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EFV-8 Club Forum / General Ford Discussion / '46 Coupe 59AB Rough Idle

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Discussion Topic: '46 Coupe 59AB Rough Idle

51woodie

08-21-2026 @ 7:38 AM

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I need some suggestions on where to look for blockage in the carb on my stock ’46, 59AB with Model 59 carb. The engine started idling rough and these are the steps I’ve taken to troubleshoot the condition.
Vacuum at idle is steady at 20”, shorting individual plugs gives little change in RPM. Idle is 525 RPM
Compression is 120/125 on all cylinders. Plugs 1, 4, 6, 7 are a bit sooty, plugs 2, 3, 5 and 8 are dark tan.
Left idle mix screw in and out slows idle, right screw in slows idle but out makes no change.
I changed the carb out with a spare and the rough idle is gone.
My assumption is there is a blocked passage somewhere, and the question is where do I start to look as I disassemble the carb?

ford38v8

08-21-2026 @ 1:08 PM

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As both left and right idle screws are fed from the same source, I would assume the difference between the two are the condition of the screws themselves and the seats they act with. Something to keep in mind when adjusting those screws is to start with a fully warm engine, adjusting each screw separately while maintaining idle speed with the idle adjustment at the side of the carb. Each is re-adjusted after the other has been adjusted closer to optimum.

Alan

51woodie

08-21-2026 @ 1:54 PM

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Thank you for your reply Alan. What causes me to think there may be a plugged passage somewhere in the right side, is that the engine was running well on my trip to town, then started to act up when I stopped to turn into a driveway. The carb was rebuilt by Charlie NY about two years ago, and the idle mixture screws were never adjusted after I got it, as the engine ran perfectly when the carb was installed. Also, I believe the difference in the plug colors indicates a fuel mixture issue.

carcrazy

08-21-2026 @ 4:07 PM

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The problem is with the right side. The right carburetor bore feeds cylinders #1,4,6&7. Look for a blockage somewhere in the idle passages for the right bore of the carburetor. The right side idle tube (jet) is most likely plugged by some dirt or debris that was in the gasoline. You will most likely find the blockage at the bottom of the right side idle tube where fuel enters it from the float bowl.

This message was edited by carcrazy on 8-21-26 @ 4:10 PM

TomO

08-21-2026 @ 6:22 PM

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The right side is apparently running rich. The idle air mixyture bleed is probably blocked. See the You Tube video for the idle circuit of the Holley 94 Carb.

https://www.youtube.com/watch?v=oqlcX6N_brM

Tom

51woodie

08-25-2026 @ 12:00 PM

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Tom,
Thank you for the video link. I watched it and a few others, so I'm sure I can get the carb sorted with no issues.

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