Thread: Carb settings? Ford 428 FE with Holley DP 650 in a Cobra clone

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All,

I'm "going back" in time, and tuning my first Holley DP in about three decades. (I've been running EFI since then). I know it's impossible to "tune a carb" via a forum posting, but I'm interested in thoughts about ballpark settings, jets, etc.

Some basics: 428 FE, ported heads, stock valves, headers and (loud) sidepipes. Carb is an out-of-the-box 650 CFM DP (prefer this over a 750, since I'm mostly street-driving this clone). Lopey idle at 750 RPM, and 12" vacuum. Top-loader four speed. Manifold is single-plane, Edelbrock. Cam is Crane, .533" intake/.566" exhaust,208/218 duration. The car weighs about 2,400 (wet). About a 5,600 RPM redline (street driving).

Carb currently has 30cc accelerator pumps, 65 primary jets and 73 secondaries (and I suspect both will be pretty lean on the 428). Nozzles are TBD--not yet picked yet.

Thoughts--about jet size (primary and secondary), nozzle size (primary and secondary), and carb cam (color)? May 2013-26.JPGAgain, not expecting to get a "final tune" via this posting, but if any of you 428 rodders are out there, I'd welcome your thoughts.....

I've done many carbs for many different applications and I do seriously brlieve that a 650dp shouldn't be used on anything except very remotely possibly maybe a dual quad set up------

I thing most of the street driven Ford FE 427-8 worked very good with the 950 vacume seconary carbs-they had big enough primaries that let you drive pretty modestly thru the gears------

I don't think a 650 would give the flow that a 428 needs, I ran a 406 FE with a 780 vacuum secondary carb. It would handle the engine fine other then leaning out just a bit on the big end, but it was in a much heavier car, too.

201 and 218 @ .050 in a 428 and you say it's "lopey"? I would have to hear it to believe it. That cam is just about as small as that carburetor for a 428. Ford was notorious for using small carburetors, but even the stock carburetor on a 428 was bigger than that.

That said, a couple of things for you. First, the accelerator pumps and jets can be thrown in the ditch when considering tuning the carburetor for curb idle, because neither circuit is even in the equation at idle. Curb idle RPM and air mixture is the first thing you want to iron out with the carburetor, BUT before you do that, you should make SURE the timing is optimal, first.

I bring the ignition timing up, because you have a BIG red flag of low idle vacuum signal for a BIG engine with a TINY camshaft. That engine should be close to if not over 20 hG ov vacuum at idle. That's a very, very mild cam and should have a healthy vacuum signal. 12hG ain't it.

I bet you a whole dollar the timing could use advancing. Seen it a thousand times. First, what's the static compression ratio? I'm talking about the KNOWN MEASURED compression ratio, not the "It's got 10:1 pistons in it" stuff. Better yet, stuff a compression tester in a spark plug hole and tell us what you have for cylinder pressure. THAT could get us even closer.

If that 428 doesn't have any more than 150 PSI cranking pressure and you have only 12hG vacuum at idle, You can raise the initial timing up to around 20*, but you'll need to limit total timing to between 32-36*, depending on your cranking compression reading. Get that first and let us know.

It is IMPERATIVE to get the timing worked out FIRST, then proceed to the carburetor or you'll be chasing your tail and never get it right.

I owe all of you some more info....!

Denny, Rusty.... Sorry, but I "fatfingered" the cam info. The cam is a Crane H-272-2 (aka 343941) hydraulic flat-tappet. The CORRECT specs are advertised as 272 Intake, 284 Exhaust. (I tried to correct my first posting several days ago, yesterday, but for some reason I could not log on until just now).

I should also mention this car is being revived. It's been in storage for 24+ years, and that cam was pretty popular back in the 1980s and 1990s. In fact, the profile is very similar to the Ford Factory Performance camshaft C8X-6250-C that ran in the Cobra Jet 428's. Summit still sells this particular cam, and Crane characterized it as "good low and mid-range torque, good idle, off-road and performance cam for 2600-3000 cruise RPM...."

As for the carb--I'd agree if I was spinning the mill to a 6,500+ redline, I'd run a much larger Holley. In fact, I do have a 750 CFM that I'm currently rebuilding. This carb (the 750) will probably be the one I'll ultimately run. It's a LIST 4779-2. which makes it "period correct" for the car, and is one that came with the "high performance" FE engines.

Right now, I'm a little concerned about soft valve springs, since the car was stored without the lifters being loosened. For that reason--until I get into the engine internals later this Fall--I'll granny-shift and hold the revs to about 5,200 RPM to avoid any float. With that redline in mind, a 650 CFM DP should work for the remainder of this summer, even with a have a Ve of about 92-95%. I'll hang the 750 on it, after I do a tear-down and measure the valve springs.

Rusty, agreed about the curb idle. I realize the shooters, accel pump and cam do not play in setting idle. Having said that, I have adjusted the carb to "square up" the transition channel (about .040") to minimize their contribution to ldle. I have the idle mixture jet set for max vacuum (about 13" of Hg). Cranking compression is 150-160 for all cylinders, and I've done a leakdown test on the manifold. Good thoughts, about the timing--and that will be my next check. Thanks for comments regarding ignition timing!

(Sorry for the typo on the cam.....)