Reading Plugs
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Senior User
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From: Flaherty, KY
Reading Plugs
Are reading spark plugs an accurate way to determine if the mixture is correct? When I rebuilt the carb on this truck (in sig) I went from a 61 jet down to a 59 because of elevation here. I wondered if that was too small, but the plug I pulled is a nice tan. Is this indicative of a good mixture?
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From: Flaherty, KY
I've put about 4-5k on the truck since I rebuilt the carb. Plugs, wires & all other maintenance items were changed at that time. I swapped clusters in between so not sure of the exact mileage since all that work. Maybe I was spot on for the carb jetting.
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From: Northeast, OK
Jim knows a lot more about this than I. Heck, lots of people do. But in my opinion there are several variables in plug color, with the main two being air/fuel ratio and the other being the heat range of the plug. In other words, a hot plug can burn the deposits on the insulator off, making it look like your air/fuel ratio is lean. Or a cold plug can make the plugs look like the AFR is rich.
If your engine is fairly close to stock then the plugs recommended by the factory will probably be correct. If that plug is one of those then you are close. However, I think you might be just a tad rich. But I would rather be rich than lean.
If your engine is fairly close to stock then the plugs recommended by the factory will probably be correct. If that plug is one of those then you are close. However, I think you might be just a tad rich. But I would rather be rich than lean.
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From: Flaherty, KY
Plugs are Denso T16PR, comparable heat range to the stock Motorcrafts and gapped to .044. Before I rebuilt the carb and made adjustment the plugs were filthy black, caked with carbon, and the truck idled like it had a hot cam, and belched black smoke. Seems like I'm making progress.
The mixture is controlled by the jets, correct? Do the idle mixture screws have a noticeable effect on plug color? I didn't set the mixture screws precisely at all, since I can't get a wrench in there. I just backed them off 1-1/2 turns each before I installed the carb. It's idled fine since.
The mixture is controlled by the jets, correct? Do the idle mixture screws have a noticeable effect on plug color? I didn't set the mixture screws precisely at all, since I can't get a wrench in there. I just backed them off 1-1/2 turns each before I installed the carb. It's idled fine since.
pedant
Joined: Jan 2008
Posts: 23,576
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From: EXTREME southwest CT
Idle shouldn't have an appreciable effect on plug color unless you spend a lot of time idling the truck.
As Gary said, I'd rather see them a little rich. (especiallly given your altitude)
There are no real jets in a 4180.
Metering plates are non adjustable and the passages are unique to this carb.
More info on how they are internally can be found in a HotRod reprint at the Flemworld mustang website.
4180's were also found in the mid '80's notchback H.O. Mustangs.
As Gary said, I'd rather see them a little rich. (especiallly given your altitude)
There are no real jets in a 4180.
Metering plates are non adjustable and the passages are unique to this carb.
More info on how they are internally can be found in a HotRod reprint at the Flemworld mustang website.
4180's were also found in the mid '80's notchback H.O. Mustangs.
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From: Flaherty, KY
I think I've stumbled upon that website, which helped me gain some familiarity with this carb...
However, the primary circuit has its own jets, no? I know I replaced two of them, but the secondary side had a metering plate with no jets.
However, the primary circuit has its own jets, no? I know I replaced two of them, but the secondary side had a metering plate with no jets.
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pedant
Joined: Jan 2008
Posts: 23,576
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From: EXTREME southwest CT
Yes there are replaceable jets on the primary side.
You already said you stepped down.
I wonder if Ford has high altitude instructions for this carb?
If it is running well and the plugs look like that I doubt you can do better without installing a wideband and observing a/f in real time under actual driving conditions.
You already said you stepped down.
I wonder if Ford has high altitude instructions for this carb?
If it is running well and the plugs look like that I doubt you can do better without installing a wideband and observing a/f in real time under actual driving conditions.
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Even if it isn't a friend, just find a shop that does engine tuning. There used to be one next door to a privately-owned & operated emissions inspection station (1981 & older only) a bit north of Denver but that'd be a fair hike from Pueblo, I'd have to imagine there are still such places near you (or maybe COS if need be). This is necessarily a common thing needing done at this altitude.
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From: Flaherty, KY
Good advice. I'm sure the tuning shops are dime a dozen in Colo Spgs, what with all the younger military crowd. I'll start looking around. I thought maybe reading the plugs was accurate enough, but I figure do it right once, and I won't have to mess with it after that.
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