Last year I switched to a Holley 390, and the driveability improved significantly.  Also, latest UOA indicates that there is MUCH less fuel dilution and that wear is back to where it should be.  Yes, it lost a LITTLE at WOT, and the intake roar isnt quite as loud with the smaller carb, but the throttle response and better wear internals more than makes up for it, IMHO.

Hope this helps!

Best,  

Much as I like the 390, and I do, You are the first person I ever heard with a problem that Edelbrock couldn't or wouldn't solve. Most folks like the way they work. Not sure what your problem was, but something was wrong. Not Jets either unless they were not matched to the needles.

Seems to be a misconception out there that large carburetors make an engine run richer.  In reality, the engines actually run leaner if the carb is oversized.  But I’ve had good luck with carbs up to 780 cfm driving around just fine with the Y engines.  It’s just when the secondaries are opened that may force the issue on carb sizing but with an oversized vacuum secondary carb, air flow is diminished enough that the secondaries are either late in opening or will not open at all.  It goes without saying that almost any double pumper carb will give the engine too much air when opened at too low an rpm but a vacuum secondary carb is self-compensating in this regard.  With smaller primaries, air flow or velocity through the venturies is higher which in turn gives a better throttle response at low rpms at the expense of high rpm air flow.  Atomization is also obviously better with the smaller venturies which in turn helps with fuel efficiency.

I’ve had most of the popular carbs on my 272 and the 525 cfm Road Demon Jr. is the clear winner.  I’ve run several of the ’57 ECZ carbs (390-410 cfm) and these do not have the benefit seen from the advances that have occurred in metering and circuitry that’s employed in the newer carbs and although these older carbs could be tuned to satisfy either the economy or performance characteristics desired, would not do both simultaneously in a satisfactory manner.  I did run a 500 cfm Carter for a fair amount of time and although it got good fuel economy, it was always touchy when the secondaries were first opened at the lower rpms.  Part of this is due to air leakage around the weighted secondary blades and the lack of intial fuel to compensate for this.  But beyond this, still a good all around carb.

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It seems to behave as a large two barrel - primary id is pretty big - with a generous secondary to back it up - but the secondary boosters are pretty restrictive and keep up the flow velocity. The bowls are bigger than they look too, which allows for pretty heavy fuel flow. Gets my vote......got mine as a factory "rebuilt" (warranty return) for $269 with shipping from an ebay store.

I don't have it under my nose - but there is a Holley part number for a pair of .130 orfice needle and seat assemblies (for an alcohol carb) that is reasonably priced and easily solves the problem. They screw right into the Demon carb - just try to match the installed height you found on the assembly you are replacing, to avoid a flooding situation when you start it up to set the float level. I did mine one at a time to avoid a double trouble overflow situation. Both could be installed dry by removing the bowls and setting them in the inverted position - I was just lazy....

One thing about the Demon that has caused me a bit of trouble - it has a fairly stiff integral throttle return spring. This has raised some difficulties with the stocker Ford-o-Matic kickdown system - but a stick shift car would never notice.

Don't settle on a spacer until you have the carb and get a look at the throttle bores - they are significantly larger than the bores in the late four barrel manifold (9425-B). It will require modifications to the manifold - or some massaging of a phenolic spacer to create an adapter capable of "funneling" the larger carb bore down to the manifold size. 

Use a plastic spacer under the carb. 1/2 or 1 inch max performance is a single square hole spacer.  Smoother operation is with a 4 hole spacer and is recommended for street.

These modifications will support over 300HP with other engine mods. And with a proper metal spacer this could probably be done to '55 and '56 manifolds also so that a more modern carb could be used.

Charlie.  The camshaft in my 272 is 216° @ 0.050” with the lobe centers being ground on 108° and the 525 cfm Demon Jr. is fine with this.  There will be a certain point where the manifold vacuum drops off enough that the idle bleed holes will need to be tinkered with and this is where screw in air bleeds would be nice.  But those screw in air bleeds would appear to only be available on the higher end carbs and those of a larger cfm capacity without going to the trouble of retrofitting a non-equipped carb with them.  With the pressed in bleeds, it’s trial and error on addressing idle and low rpm fuel mixture issues with a much larger than normal camshaft.  Changes are typically made in 0.003" increments.  But  if it makes you feel any better, the Road Demons and Speed Demons basically have the same size idle bleeds and I have ran both on a Y with the 254° @ 0.050” camshaft without major differences in idle quality.

Ted,

I didnt intend to reinforce/perpetuate that myth; sorry if it came across that way.  I certainly understand/agree with your (and others) previous explanations on bigger carbs running leaner.  I was just commenting on how my 500 cfm showed a LOT of fuel dilution on a UOA vs. the 390.  May have been as much an issue of the carb itself as the size...

I have yet to find a setup that is really all I want it to be; for starters I cant really pull much more than about 10 in hg at idle... but that is a question for another thread!

The gaskets, needle seats, jets, and power valves are basically interchangable between the Holley and Demon carbs.

As a rule, never wire the electric choke to an ignition circuit.  This creates a voltage drop which potentially either causes the ignition module to fail prematurely or just creates a situation where the engine misses due to not having enough voltage to the ignition coil.  An electric choke needs to wired to a circuit independent of the ignition but still turns on and off with the ignition switch.