a fire prevention measure. Other enthusiast websites have similar recommendations to perform this measure, with the added suggestion to also modify the exhaust manifold heat riser valve by removing the round metal plate from the shaft inside the heat riser.
Is it advisable or nessessary to remove the plate from inside the heat riser when plugging the hot slot holes?
Perhaps JohnZ can chime in on this with his thoughts.
As always, Thanks!
There are two different "heat" circuits on intake manifolds with the "hot-slot" channel at the front of the carb pad; both are controlled by the heat riser valve. The primary circuit directs hot exhaust gases from the right bank through the heat riser crossover passage in the intake manifold to the left side head, where it merges into the exhaust ports and exits through the left side exhaust manifold. This crossover passage goes UNDER the floor of the intake plenum, warming it, which warms the incoming fuel/air charge for improved cold driveability so the fuel droplets don't condense on the cold walls of the manifold and intake ports; it also gradually warms the intake manifold casting, which in turn eventually warms the carburetor itself to prevent "carburetor icing".
On cars with the "hot-slot" channel, the holes at the ends of the channel are cast/drilled into the crossover passage that goes below the manifold plenum floor; the idea was to get hot exhaust gases to the carburetor baseplate right away to prevent carburetor icing, which is why those cars also use a stainless steel baffle between the carb baseplate and the carb gasket, to protect the aluminum carb baseplate from direct exposure to the hot gases. This design resulted in a number of damaged carburetors and carburetor fires (and a huge safety recall), and was abandoned after 1969.
Plugging the holes at the ends of the "hot-slot" prevents overheating and damaging the carburetor, but it leaves the crossover passage functional; this is important for proper function of the remote choke thermostat coil mounted in the well in the intake manifold on the passenger side; if that well doesn't get hot from the gases going through the crossover passage, the choke won't fully open.
In most cases, the best compromise is to plug the "hot-slot" holes (to protect the carburetor), and either remove the flapper valve in the heat riser or just wire it in the fully open position; that will still allow some exhaust gases to heat the choke well so the remote choke thermostat works.
Some guys choose to plug the ends of the crossover passage entirely, with gaskets with stainless inserts; that stops all flow of exhaust gases and keeps the intake manifold (and carburetor) cooler, but it also means the choke won't work any more (won't open fully due to no heat in the choke well); if you choose to do that, you MUST either remove the heat riser, gut it, or wire it open, or the exhaust gases from the right bank won't have anywhere to go when the heat riser is closed.
