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Old 06-13-2010, 02:48 PM
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Ford 4100 carburetor

I have a 66 Thunderbird with the original 4100 series carburetor atop a stock 390 FE motor. I, with the help of a reliable and knowledgeable enthusiast rebuilt the carb with a good quality kit. The secondaries will only crack open slightly and stick there until I push the rear armature back in. The idle circuit is fantastic. The primaries give everything they've got and work flawlessly from idle up through higher acceleration. We tested the carb on another 2 engines in fine working order to the same conclusion. I was told that the rear cover for the secondary diaphragm could be slightly warped and that this could course a leek. Any ideas on this? Yes we did soak the disassembly for 24 hours carb parts cleaner and thoroughly clean everything.

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Old 06-14-2010, 03:24 PM
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There is a secondary diaphragm check ball called for but that's often missing (the correct size check ball, that is) from many kits. If the ball is missing or the wrong ball installed, or an old check ball is still in there and has become stuck, this will cause the secondary to not open as it should. The check ball is 1/8" and the passage it sits in is just a whisker larger, so it's possible for it to well and truly get stuck in the passage.

Otherwise, the actuation is by the diaphragm/chamber- so all must be sealed to hold vacuum and not leak.

It's possible to rig a test to pull a vacuum at the port in the secondary side that feeds the diaphragm to see that it's sealed adequately.

Please be aware that jazzing the throttle at rest- no load on the engine- will NOT allow the secondaries to open. The vehicle needs to be under way, and under a load before the secondary will open.

The choke lock-out has to be in adjustment and free to move, otherwise it can keep the secondary from opening.
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