1975 New Yorker with 440 engine and Holley 4175 spread bore carb. Replaced timing chain and engine starts and runs smooth until engine comes off choke. Then the idle is not so smooth. Tried adjusting choke but that doesn't seem to be the problem. Now I'm thinking that because I did not run the engine for maybe 7 months that the idle circuit became clogged. I'm currently spraying the air bleeds on top of carb with brake cleaner to try to clean the idle circuit and then maybe putting a can of carb cleaner in the tank and doing some "spirited" driving. Any other ideas to improve the idle? Any thoughts appreciated. Paul
cleaning a clogged idle circuit
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Might as well use carburetor cleaner to clean a carb. It's made for fuel and those deposits.
Always use flammable cleaner with a carb on the car. Many brake cleaners are non flammable. Your spark plugs won't like that.
Always use flammable cleaner with a carb on the car. Many brake cleaners are non flammable. Your spark plugs won't like that.
Sometimes unscrewing the idle mixture screws and spraying carb spray in the carb can help.
Knebel
Senior Member
Here is what i would do, given you put the work in and the car sat. Id order a rebuild kit for the carb and an ultrasonic cleaner. Completely fill the ultrasonic cleaner with berrys injection cleaner and then put all the carb parts into it and give it a good cleaning.
Unlike the similar Edelbrock/Carter carbs, the restrictions in the Holley carbs are in the metering block. NOT accessible from "the outside" of it, but through drilled passages. What you are spraying on top, in the venturi, are the "air bleed" correction/fine tuning calibration items. THOSE bleeds go rich when they get deposits on them to decrease their size.
Get some fuel cleaning additive with the "PEA" additive in it to clean things up. Gumout is one brand that has that additive in it. It also cleans the combustion chambers, too.
As to a rebuild, that can be done . . . but my experiences with my 4175 is that sometime in the 1990s, Holley changed their metering block/plate gaskets, plus the bowl gaskets, such that they look like they have some sort of shellac coating on them. With ALSO glues the two parts they are between, together! It might be difficult to get the float bowls off, but impossible (without breaking something!) to get the metering block off of the main body. I did get the rear metering plate off, plus the metal plate it seals against, but had to sand the paper gasket OFF of the metal plate. I ended up getting another carb! Just be advised.
I'd recommend the fuel cleaner first. No real need for "spirited driving", to me, unless that's what you desire to do. IF you want max cleaning, you might do some shorter trips instead, to allow the cleaner to soak the suspected deposits, between times. "Spirited driving" can help any carbon deposits in the combustion chambers, I suspect.
I concur on spraying a good carb cleaner into the idle mixture holes (with the screws removed). Look at the pcv passages, too. Brake cleaner might work, but not as well as carb cleaner will.
Just my experiences,
CBODY67
Get some fuel cleaning additive with the "PEA" additive in it to clean things up. Gumout is one brand that has that additive in it. It also cleans the combustion chambers, too.
As to a rebuild, that can be done . . . but my experiences with my 4175 is that sometime in the 1990s, Holley changed their metering block/plate gaskets, plus the bowl gaskets, such that they look like they have some sort of shellac coating on them. With ALSO glues the two parts they are between, together! It might be difficult to get the float bowls off, but impossible (without breaking something!) to get the metering block off of the main body. I did get the rear metering plate off, plus the metal plate it seals against, but had to sand the paper gasket OFF of the metal plate. I ended up getting another carb! Just be advised.
I'd recommend the fuel cleaner first. No real need for "spirited driving", to me, unless that's what you desire to do. IF you want max cleaning, you might do some shorter trips instead, to allow the cleaner to soak the suspected deposits, between times. "Spirited driving" can help any carbon deposits in the combustion chambers, I suspect.
I concur on spraying a good carb cleaner into the idle mixture holes (with the screws removed). Look at the pcv passages, too. Brake cleaner might work, but not as well as carb cleaner will.
Just my experiences,
CBODY67
I don't think it's a clogged idle circuit.
I would try adjusting the idle mixture a bit. From what you are saying, when the choke is on, it idles OK. When the choke comes off, it's idling rough. So that would be my first place to go because it's coming from a rich condition with the choke on and then it starts idling rough with the choke off. Look for the Holley instructions on setting the idle mixture with a vacuum gauge. A google search should come up with that instruction sheet if you don't have one.
Another thing to try (do these one at a time) is to bump the timing up a couple degrees and see how it runs.
Always do the simple stuff first.
Ripinator
Old Man with a Hat
Ninety - five percent of all carburetor problems are electrical . . .
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