Random carburetor annoyances

[1987cp]

Since Pirate started bashing AFB carburetors in Tom's carb-conversion thread, he's got me doing some thinking about my continued use of AFBs in the future.

The obvious downsides are of course the fact that the Carter-design metering rod system doesn't allow quite as much adjustment as may be desired, and the fact that the secondary sides lack metering rods. That on top of the fact that the AFB doesn't provide for convenient tuning of the secondary air doors means a less adjustable carburetor overall - bad for fuel economy, and it's looking like it can sometimes even be bad for meeting the actual fuel needs of even a mildly modified engine.

Another annoyance I've noticed with AFBs is the lack of replacement parts. I may be wrong, but I get the impression that with, say, a Holley, you can start with a main body, and build up the entire rest of the carb out of a catalog, from metering plates to boosters to fuel-enrichment valves to quick-change covers for secondary springs on vacuum-secondary models. I e-mailed Edelbrock a while back asking for a part number to order replacement boosters, such as if someone wants to convert a 1404 to a 1406 or vice versa (the boosters are the primary difference between the two models), and received no reply. I've more recently realized that if the stepup plungers, power pistons, fuel enrichment plungers, or whatever you want to call them get damaged or go missing, there doesn't appear to be any way to get new ones (I haven't completely exhausted all options, but a complete lack of supply options does seem so far to be the case).

I can't afford any more carburetors at present, and I'll admit that my initial purchase of AFBs was in part motivated by their lower retail price in addition to the reputation for ease of tuning for someone completely new to the world of carburetors (as I was at the time of my first purchase), but these various considerations (and probably others I'm not thinking of, such as the fact that not a lot of people know much about the things in the first place) have me wondering slightly whether I should really continue running AFB carbs in the future.

[gadget73]

I can't get mine to start worth a crap. Not sure if its an issue with the fuel pump valves allowing fuel to drain back or what, but they're always empty when I try and fire off the boat. Takes way too much cranking and pumping to get it to light off. Honestly I have had better luck with marine Qjets, but I'm not interested in springing for a pair of marine carbs at the tune of $450 or so each.

The AFB is a decent carb, but its kind of designed for an OEM sort of application where a set it and forget it deal is a good thing. They'd have been jetted and rodded per manufacturer's spec, and then left alone. They're not infinitely adjustable, but they're so simple that anyone can take one out of the box, play with it a little, and make the car go. Holleys are a lot more work just because they are so adjustable. You can change everything on a Holley, but unfortunately sometimes you actually do need to change everything to make it how you need it.


If someone actually made a spread bore intake for a Ford, I'd suggest fooling with a junkyard Qjet or a ThermoQuad. Those have adjustable air doors, and at least in the case of the Qjet, metering rods on the secondary. I think the ThermoQuad has that too, but I don't know those all that well. I mostly know they're huge, and had plastic bodies that were prone to cracking.

[Pirate]

The AFB/Edelbrock carb is a simple, basic carb, provided that the carb is properly selected for its intended application, almost any halfwit can make it work.

The problem is when you attempt to fine-tune it.

It took a few weeks of driving and tuning to get my last Edelbrock-equipped vehicle running and driving correctly; however, the absolute best gas mileage I could get out of my '85 Mustang GT was around 16 mpg on the highway, with 3.55 rear gears, and driving 65 mph. I could make it run leaner, and get better gas mileage, but to be honest....it ran like crap in the process.

The edelbrock was actually one of the most difficult carbs that I've ever had to tune; the primary side is pretty simple, it's the secondary side that's a pain in the arse. I ended up adding some stick-on lead weight to the flapper door weights to get it to slow down a bit, as they are a bit light (lighter weight is to induce running rich when the secondaries are in operation, it's safer that way).

Oddly enough, the more expensive Edelbrock (Thunder Series AVS) does have an adjustable secondary air door....but it's almost double the cost of a regular Performer carb.

Woo-hoo.

The problem is that you still get the same, basic, 1970's Edelbrock 'technology', with crummy venturi/venturi booster design, with you being charged almost $150-$200 only for the added benefit of an adjustable secondary air flapper door?

I don't think so.

For a basic carb, I'd start off with the Barry Grant unit. Yes, it's more money, but at least the venturi/venturi booster shape/design are somewhat modern, and they're fairly easy to adjust, provided you get at least the regular Road Demon. The Road Demon 'Jr.' is simply an overglorified piece of crap Holley 1850 600 cfm unit, in that it has a secondary metering plate (with fixed fuel 'jetting' orifices) instead of a metering block where you can actually change the jets.

