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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 07-19-2012   #1 (permalink)
Potrice is offline Rookie


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Default 1966 mustang 625 CFM street demon carb too big for my stock 289?

1966 mustang convertible, 289 4v, C4 3speed automatic (rebuilt), 2.80 gears, 12 timing, electronic ignition, new MSD Blaster 2 coil, new spark plugs and timing chain. The engine is stock and was never rebuilt.

I was previously using the original Ford 2v carb and just installed a brand new 625 cfm street demon carb. Here's a link to the carb I'm using.

Demon Carburetion 1901 - Demon Carburetion Street Demon Carburetors - Overview - SummitRacing.com

I Installed the carb straight from the box and the car is running good but I noticed a HUGE drop in MPG. Seems like the stang is using twice the gas it was when using the original Ford 2v carb. Is the car running too rich? I'm also a very conservative driver and my tires have more than enough air in them.

Is anyone else using a Demon 625 cfm? Is it too big for a stock 289? Feedback would be appreciated!

Thanks!
Patrick
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Unread 07-19-2012   #2 (permalink)
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I would say its to big, did you replace the manifold or did you just get one of those 2-4bbl adapters. I had a 600 on a fully modified 289 but I had a cam heads the works. It's not the end of the world if you willing to do some mods.
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Unread 07-19-2012   #3 (permalink)
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I am sorry, I didn't see you originally had a 289 4v. That changes alot of things, are you running a noticeably rich mixture, check to see if the spark plugs a getting fouled up and the exhaust is really black, if the motor has never been rebuilt check the compression
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Unread 07-19-2012   #4 (permalink)
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The car came with a 2bbl carb and I changed the manifold. Thanks for the information 90lxconvert. It really sucks if the carb is too big and I have to change it...the throttle response is killer with this carb...argh!
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Unread 07-19-2012   #5 (permalink)
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I put a similar Edelbrock carb (600-650?) and Ford manifold on my rebuilt 289 that cam with a 2v carb.
I had the motor dynoed and ashed if the carb was too big and he said no. It runs great.
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Unread 07-19-2012   #6 (permalink)
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I gotta ask, why didn't you look into it before buying it? A 625 for a stock 289 is quite large. Check out a horsepower book, or even go to Holley. My horse power book (David Vizard) as a rule of thumb state 1.8-2 CFM per engine CID for a performance engine. That comes to 578 CFM for a 289 using the high end of that basic CFM estimate.

Go to Holley's interactive carb selector, even telling them you run a highly modified 289, they still suggest a 570 carb.

Holley Interactive Carburetor Selector


An oversized carb decreases your air velocity. When the air is moving too slow it reduces power. The same phenomenon is observed with exhaust pipe diameter. Your engine performance is probably going to be optimized with a smaller carb.
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Unread 07-19-2012   #7 (permalink)
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Your 625 will work on that 289,,,The jets on the demon 625 are large for that engine,,( this accounts for your mpg loss),,I would try #60 primary and #68 for the secondary,,,and see what happens,,work from there,,jets are cheap,,and easy to change,,also get a set of re-usable carb gaskets,,in case you have to change the jets a few times,,,if you do a little research you can find out how carbs work in relationship to engine size,,cfm and fuel delivery,,,
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Unread 07-19-2012   #8 (permalink)
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I used a demon Jr. 625 on my stockish 289 and added a 1/2" phenolic spacer. It ran really well and mileage was a little better than my 2bbl. when i kept my foot out of it. I'd try the jets as Seph10 suggested to fine tune the carb to the engine.
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Unread 07-20-2012   #9 (permalink)
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joseph and jon are both correct. the 625 should work really well on a 289. try the jetting that joe sugested and double check everything else. it should not have killed the milage like that.
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Unread 07-20-2012   #10 (permalink)
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Lizer is right on the money. If a bigger carb worked as well as a smaller one on a stock engine, Ford could have saved a lot of money by just slapping 750's on everything from 260's to 429's, but physics just don't work that way! You will always get the best performance with an appropriately sized carb, not just a big one tuned and jetted down for a smaller motor. A stock 289 has no hope of ever, ever, pulling anything even close to 625cfm through the carb at WOT.
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Unread 07-20-2012   #11 (permalink)
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a little extreme, don't you think joe....45 CFM bigger is not going to kill it. this is the real world, not a theory. he already has the carb. better plan is to figure out where his milage went!
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Unread 07-20-2012   #12 (permalink)
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I only suggested re-jetting to save him some money,,no sense buying a new carb when you don't need one,,as potrice stated he is a Conservative driver,,,re-jetting should work fine,,if he was going to race the car and need to get every ounce of power out of the car,,then I would change the carb,,,he can re-jet the carb, the engine will run good and he should regain some of the mpg back,,of course he won't get the mpg he did with the 2v carb,,but he can come close,,,
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Unread 07-20-2012   #13 (permalink)
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Here is a link to a CFM calculator. Since your engine is basically stock The 625 is a bit large for your engine.
Carb CFM Calculator

I ran a 625 on a 383 Mopar years ago with good response and decent fuel mileage.
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Unread 07-21-2012   #14 (permalink)
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+1 for down jetting! Too late to throw the baby out with the wash!
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Unread 12-08-2012   #15 (permalink)
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The Street Demon 625 IS NOT based on a Holley carb, it's a hybrid of AFB and Quadrajet engineering principles. Therefore, you cannot compare its CFM flow with a Holley, and you also cannot conclude that the carb is too big based on online CFM calculators.

2 things to keep in mind: 1) the small 1 3/8" primary bores are going to do most of the air-fuel work, 2) the CFM flow from the secondaries is controlled by the air valve above their butterflies. The primary bores have a triple-stack venturi, design concept taken directly from a Quadrajet, allowing crisp throttle response and good fuel ecomomy. The air valve's purpose is to limit CFM flow to the secondaries. This means the OP's 289 will "suck" what it needs, - it's not going to get force-fed the full 625 CFM if it doesn't need it.

Proper jet/rod combos and power piston springs should correct the fuel economy issues, as well as possibly improving power & performance over the previous carb.
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