Vapor lock

[shelbydoug]

Holley Sniper EFI system is the ultimate solution if you want to drive your car a lot in the summer heat.  It will cold start like your daily driver, no more flooding/fuel smell after hot shut off, and a perfect air/fuel ratio all of the time.  Plugs will never foul again. 

https://www.holley.com/products/fuel_systems/fuel_injection/sniper_efi/sniper_4bbl_tbi_kits/

One item that seems was never dealt with well on the '67 GT500 in particular is isolating the carbs from the engine heat. What happens is when you shut off a hot engine, (a normal operating temperature) heat will rise out of the engine, through the intake manifold and into the carbs. What it can do is make the fuel in the bowls boil and peculate like in a coffee pot.

In severe cases this will push the fuel up and out of the vents and into the intake manifold. There's your flooding.

The intake, then the carbs will act as heat risers. Weber carbs are great for this but the fix is really simple. What you need is just an 1/8"thick asbestos carb gasket. If you can get phenolitic isolators that will work too. You are trying to reduce the heat sink effect into the carbs. You can't eliminate that.

This perculating scenario, also affects the floats and the inlet valves. The pressure on the floats due to the rising lift created by the boiling fuel can jamb the inlet valves closed. Right there, that's your "vapor lock". It is only temporary or even momentary. Fuel pressure will push them open eventually. Look no further for the vapor lock. That's it right there.

That's how you get "vapor lock" and engine flooding simultaneously.

Just isolate the carbs. That's generally enough and will stop fuel from leaking out of the throttle shafts also. That's all there is to it.

You might want to think about isolating the fuel log tabs from the intake manifold also with phenoletic flat washers.

FE's in Mustang's and Cougar chassis' are particularly susceptible to this phenomenon because of the close quarters of the engine to the chassis. Small blocks are probably about 95% less susceptible to it.

My Pontiac 389 with the Carter AFB on it was the only other car I can think of that had this problem. I was 16 and no one ever heard of this kind of an issue back then. I wish I knew this back then 'cause that engine caught fire as a result on a hot attempted restart.

I don't feel too bad about it though. Even Inglese had to learn this when he saw the gas running out of the hood scoop and down the front of the hood on his 427 Cobra from the Webers! Easy fix though...if there's no fire? Just isolate the carbs. 

I'll bet 'ya a nickle that this is the cause of 95% of carb fires?

[2112]

Could a person find a asbestos gasket today?

[shelbydoug]

Could a person find a asbestos gasket today?

I have them for the Weber carbs but that term is not used. I don't know what the substitute is but is likely of ceramic content like ceramic coatings on headers. A man made concoction no doubt superior and safer.

You want to search for "phenolitic  gaskets". Coming from me, that's likely spelled wrong.

[1967 eight barrel]

Charlie: I bought them from Summit with the exception of the AMK kit.  You have to change the carburetor studs too.  I used the AMK kit nuts on the Edelbrock stud kit. The carburetor stud side is 5/16 NF thread and 7/16 nuts instead of the 1/2" nut that is supplied by Edelbrock.

1. 2 X Canton Spacer 85-154 .250 Phenolic .
2. 2 X Edelbrock stud kit 8024  1.750. (Put the nuts and the washers your hardware drawer.)
3. 4 X Fel Pro SFL1901 base gaskets.  ( 2 per carburetor. Each side of the spacer)
4. AMK 3475 2X4 special nut and stud kit.
                                                                   -Keith

[charlie D]

Kieth, thanks for the part numbers.

[shelbydoug]

You don't need to go crazy with the spacers. The thinnest one I can find for the Holley's so far is 1/4". Here's a picture of the Canton 85-154.

[2112]

https://www.summitracing.com/parts/ctr-85-154

[1967 eight barrel]

The 85-154 is .250. That's 1/4"  I don't recommend anymore than that because of air cleaner variances and hood interference.  It'll stop the fuel bowl boil.

                                                                                          -Keith

[shelbydoug]

The 85-154 is .250. That's 1/4"  I don't recommend anymore than that because of air cleaner variances and hood interference.  It'll stop the fuel bowl boil.

                                                                                          -Keith

I'd like to find thinner but so far no good. The ones for the Webers are basically thick gaskets and work fine.
I agree. Thicker then 1/4" and you are too close to the hood.

A few years ago I noticed that a Pro Stock racer that I knew was using plywood spacers. Apparently that is common place in drag racing now?

It's got a better thermal transfer rate then the polymers do.

[shelbydoug]

Holley offers a heat isolating gasket. It's part # 20-130. It retails at around $9 each. It's black and 1/4" thick.

[vtgt500]

Phenolic is a rather generic term.  Includes laminates made from paper, canvas, linen, and glass weaves bonded by a thermoset resin. All work well as a thermal insulator.  However, the canvas and linen product have good strength, preferred machinability, and reasonably good fuel resistance.  Wood or plywood is a foolish idea.  Porous, warps, can not be machined flat enough for a reliable sealing surface, and it only used because it's cheap, and car hobbyist are generally pretty stupid.

[shelbydoug]

Phenolic is a rather generic term.  Includes laminates made from paper, canvas, linen, and glass weaves bonded by an epoxy resin. All work well as a thermal insulator.  However, the canvas and linen product have good strength, preferred machinability, and reasonably good fuel resistance.  Wood or plywood is a foolish idea.  Porous, warps, can not be machined flat enough for a reliable sealing surface, and it only used because it's cheap, and car hobbyist are generally pretty stupid.

I've used both the fabric and the hard epoxy versions. Neither is a perfect beast but my preferences are for the black fabric one. I've broken corners off of the epoxy units. They tend to be a little fragile.

None are going to give you 100% heat isolation.

The original factory configuration provided for no heat isolation so anything you do is going to seem 100% better.

PROBABLY any car will benefit for using some sort of heat shield? I remember using the GM Z28 heat shield for a while as well on a single 4v application. That one tends to be more of cosmetic device and causes issues due to limiting access to the intake manifold itself. Those are an aluminum plate with a thick gasket on the top and bottom.

Holley sells a generic version, Holley 108-70

There are generic "epoxy" versions of them that I suspect are going to get broken sooner or later since that material doesn't bend well and cantilevers over the intake.

[rbarkley]

This is great information!  I have yet to start my MR 427, but will apply what you guys mention in this thread.

Question - All else being the same (motor mount etc.,) does anyone know if the 427 w/MR intake is the same height as the factory 428 PI with dual quads?

Before buying the gaskets/spacers, I want to make sure the wing nuts holding the air lid do not punch through the hood!

Thanks

Ron

Pic with the 427 lid, but I plan to replace it with the cobra element.

[shelbydoug]

The '67 Shelby GT500 C7ZX dual quad intake is a medium riser intake. I'd recommend that you measure the clearance by using a compressible material like play doh.

[1967 eight barrel]

The C7ZX-9425A is a 427 MR intake. The air cleaner clears the hood with plenty to spare with the .250 spacers I used and recommend.  No more hard hot start ups and it's not boiling over into the vents and dripping onto the butterflies.  I am thinking about having the spacers machined to the profile of the carb base on the intake instead of leaving them in the Holley square bore base profile. It will make them less noticeable. I have a relatively "hot" motor. It is always under 200 degrees. So no issue with over heating.  Another point of interest is the fact that I had no issue with the fuel percolating with VP racing fuel. When I started using the 50/50 mix with pump 93 and VP 110 the issue started. So I am an in agreement that it has everything to do with the alcohol in the pump fuels.

                                                                                -Keith