The base Holley 1850 600cfm unit doesn't even have externally-adjustable needle-and-seat assemblies that allows you to adjust the float level from outside the carb....now, it's pray you get it correct if you adust anything, as the float bowls now have to come off to get it right.

More later....

[gadget73]

The AFB isn't crappy 70s technology. Its the pinnacle of late 1950s technology. I got bored the other night and found out they introduced that thing in 1957 on some Dodge product.

[Pirate]

The AFB isn't crappy 70s technology. Its the pinnacle of late 1950s technology. I got bored the other night and found out they introduced that thing in 1957 on some Dodge product.

Well, I was trying to give it some credit, lol.

[ss_mini14]

if i remember right most edelbrock carbs are rebadged weber carbs. you'll find the trademark "W" cast in there somewhere. i hear they are quite easy to tune if you are fluent in weberisms. i am not fluent so my toyota runs richand gets maybe 12 miles per gallon

[1987cp]

It's actually an evolution of the original iron/zinc/aluminum Will Carter Four Barrel that first appeared in the '40s atop a Buick straight-eight. I still haven't figured out where Weber comes into it, maybe they just cast the main bodies.

I had forgotten that the AFB was introduced in '57 and the AVS in '66. Now I feel like I'm driving a real antique!

As far as using it as a set-it-and-forget-it carb, I imagine some of you read my post from several months back where an old carb guy on eBay blew about six or seven gaskets when I asked questions about retuning an old rebuilt AFB carb he was selling, because his carbs are delivered correctly calibrated and should never require adjustment!! After he sent three fairly abusive e-mails in a row, I was no longer interested in asking how he magically knew exactly what I was driving and what was in it, and how the carb was to magically adapt if I were to change something. :p

BTW Thain, I noticed a 650cfm QuadraBog on one of those marine websites. Since manufacturers used large-cfm-rating Q-jets on very mild (comatose?) engines, that got me wondering what this smaller model would do on a street motor.

[Archangel]

AFAIK the 650 Q-jet is somewhat of a rare animal, most were 750s, 850s, and 950s. By the way my old Chev ran freakin nice with the Q-jet, it definitely had some power, and it actually did a bit better on city fuel economy that my Lincoln. Don't recall why I replaced it with the Holley, I think I cracked something at the main body or something, but it was a pretty decent carb even with my home-made choke setup.

[gadget73]

Qjets work well, but I don't think there is a spread bore Ford smallblock intake to really take atvantage of them. I kinda like the things, but a lot of people don't. Fairly simple to rebuild too, only minorly difficult thing is getting the metering rods in position through the top gasket. I really don't like how they come through like that, but it is what it is.

[351m]

There is a spreadbore intake for the 351W but I can't remember who makes it, as far as the 302, there is nothing that I have found available.

The reason the Q-jet and Thermoquads were so large was because they had such tiny primary bores. If you put a smaller one on you are into the throttle more, and into the secondaries sooner, thus back to burning more fuel.

One thing I was always told growing up... If you own a Dodge or Chev and want mileage AND power, get a 4 barrel. For the Fords, stay with the 2 barrel or for more power go with the 4.

As far as the bodies on the TQ cracking, the only time I have ever seen one crack was when someone fucked it up working on it, the deliver very good performance, mileage, and the reliability seems far better then any Q-jet.

[1987cp]

I've seen 'em for 302s, but I think not for 351s, which I would think would be more likely to really take advantage of the things. Quick check at Summit: all three typical Offy designs (360 single-plane, 360 dual-port, Port-O-Sonic) are definitely offered new in spreadbore for the 289/302 .... for 351ci it's only showing Cleveland offerings, and those only in the two "360" models. Not promising, I guess, if one wants to run a more conventional 180-degree manifold without an adapter.

I know Carter Thermoquads are good carbs too, once one gets away from the smog-hell stigma, but aren't they even more obscure and harder to get parts for than a Quadrajet?

[Nathan in MI]

As far as I know, you should be able to use jets and metering rods from an AVS carburetor in a Thermo-Quad, since a TQ is basically just an AVS with plastic fuel bowls.

[1987cp]

Oh yes, quasi-n00b question, what determines at what vacuum levels or throttle openings Holley power valves open?

[Pirate]

Oh yes, quasi-n00b question, what determines at what vacuum levels or throttle openings Holley power valves open?

It's similar in function to a AFB, in that the power valve is held closed until the vacuum drops below a certain level. Most Holleys come equipped with a 6.5 power valve (stays closed until it drops below 6.5 inches of vacuum), but of course, you can go up or down from there.

[1987cp]

Oh dear. So to tune properly you have to buy several different complete power valves at $8.69 each?