Mailbag: Rich-Running Ford 302 Won’t Shut Off in Mustang Cobra II—Please Help

[Image/ NRMA Motoring and Services, CC BY 2.0]


Q: Under the hood of my 1976 Ford Mustang II Cobra II is the original 302 bored .030 over.

It has flat top forged pistons, Sealed Power chromoly rings, a reconditioned .020/.020 crankshaft, performance reconditioned rods, high-volume oil pump, a Summit cam with new lifters, matched valve springs and locks, Summit pushrods, and a double roller timing set.

The heads are 1970 351W with new valves, hardened seats, a three-angle valve job, and COMP Cams Magnum roller tip rockers.

The engine also has a Weiand dual plane intake, a Holley carb, a Carter race fuel pump, a Crane HI-6 ignition, ACCEL Super Stock coil and distributor, and Bosch plugs gapped at 0.35.

The timing is set at 10 degrees initial and 32 degrees total.

At idle, I have the idle screws all the way in, and the engine will not shut off and runs very rich. The carb has a 4.5 power valve with a vacuum gauge, and in gear it has about eight inches of vacuum.

The car has been sitting for about eight months with 90 octane treated fuel.

Could the fuel be bad? Do I have the correct power valve? Got any other ideas?

A: We believe your power valve is correct.

Your problem is probably carb-related, though.

We recommend disassembling the carb and checking all of the passageways in the baseplate and metering block.

Also, check the main body for obstructions in the air bleeds and the small holes at the bottom of the casting.

If this doesn’t turn up anything, try adjusting the secondary butterfly angle.

There’s a small screw located on the bottom of the carb near the secondary shaft on the passenger side. Turn this a half-turn in to open up the secondary butterfly a bit.

This will allow you to close the front throttle blades some, which will give you more control from your mixture screws.

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  1. Had same problem on my 67 Fairlane 289. Turned out being wrong washer/casket for power valve. Ordered new power valve and noticed the gasket was different.

  2. Throttle shaft worn out

  3. Huntley Hennessy says:

    Most likely power valve related. Blown power valve diaphragm or wrong gasket installed or damaged. Sometimes the gasket will split when over tightened.

  4. J j e s o n e k says:

    I agree with the text your front butterflies are not closing enough to cut the air off and make it stop running and your fuel adjustment will be more available on your idle screws on your jets

  5. Gilbert Crook says:

    Very possible that the secondary butterflies are not closed. They may be not closing completely or the set screw for the closed position is too far open.

  6. Power valve blown diaphragm, all it takes is one backfire!

  7. Replace the power valve, double check the float level. Make sure you have no major vacuum leaks,you know basics stuff.js

  8. Davidmbunnell says:

    Fuel pressure? I see no mention of what it is. It has to be within the carb limits. I need to know what model Holley. The manual for it will tell you how much the pump pressure limits are.
    What fuel pump are you using. . The information is too vague. Pump output cannot exceed the carb limits. If so it has to have a quality regulator with a quality gauge to set it mounted as a matching set of components.

  9. The current assumption is that you have the throttle plates ope. To far and are off the idle circuit and into the primary circuit. Pull the carb off and check power valve to ensure it’s not blown. Reset the idle mixture screws to 3/4-1 turn out. Reset the the primary and secondary throttle plates to only cover half of the idle transfer slot. This gives you base line to adjust the carb from.

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