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Discussion Starter #1 (Edited)
Just bought a 1966 GT coupe with 289 4bbl. Ran good for a few weeks and now having carburetor issues. Edlebrock Carb Model 1403. Will go from running great to really struggling. Restored very well by previous owner. Distributor looks good. No moisture, contacts good, timing good. Put new plugs in and cylinders 1,2,5,6 have heavy deposits on them after only a few miles of herky-jerky driving (really becomes an issue when giving it more than just very gentle throttle). The others remain clean. Any thoughts out there?
 

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It is suspicious that there are two cylinders, that are next to each other, that seem to be having a problem firing. Two cylinders on each bank, both towards the front of the engine, no less!
Before proceeding any further, you need to do a compression/leak-down test on the engine to verify that there is acceptable compression in all of the cylinders.
 

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I once bought a 60’s Mustang for my son as his first car, and the engine had “just been rebuilt” according to the seller.
What he did not tell me is that he used original 60’s heads and valves when he did so, which are the WRONG parts to use. Today’s unleaded fuel requires hardened seats and valves, which are different parts. This required us to completely rebuild the top end (heads and valves) when the new/old parts lost compression in a couple of the cylinders.
I hope this is not the case for you, but your symptoms sure sound familiar!
 

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Discussion Starter #4
Will check out the compression but just heard from someone that regularly works on classics having the same issue with the same carburetor on a 1966 Mustang and thinks he has it pinpointed to an issue with the carburetor, most likely a plunger that has gone bad just letting fuel flow in. Hopefully that will do the trick versus rebuilding the heads and valves!
 

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I hope that it is just the carburetor. But the cylinders that are misfiring seem odd to me... Pretty much the entire front of the engine.
Edelbrock carbs are fairly user friendly to work on, so it shouldn't be too difficult to determine if it is dumping fuel.
I would expect that the exhaust would smell like an overly rich mixture in any case, so you will just have to do some good old fashioned diagnostics to narrow it down.

Keep us posted!
 

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Discussion Starter #6
Appreciate the suggestions! Ended up being a carburetor issue, accelerator pump in particular. Decided to just put a new one on and will rebuild the other as a "spare". New points and a new condenser along with the carburetor, it is running better than ever! Still a few tweaks needed to have it dialed in perfectly but we're almost there!

1966 GT.JPG
 

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Really wish I owned that car! My original 65 K, bought in Sept., 64, ran like a champ, cross-country many times. How I miss it. Those days back in '66, '67, in rural Utah, heading for Las Vegas, two-lane roads in the middle of nowhere, little Mormon towns with names like Parowan, Panguitch, crossing what then was Capitol Reef National Monument, most beautiful place I haver seen on this earth!
 
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