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Help! Quadrajet - Heat Insulator between intake and carb - guidance needed

68BigBlock

Active member
Joined
Oct 28, 2007
Messages
29
Location
Davidson, NC
Corvette
1968 Corvette Coupe- 427/390
I have a 68 Big Block, 427/390 numbers matching car with Quadrajet carb.

;help Car is vapor locking on me during summer drives when I stop engine and attempt to restart. I run mixture of 100-octane racing fuel and 93-octane, non ethanol.

Car has the "smiley face" aluminum intake set up - I've made all the gaskets updates per LARS guidance back in the winter.

Seems heat is boiling the gas out of the carb -- let it sit for 25-30 minutes and it fires right up.

My questions:

1. I believe I need some type of phenolic gasket/heat insulator between intake and carb -- can anyone who's had this issue and corrected it with this solution please comment on what products you used and where sourced?

2. On the topic of the insulator, what is the maximum thickness I can use and not create a hood closure issue, or worse yet, jamming the air filter lid into the hood when punching it? Will the clearance allow for 1/2" insulator or is 1/4" sufficient? Again, anyone who's solved this problem would appreciate your comments and how you did so?

Many thanks in advance!
 

Vettehead Mikey

Well-known member
Joined
Aug 26, 2003
Messages
3,486
Location
Not that far from Ottawa
Corvette
1973 Coupe
Vapour lock occurs in the fuel pump and pump inlet fuel lines, NOT the carburetor. The fuel in the pump essentially turns to vapour from engine or exhaust heat and the pump looses it's prime, A heat shield under the carb will not help the issue.

You're probably thinking of percolation which is gas boiling out of the carb.

First step is to determine which of the two problems you have. Best way is to look down the carb throat after hot shutdown and watch for signs of dribbling or boiling over. Alternatively, when you attempt a hot restart, see if the carb accelerator pump has enough fuel to function.
 
Joined
Mar 9, 2009
Messages
1,026
Location
Yemen
Hot Weather Problems With QuadraJets.

Boiling fuel out of the small fuel bowl has always been a problem with Quadrajets when the weather gets hot and there's not much you can do other than using a 1/4" fiber spacer between the carburetor and the intake manifold. One way to help the situation is to remove the intake manifold and place a .010" stainless steel shim between the gasket and the heat riser passage on both sides to prevent exhaust gasses from passing thru the manifold. Vapor locking was an issue back in the 40's and 50's when 75 octane gasoline was used combined with the suction of the fuel pump and high elevation as the low pressure allowed the gasoline to boil into a vapor; hence the name "vapor locking". The use of the fuel pumps with the 1/4" return line really helped the situation as it kept a portion of the gasoline moving back to the tank where it could shed heat.

Try the 1/4" thick gasket first and see if it helps.
 

SVO

Well-known member
Joined
Jan 11, 2011
Messages
332
Location
Texas Gulf Coast
Corvette
69 L68 Vert, 72 LS-5 Cpe, 72 LT-1 Air 'Vert
Originally, your car was equipped with a thick carb base gasket from the factory, which offered some heat insulating value. A thick Q-jet gasket is available thru most of the Corvette suppliers. In addition, GM offered a thick gasket/aluminum heat shield for '70 model California Corvettes. It is also available thru most Corvette suppliers. Either of these, when installed, will not give you hood clearance problems. If you are looking at aftermarket carb spacers, you can put a lump of modeling clay on the high point of your air cleaner and gently shut your hood. Open the hood, measure the thickness of the clay, which will tell you clearance between the A/C lid and the bottom side of the hood. You need to maintain at least 1/4" to 1/2" of clearance between the hood & the A/C lid.
 

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