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Hazy Heads Up Display and Older Carburetors - Technically Speaking

James Berry Jun 16, 2014
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1999-2013 Corvette Heads Up Display Appears Lazy

Q: Hi James, I'm an '11 Corvette owner and love the magazine and the Corvette lifestyle. My problem is that my Heads Up Display (HUD) image appears hazy. If I look at the image on the windshield closely it appears to be two separate images. Thanks for any help with this problem

Via U.S. Postal Service

A: Jim, as you know, the HUD image is projected on the windshield glass. The windshield is composed of two layers of glass giving you two reflections. The two images must be aligned. If the images are not aligned it is possible to have a double image. These images are aligned using a wedged shaped plastic laminate placed between the two layers of glass.

The most common problem with some Corvettes built from 1999-2013 is if the dimensions of the wedged shaped plastic laminate are not accurate, the image will not align correctly, thus the HUD image may appear blurry.

To check for this problem simply apply Vaseline to the outside of the windshield in the area of the HUD display. If the ghost image disappears the most likely problem is the windshield. I usually see this in windshields that have been replaced with an aftermarket component. If the windshield has been replaced it's possible that a HUD-equipped windshield may have not been installed.

Jim, I would almost bet your windshield has been replaced due to an accident or a chip. There also could be a problem with your HUD unit but that is an uncommon problem. Good luck and thanks for the question.

Holley Power Valve Repair For Older Carburetors

Q: Hello James, I'm a long time Vette magazine subscriber and love all of the technical articles that the magazine offers. I own nine Corvettes of varying generations that I try to maintain myself since I have retired. My wife says they keep me out of trouble.

This is my question, I have had two of my Corvettes equipped with Holley carburetors that will occasionally backfire through the carburetor when starting and once this happens the engine will start fouling spark plugs and a cloud of black smoke will emanate from the exhaust. This normally occurs after the vehicles have been stored for several weeks.

I took the carburetor apart and installed a rebuild kit which seemed to fix the problem until one day when I flooded the car and it backfired on start up. Now this carburetor is doing the same thing again. Do you have any ideas what could be causing this and how I can repair it? Thanks.

Via email

A: Paul, Holley carburetors utilize a vacuum-operated power enrichment system which supplies additional fuel to the main system during heavy power load situations. The problem with older Holley carburetors built before 1992 is the power valve rubber diaphragm has a tendency to tear. Holley repaired the imperfection and any Holley carburetor built after 1992 utilizes a power valve blow back check system that effectively eliminates this problem.

On older Holley carburetors if the engine backfires and pops back through the intake manifold it will generally destroy the power valve. When this happens, the torn rubber diaphragm will allow engine manifold vacuum to pull fuel through the valve, making the engine run extremely rich. (Image A)

Rubber Diaphragm 2/6


The test for a blown power valve is simple while the vehicle is at idle, turn the idle mixture screws all the way in. If the engine stalls and dies the power valve is not ruptured and most likely there is a different underlying problem.

The power valve is made up of a small rubber diaphragm and a small coil spring. At wide-open throttle the intake manifold vacuum drops to almost zero. When this occurs, the power valve opens and directs additional fuel into the main power circuit this enriches the air/fuel mixture during wide-open throttle.

Replacing a defective power valve is a relatively easy repair. It requires removing the primary float bowl and metering block. Once these are apart it's a simple job to remove and replace the valve. We will take you through a simple step by step procedure for replacing the power valve. (Image B)

Power Valve 3/6


1. The carburetor does not have to be removed to replace the power valve, but it will make for less of a fuel spillage on top of your engine.
2. On a cold engine, drain the gasoline from the float bowl by removing one of the lower bowl screws and catching the gasoline in a small plastic container. This will minimizes fuel spillage on the intake manifold.
3. Remove the fuel line connected to the float bowls.
4. Remove all four screws that connect the float bowl to the carburetor main body. Notice, there is a gasket on all of these screws. Now, completely remove the float bowl.
5. Gently pry off the primary metering block from the bowl.
6. With the metering block off the power valve will be visible, it is the round brass fitting that sits just above the two main circuit jets in the metering block.
7. Loosen the power valve from the metering block with a one-inch wrench or socket. Note: there will be a small round gasket between the power valve and the metering block.
8. We have found that sometimes even a new power valve can be defective. You may want to test your new power valve before installation. (Image C)
9. The simplest tool to test a Holley power valve is a vacuum pump. You will need to use the small rubber suction cup that is supplied in most quality kits. Wet the suction cup and place it on the power valve. Apply vacuum. If the valve is good the vacuum pump should hold vacuum (generally around 15 to 20 inches of vacuum). If the valve is defecated it will not hold vacuum.
10. All gaskets that have been disturbed will need to be replaced when servicing the power valve. Take care when cleaning not to damage any contact surfaces.
11. It's not recommended to mix brands of parts when servicing your Holley carburetor. Use Holley brand replacement parts. Some aftermarket parts and gaskets are a little different and can cause problems. If you chose to use a different brand of part buy all components required in that same brand.

Testing Power Valve 4/6


Holley does offer a power valve block-off plate that eliminates the power valve circuit completely. Jet size must be increased in order to compensate for the loss of additional fuel under wide-open throttle conditions. We do not recommend blocking off the power valve as this can cause rich conditions and driveability problems throughout all load ranges.

The latest advancement in carburetor power valve technology came nearly a decade ago. Holley decided to add a check valve system into the throttle plate to protect the power valve from engine backfire. This check valve is designed with a manifold reference hole and an anti-backfire check valve that will seal the power valve quickly should a backfire occur. Once closed, the check valve system interrupts the pressure wave generated by the backfire, thus protecting the power valve diaphragm.

If you have an older carburetor or a numbers matching carburetor on a restored car you can employ this same concept by drilling a relief hole in your original base plate and installing an anti-backfire check valve. (Image D & E)

Base Plate 5/6


Backfire Check Valve 6/6


Paul, on vehicles which have been stored without starting for several weeks or months I find it a good idea to remove the coil wire and spin the engine over until the carburetor bowls are full. You can tell when the bowls are full by looking into the carburetor butterfly. With the key off, open the throttle to see if there is a squirt of fuel from the accelerator pump. If so reinstall the coil wire and start the engine.

If you want to have your carburetor re-plated or an older carburetor updated you will find this is left better in the hands of the professionals who specialize in restoring or rebuilding carburetors, such as the guys at Daytona Parts Company you can give them a call at (386) 427-7108 or online at Thanks and good luck.

Got a question for our Tech Corner expert? Just jot it down on a paper towel or a lightly soiled shop rag and send it to us at VETTE Magazine, Attn: Technically Speaking, 9036 Brittany Way, Tampa, FL 33619. Alternatively, you can submit your question via the Web, by emailing it to us at Be sure to put “Technically Speaking” in the subject line.



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