Carburetors for Lumps

Apologies if this has already been specifically addressed but I’m interested in what other lumpers have feeding their lumps, ie pros, cons, preferences for particular carby’s etc. My preference is for economy and smoothness, and I’m struggling a bit to find the ideal setup. I’m really not inclined to go FI. Yet

When I bought the car it had a 307 sb with a street demon. I was actually pretty happy with it but when I changed to the 350 the engineers were very derogatory of the Demon and convinced me to go with a Holley Avenger 650 cfm. Now I’m not so sure because no matter how it’s jetted the economy is appalling.

The 350 is a one step up from standard 308hp crate so I’m not looking to burn rubber. I’m now considering putting the demon back on or fitting a Rochester quad.

A 650 is grossly too large for a 307: on the street, a 390 is puhlenty.

I agree but it now has a 350 sbc. The Demon was 625 cfm on the 307 and worked pretty well but 390 seems a tad small? :-o GM recommends a 570 which seems appropriate. I wish I had trusted my own instincts and gone with that.

Carburetors are not as efficient as fuel injection, but they used to be all there was and we didn’t think twice about it.

I never had much luck with Holley’s although they are pretty easy to work on, just swap some parts around. When I became more interested in some sort of economy, I went back to the factory (Rochester) Quadrajet. It came in two sizes with the smaller CFM being the most common. They have a relatively small primary venturi and huge secondary. Cruising in town and steady highway driving lets you stay on the primary’s, but when you are ready step it up a little, those huge secondary’s can pin you in the seat.

That being said, none of the carburetor’s are perfect out of the box, unless you just want the engine to run and not much more. Fiddling with and improving them takes some education and dedication. There are (were) good books on the subject.

If you were pretty happy with the Demon on your 307, put it on the 350 and see how it works. The engineers you mention, were they affiliated with Holley by any chance? I don’t really know what engineers you are referring to, but take their advice or comments with a grain of salt, learn and understand how a carb really controls air/fuel and make your own experiences.

A carburetor uses vacuum created from the intake stroke of each piston to control the amount of air/fuel drawn into the cylinders. A lumpy cam doesn’t allow for good vacuum (at low speeds and idle) so the carb is most always setup rich (poor mileage). A dual-plane stock manifold will usually give you the best low to upper mid-range performance.

Just a comment: aftermarket fuel injection is a serious replacement for the carburetor when you’re ready. Much finer control of the fuel/air, more precise metering, simpler mechanically, not as simple electrically/digitally. Costs a lot more (although I think the price of a new carburetor is astronomical).

Good luck


Great response thanks. The guys advising about the holley were not the ones working on the car. They were running a performance shop a few doors down from where the car was.

No connection to Holley but like everyone else I’ve come across who either had not seen or didn’t know what a Barry Grant Demon was, either laughed at it or wouldn’t touch it. I just had to make a choice to get the car going.

I’ve always regarded a Holley as a blunt instrument and in hindsight would probably have bought a new Quadrajet.

This car has spent well over a year in the shop and the Jag guys have really worked that Holley over. They’re one of the best Jag shops in the country and look after a few lumps.

I’ll check out FI but would like try a Rochester first. I’ve spent so much money on this car I now have a severe gag reflex when I open my wallet.

That explains the “engineer” part of your first post. Performance shops love Holley’s and your assessment as a blunt instrument is perfect. Do your Jag guys feel comfortable tuning a Quadrajet? Another carb I’ve worked with is the Qjet replacement make by Carter. I had it on and off the car a few times, but never liked it as much as my tuned Qjet.

The FI opinion is just something I wanted to put out there.

I’m sure my Jag men would be capable but as they are an hour away I’d be more inclined to take it to a lump person around the corner. It doesn’t matter if there’s no affection for Jags if their expertise is Quads on lumps.

Its 2. 45am here and there’s a massive wind storm. It sounds like half the roof just went. Flamin’ heck! Now the rains coming.

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I’ve been a fan of the Quadrajet for almost 40 years. Find a book by Cliff Ruggles that explains
everything. When you are ready, you can call his business phone and describe all of your engine
parameters and he’ll suggest jets and metering rods that are spot-on.
The trick with them is that in stock form they don’t move enough fuel at idle, so people end up opening
the throttle plates so far they get in to the main circuit. You have to open them up and enlarge an idle
passage, but then they are great for performance and not bad for economy if you can stay out of the

I’ve got them on Pontiac, Chevy, and Dodge motors with cams around 225-230 duration at 0.050.
If they bog or don’t run right, it is because they haven’t been set up properly, not because there’s something
wrong with them. Ones without a divorced choke are easily converted to electric choke, if they don’t already
have one. My preference is to avoid the later models with the electronic mixture solenoid.


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I’m writing this whilst waiting for emergency services to come and nail my roof back on :-o.

Ironically both my jag guy and myself were originally wanting to fit a Quadrajet but were persuaded to go Holley by the V8 shop a few doors down. At first glance pricing for a recoed Quad for a 350 is pushing $1500 here which seems excessive and is the same price for a FI kit not fitted.

Refitting my Demon is looking a more sensible option at the moment but I’ll still look around for a Quadrajet for the economy.

$1500, holy crap!!! I’ve got a gold mine somewhere out in the storage shed. That’s ridiculous!


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Yeah, and I think about all the Quardratoilets I tossed out in favor of Holleys… :slight_smile:

And it’s on sale!

Oh, geez… plus 20 dammits.