1968 Corvette 427 390 Intake Gasket Restrictor Plates

[Joseph M.]

Hello,

I have my intake off of my 1968 Corvette 427 390HP and I was wondering if it would be a good idea to install the restrictor plates on the intake gaskets so to reduce the amount of hot exhaust gas that make its way up through the original intake to my matching # Q-Jet.

[Joe L.]

Re: 1968 Corvette 427 390 Intake Gasket Restrictor Plates

Hello,

I have my intake off of my 1968 Corvette 427 390HP and I was wondering if it would be a good idea to install the restrictor plates on the intake gaskets so to reduce the amount of hot exhaust gas that make its way up through the original intake to my matching # Q-Jet.

Joseph-----

In many ways it's a good idea, but if you do it your automatic choke will no longer function. The divorced-style choke used for your application depends on the heat from the cross-over to open. So, if the heat cross-over is blocked, the choke will never open. It will have to be removed or disconnected and the choke valve held open in some way.

Also, if you block the heat cross-over then you MUST either remove the heat riser valve and install a spacer in its place, permanently wire it open, or "gut" the valve by removing the butterfly valve.

An "intermediate" approach you can take is to block the holes in the intake manifold carburetor flange at each end of the "heat slot". This will accomplish about 80% of what you're trying to do with carburetor preservation without affecting the choke operation. You can either drill and tap the holes for internal hex head pipe plugs or use drive-in cup-shaped plugs.

[Joseph M.]

Re: 1968 Corvette 427 390 Intake Gasket Restrictor Plates

Joe,

Thanks once again for your excellent advise! While you do mention that it is a good idea to plug these ports off under the carb, do you generally advise to do this or only for certain types of problems? And yes I am running the original factory choke so restricting the cross-over port will only cause more headaches. In looking at the intake, it appears that the hole size is around 1/2 so I'll pick up a set of 1/2" hex pipe plugs tomorrow unless you suggest otherwise.

Thanks!
Joe

[Joe L.]

Re: 1968 Corvette 427 390 Intake Gasket Restrictor Plates

Joe,

Thanks once again for your excellent advise! While you do mention that it is a good idea to plug these ports off under the carb, do you generally advise to do this or only for certain types of problems? And yes I am running the original factory choke so restricting the cross-over port will only cause more headaches. In looking at the intake, it appears that the hole size is around 1/2 so I'll pick up a set of 1/2" hex pipe plugs tomorrow unless you suggest otherwise.

Thanks!
Joe

Joe----


I ALWAYS recommend that whenever an engine is equipped with an intake manifold with the "heat slot" in the carburetor flange that the holes supplying exhaust gas to the slot be plugged. This "heat slot" was a carburetor destroyer. I wish I had figured out a lot sooner than I did that this thing needed to be eliminated.

While the holes are about 1/2" OD, you don't want 1/2" pipe plugs. In pipe size, a 1/2" pipe plug is about 3/4" OD. If I recall correctly, the plugs I use are 1/4" NPT.

[Joseph M.]

Re: 1968 Corvette 427 390 Intake Gasket Restrictor Plates

Thanks again!

Joe

[Duke W.]

Re: 1968 Corvette 427 390 Intake Gasket Restrictor Plates

Another option is to just block one side - the side opposite the choke heat stove, which will allow some exhaust heat to reach the stove/carb base for near normal choke and carb operation.

If you don't drive in subfreezing weather, this reduction in carb/choke heat should work out okay - not enough reduction to affect "cold" operation, but not so much heat to cause fuel percolation and carb base plate distortion.

You must also block the heat riser open with just one side of the passage blocked.

Duke