Part of the warmup system is the heat riser which directs the passenger side exhaust under the carb. When the engine is cold, the heat riser spring recoils and forces the internal flap shut, making the only path the for the passenger side exhaust to take, is to flow under the carb and exit out the driver side of the manifold into the open exhaust pipe.
This redirect of exhaust quickly warms the manifold and accelerates the procedure of getting things operating smoothly. A few minutes after the cold engine starts, the heat from the passenger side exhaust builds up enough to uncoil the heat riser spring, it opens the heat riser and allows the gasses to flow down the passenger side exhaust pipe. After this, the underside of the manifold still gets some flow from the alternating exhaust pulses, but not a direct flow.
Your engine is designed to operate at a certain temperature. The quicker it achieves this temperature, the smoother and more efficiently the engine operates. Disabling parts of this engineered parameter will cause a few changes;
A) It will require a lot more time for the engine temperature to move to the manifold and stabilize operation.
B) During the (now) elongated warmup period, the performance will be noticibly poor because the fuel can't vaporize.
C) An electric choke compounds the problem because it operates on a electrically heated coil that will open all the way very early, and leave you with a lean burning engine which will balk at moving off the line.
An older worn engine may be very problematic in this situation. The only way to overcome this is to use a manual choke to keep the fuel ratio adequate to run thru the cold manifold.
D) If you block off the manifold stove, you MUST remove the heat riser or block it open so the exhaust has somewhere to go.
Even if an engine with a blocked manifold gets up to operating temperature, the manifold can still be too cold in relation to the engine, for quite some time.
If you can still use the older mechanical automatic choke, it works off the temperature of the intake and slowly opens as the manifold heats up. In this instance, the enriched mixture leans out as the engine warmed up.
The Avanti's used manifold gaskets with much smaller holes for the choke stove, than the standard V8. This was due to the underhood temperatures on an Avanti being more critical than the metal cars.
The space around an Avanti engine is very small, ventilation is much less and the engines were higher compression, creating more heat to begin with. Bottom line is, that you need some heat passthrough under the carb for the engine to run efficiently, ie smooth operation and decent fuel mileage.
What should be done to prevent boil out is to raise the carb with insulating gaskets (Mr. Gasket #98). This is required if an aftermarket Edelbrock carb is installed, due to the extra linkage, which will hit the manifold if it isn't cut off.
You can install Avanti manifold gaskets or modify standard ones to reduce the size of the stove opening, but never eliminate them.
Vapor lock can be significantly reduced by re-routing the line from the fuel pump to the carb, away from the engine. Angled at 45' from the fuel pump outlet, toward the inner fender, up away from the exhaust headers and a couple inches over the top of the valve covers. The line should be all steel, except where it connects to the carb inlet nipple.
A secondary vapor lock preventative measure is to install an electric fuel pump, with a dash operated switch. At the slightest hint of vapor lock, flip the pump on and it will force fuel into the carb, eliminating the vapor lock.
Understand this is my opinion, but I've been through this with 3 cars and have to testify on the outcome, the way I experienced it. / RJ
