Home | Mustang Magazine Online

Diagnostic Detectives - Wiring

Thursday, 06 May 2010 07:43 |

Written by Brandon

Restoration

Diagnostic Detectives

The Case of the Runaway Idle

by Noel Jennings and Tom Shaw

photography by Noel Jennings

Welcome To A Different Type Of Tech Article

Idle hands, it is said, are the devil’s workshop. Whether it was the devil’s handiwork causing idle problems this ’65 coupe, we’re not sure, but we do know that the owner tired of his engine idling like it was possessed after a cold start, and wanted it fixed.

The 289 4V Mustang rolled into R&A Motorsports shop, where lead mechanic Adam Hoaglin confirmed the complaint that the engine runs “super high” when started and doesn’t “kick down” by stabbing the gas pedal.

“Initially, we thought the new carburetor needed adjusting. We started adjusting, but by this time the car was warm, and the problem only happens on cold start.”

So the usual screwdriver therapy on the idle mixture and idle speed screws was not correcting the problem.

A review of the 1965 Ford Mustang Assembly manual revealed that there were some missing parts — the choke tubes that route hot air from the exhaust manifold to the choke’s thermostatic spring housing were gone. A tiny vacuum port in the carb pulls warm air, heated by the exhaust gases, through these tubes to the choke spring to get it warm at the earliest possible moment, speeding the choke’s release. On this car, the steel tubes had long since rusted away, and the vacuum port had been blocked to prevent the vacuum from leaking. As a result, the choke’s warm-up was delayed, and the engine continued on high idle too long.

When properly adjusted, the choke spring closes the choke plate on top of the carb, and also sets a fast-idle mechanism on the side of the carburetor that engages to increase idle speed once the accelerator is pushed. So when the driver pumps the gas pedal once or twice before starting the engine first thing in the morning, the fast-idle mechanism is also engaged. A screw adjusts the fast idle speed, and it hits steps that increase idle speed the farther the choke closes, so the colder it is outside, the farther the choke plate closes, and the faster the cold idle will be. Once the choke spring warms up a little, even if the engine has not yet reached its full operating temperature, a jab at the accelerator should release the fast-idle mechanism if it’s clean and properly adjusted.

1. We’re working on an A-code 289 4V engine with a Autolite 4100 “cracker box” carb. It’s simple and easy to to work on.

2. After consulting the assembly manual, we learned that the choke tubes were missing in action, and would need to be replaced for the choke — and idle — to work right.

3. National Parts Depot stocks just what we need, part number 9819-1A, a complete kit with both steel tubes, insulation, connector hose, and all other needed connectors.

4. The tip of the tube was still stuck in the exhaust manifold and needle nose pliers were not budging it. The remedy was to take a drill with a ¼-inch bit and drill out the remains. There are two holes, one on the top, one on the bottom. Both need drilled.

5. To get to the bottom hole, we used a right-angle attachment. Use only as much pressure as is needed to make sure you don’t get the hole crooked or enlarged where it won’t hold the tube.

6. With the holes opened up, it was time to begin installing the new parts.

7. Start by sliding the insulator onto the tube with flanges on only one end.

8. The flanged end fits into the hole in the bottom of the exhaust manifold (inset), then rises and bends towards the carburetor.

9. The insulated tube fits into the hole on the top of the manifold. Up top, a brass compression nut slips over the unflanged end and connects it to the choke spring housing.

10. The rubber hose connects the fitting on the carb to the uninsulated tube, completing the circuit. Now the carb will draw filtered air down through the exhaust manifold and back up into through the choke spring housing and into the carb, exposing the temperature-sensitive spring to the first warm air the engine produces.

11. The last step is to clamp the two tubes together with the supplied clamp. With the missing parts in place, the carburetor was much more responsive to adjustments. With minor adjusting, the car started and idled beautifully. The kit cost less than $25, installed in about and hour, and the customer was thrilled. This case of the Diagnostic Detectives is closed.

Last Updated (Thursday, 06 May 2010 16:09)

Diagnostic Detectives - Wiring

News Blog