289 carb jetting - Ford Mustang Forum
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post #1 of 10 (permalink) Old 4 Weeks Ago Thread Starter
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289 carb jetting

Hello guys,
I've got a 1967 289 that is stock other than headers, a new intake manifold, and a Holley 4160 600cfm. The carburetor is relatively new to me and I'm having some difficulties with jetting it. When I picked it up everyone told me that the #66jets they come with from the factory are going to be really close to what I need so I accounted for altitude and started with a #63 jet. I however felt that it was way to big. The exhaust smelled like raw gasoline it was backfiring and didnt run well at idle (didnt even drive the car). I stepped down to a 60. The car still smelled like gas and backfired so I went down to a 58. They car seems to be running a little better now but it still backfires occasionally and when I tried driving it for the first time but it seems like it still has way too much fuel. People keep telling me that my jets are way too small and I should just take it to someone to get it tuned but I would really like to get it closer myself.

What size jets should I be running in a 289 with a Holley 4160 600cfm 4bbl.


67 Ford Mustang coupe
289 C code V8

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post #2 of 10 (permalink) Old 4 Weeks Ago
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Stay with the factory size. Unless you stay/live/work somewhere above 6000 feet, you don't really have to worry too much about elevation. Your "rich" smell could be an issue with the choke, ignition or timing. The factory carb settings worked for the 289s over the entire nation for many, many years.

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Coltman, you have a couple of problems running against you here. I did the same thing with my original 289, thinking that it would be better than my 'junky' factory type carb (an Autolite 4100). The first issue you'll deal with is that your engine typically uses more like 300-400 CFM with the stock cam, etc. That means with a 600 CFM carb, you are not pulling enough air through to atomize the fuel very well. The big droplet size will make your engine act lean, but still put a lot of unburnt gas out the tailpipe. This is aggravated by the fact that a Holley doesn't have annular boosters.

In my experience, you can make a Holley 4160 work okay, but never as good as a 4100 or a Summit M-series, because of the much better atomization you get. Their annular boosters start drawing fuel from a lower vacuum signal, and they also make sure it's a fine mist (mostly vapor when you get to high-flow situations). That means better throttle response and economy.


To be perfectly fair, at wide open throttle, the Holley will deliver about the same power. It's only every other situation where it's inferior.
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Originally Posted by Grimbrand View Post
Coltman, you have a couple of problems running against you here. I did the same thing with my original 289, thinking that it would be better than my 'junky' factory type carb (an Autolite 4100). The first issue you'll deal with is that your engine typically uses more like 300-400 CFM with the stock cam, etc. That means with a 600 CFM carb, you are not pulling enough air through to atomize the fuel very well. The big droplet size will make your engine act lean, but still put a lot of unburnt gas out the tailpipe. This is aggravated by the fact that a Holley doesn't have annular boosters.

In my experience, you can make a Holley 4160 work okay, but never as good as a 4100 or a Summit M-series, because of the much better atomization you get. Their annular boosters start drawing fuel from a lower vacuum signal, and they also make sure it's a fine mist (mostly vapor when you get to high-flow situations). That means better throttle response and economy.


To be perfectly fair, at wide open throttle, the Holley will deliver about the same power. It's only every other situation where it's inferior.
So what size jet did you end up running, is a 58 too small. I'm at a little over 5k feet in elevation. I have been worried the carb is too big since I bought it but got a pretty good deal on it new.

I just pulled a few plugs after driving it
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67 Ford Mustang coupe
289 C code V8

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I just ran the stock jets in mine, but I'm at more like 1300 feet elevation. Those plugs look pretty rich to me.

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Originally Posted by Grimbrand View Post
I just ran the stock jets in mine, but I'm at more like 1300 feet elevation. Those plugs look pretty rich to me.
Would you recommend dropping a size?

67 Ford Mustang coupe
289 C code V8

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As said above, I think you have a few issues here.
Obviously the carb is running too rich but that doesn't mean that the jets are too big.
Is your choke fully opened when the engine is warm?
Is the mixture set right?
What is your timing set at?
Backfiring could be retarded timing or your points are bad.

My suggestion is to take it to a shop and get someone to sort out once and for all. They money you spend will be worth it in the end.

15/5/1964 260 Manual Coupe.
23/3/1965 289 Manual Fastback.
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Take it from someone who put a 680 Quick Fuel on a very-cammed 289: You really should get an air/fuel ratio gauge if you want to get a Holley tuned in. You're probably running in the idle transfer slot at cruise, so you'd want to shrink the idle feed restrictor which requires either pulling out the old one and replacing it or tapping the metering block for screw-in bleeds. Or as another option, you can drill and tap the main body transfer slot holes for screw-in restrictors. Then with a 600 cfm carb and a stock cam, you'll be closer on the air bleeds than I was, which determine the change in air-fuel ratio as air flow changes. I had to radically shrink my idle air bleeds from .070" to .036" to get the transfer slot fuel curve right.

By only working on the primary jets, you're only helping the freeway cruise and acceleration. Around town cruising and light highway will be using just the transfer slot. It is tough to get a Holley to be just right, but it is possible. Using adjustable metering blocks/main body with screw in restrictors and air bleeds helps with tunability quite a bit. I'd also recommend picking up David Vizard's book How To Supertune and Modify Holleys. It's good not just for performance tuning, but also mileage tuning tips and tricks.
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I could tell you what a 4100 needs but that doesn't mean much for a non-factory Holley. People who don't live at altitude don't have much grasp of how different it is up here. I have an AFR meter and my 'tuning' stretch of level road is at almost exactly 5,000' AMSL. We live a thousand feet higher than that which makes a full jet size difference simply from the elevation change.

All cars in the 60s were tuned rich and when you bring them up another mile it gets much worse. They will run up here so Ford never bothered to install any altitude jets in their new cars; they were selling them too fast to be concerened with such details. The factory recommended jet size reduction for altitude is a help but its less than the ideal jet change. One of our Mustangs used a 50 at sea level, Ford said 48 for 5,000. In reality, the ideal jet for 6,150 is a 46.

From your description it certainly sounds like a smaller jet would help. But without more information from AFR data its hard to say what you need. I doubt a Holley is much different in how it works than an Autolite; it just has more adjustment you can make. The AFR of an Autolite starts pig rich and leans out as you go through the transfer circuit and only levels out when cruising at 60 or faster. (All details subject to your cars setup.) If you are operating in the transfer circuit then changing main jets isn't going to change your apparent mixture much; the transfer circuit is in charge. The main jets only change mixture when you leave the transfer circuit and are in cruise mode. I don't know where that change occurs for your carb circuit and you probably don't either. Your mixture during acceleration is determined by your power valve which is a seperate circuit that overrides everything else when vacuum is low.

The attached chart is for an unmodified 289 using a factory Autolite driving at constant road speed neither accelerating or decelerating. As such it shows nothing about acceleration when mixture will go to somewhere around 13 for max power. Notice that below 45-50 MPH main jets have no affect. The differences down there are from experimental error and/or different temperatures on the days the data was recorded.

Trying a smaller jet for a test won't hurt anything and should answer some questions. As suggested, if you want to really know get an AFR meter.
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EVERY ONE I SET UP WE STARTED AT 55 JET SIZE!!
Sometimes we even dropped that a bit. Other than 3/4 to full throttle most aftermarket carbs will drown a small block ford. Another issue is to make sure the needle and seat and float are set to fill the bowls to the correct height! Some times the float arm is a little bent so it will dump excess fuel down the venturies causing a rich mixture that doesn't occur til after start and run.
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I'D RATHER GO SLOW THAN NOT GO AT ALL
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