Carburetor causing surging idle

[thegreatjustino]

Stock '67 Bronco. 289 with the original 2bbl carb. Idles terribly and feels like it's going to die every time I come to a stop sign. Throttle response is great. Idle surges up and down by nearly 100 RPM.

I have removed the EB carb and installed the 2bbl Motorcraft from my '65 Mustang and the problem goes away. Bronco purrs like a kitten with the Mustang carb.

Is there anything specific to look for with the EB carb, or am I better off just doing a complete rebuild?

Oddly, this is the second carb and Bronco I have had this issue with. I had the carb from my '73 rebuilt by the hacks at gottafish carbs a few years back and it did the same surging at idle but did it even worse than the '67. Same situation there, problem goes away when I put the carb from my Mustang on it.

Very frustrated...

 

[Rustytruck]

First thing I would do is check for vacuum leaks at the carb. Then I would check the float level. Then I would check the powervalve for leakage.

 

[jckkys]

The '65 and '67 2100s for 289s are basically the same. Most '65 289 2100s have a 1.14" venturi and most '67 2100s for 289s have smaller 1.02" venturis. The Bronco carbs had manual chokes and all were Autolite trademark until 1971 when Ford lost rights to the trademark and had to call Ford parts Motorcraft. 1970 and 1971 2100s are identicle and were made by the same people in the same factory. None of this tell us what your idle problem is. Surging is usually a lean condition. Assuming the idle mixture is correct, dirt in the idle feed restriction may be the cause. A standard cleaning and rebuild would fix it. Some heavily worn carbs have throttle shafts that are loose in their bore. This can make the idle mixture impossible to adjust correctly. Bronze bushings fix that. This is far more common with QJets, but some 2100, 2150, 4100, and Holleys are badly worn too. Fortunately they all have 3/8" throttle shafts, so the common Quadrajet throttle repair kits work for Ford and Holley carbs. This repair makes carburetors last almost for ever.

 

[thegreatjustino]

jckkys, I've commented before about your knowledgeable posts when it comes to carb tech.

I was hoping you'd contribute to this thread when I posted. I bought this Bronco from the original owner a couple of years ago, and he couldn't remember if the original carb had ever been rebuilt. There's a decent chance that it hasn't so that's probably in order.

 

[cldonley]

My money's on the bushings. Shaft moves around allowing air to enter the carb leaning the mixture and causing the surge. Had the problem on an airplane before. Engine would try to die on final approach at low altitude...that will get your attention!

 

[bronconut73]

My money's on the bushings. Shaft moves around allowing air to enter the carb leaning the mixture and causing the surge. Had the problem on an airplane before. Engine would try to die on final approach at low altitude...that will get your attention!

Throttle shaft bushing leakage is a common issue on old Quadra jet carbs.