I'll look into the choke, hopefully that's all it is. That'd be an easy fix. It "looked" right when I installed it, so I hadn't messed with it much. Thought maybe it was more along the lines of fuel percolation or something.

Yeah, I'm really hoping both of us can up our numbers. I'll be curious how that wideband works out for you. Hopefully my next fillup proves different. If not, I'll try the stiffest secondary spring and start messing with the accelerator pump nozzle. It comes with a 40, which is really big. Seems to like it though.

 

Sure sounds like it needs a bit more fast idle speed. Assuming the choke is adjusted and working. I've never had the choke/fast idle work really well on an aftermarket carb without a little bit of fiddling.

 

That does sound like a fat squirter. Mine is .031, and I wanted to drop down! Is your pump a standard 30 cc Holley pump?

I would ck the choke flap. With one throttle movement b/f you turn on the key, it should drop to nearly closed. As soon as it starts the idle should jump to 2k rpm. Then after a moment you stab the pedal and rpm's drop to the fast idle position of 1500 or so rpm. Pull up a diagram of your carb and it'll show you how to adjust the fast idle. It really sounds like the choke/fast idle need to be adjusted. With your bronco, and its gearing, I believe you should be up around 15-16 mpg with a mix of highway and city driving, if not a tad better.

I'm very curious about using the wide band gauge too. It will be nice to have a guide for a change.

 

Curious to see how this turns out. I ran one of these carbs 20 years ago on a Dodge van with a 360; Holley marketed it as a Thermo-Quad replacement. I could never get that brand new Holley to get as good gas mileage as the totally worn-out TQ; consistently got ~20% lower mpg.

After all of the carbs I've tried, I'm still waving the Quadrajet banner.

 

But yet you went with a Eddy 500 on your 300. Why, if you 'wave the QJ banner,' did you use an Eddy? I'm curious.

 

Like you, kept looking for something "better"; refused to believe that a 6 cylinder couldn't get mileage to match the 302 I had previously. The QJ, that I had Sean Murphy Induction build for me, was pretty much perfect as delivered. I sold it as soon as I got the Edelbrock, which, in hindsight, was a mistake. I have the 500 dialed well enough that if I replace it, it'll be with EFI.

 

My issue wasn't gas mileage. Of course, I wanted more, but who doesn't. With the 600cfm square bore, I was getting 13 - 15 in town and 17 - 19 on the interstate. I had no complaints about that.
My issue was, no matter WHAT I did, I could not get a full stall bog out of the take off from idle. More often than not, the only reason I didn't stop in place was because the momentum of the truck restarted the engine and kept it going until the mixture richened up. I just couldn't tune it out. I eventually learned to ever so perfectly finagle the gas and clutch so I wouldn't stall it, but I couldn't let anyone else ever drive it or they'd be a hazard. I don't know why or what caused it because if I swapped the carb over to my other 300 with a Offy C and 3.55 gearing, it'd drive like a dream (other than that, the two Broncos are identical. Exhaust, ignition, tires, tread, transmission, everything).

So, I was getting this for drivability, which it now definitely has in spades. With the 3.00 gearing, it has a snappier off the line throttle response than the one with the 3.55s.

I just didn't expect the gas mileage to drop that much, and honestly, it really shouldn't. That's WHY the spreadbore design was introduced as far as I know!

 

I know Holleys have more adjustability in the accelerator pump with cams and whatnot, but how does nozzle size affect performance? On Edelbrock carbs the nozzle size doesn't change the amount of fuel being sprayed, just how quickly it happens. At least that's what Edelbrock claims. Same with the Holley?

I've still got that O2 sensor if you want to borrow it before you play around with the carb.

 

I've never had the drivability issue you describe. A mild hesitation at tip-in until it's completely warmed up, but that's it. Sounds like your previous carbs have been way lean, either in the idle circuit, accelerator pump, or both. Your mileage figures are killer, though; I've never seen more than 17 on the freeway, and even that's a challenge.

 

Yeah, I'm not sure what it was. It'd cruise great, it'd accelerate great, it had ample power. It just would NOT get moving from a dead stop without horrible issues. Once it was moving, it drove great.


I don't have any new figures to post, as I don't have that many miles on it since the last fillup. It's been down for a week while I was waiting for a new distributor to come in. My old one was leaking oil and would wobble pretty bad if I loosened the hold down bolt. Finally had some time where I could park it for a few while I swapped it out.

But, three things I've noticed is that the fuel gauge seems to be going down slower with the 61s instead of the 64s, the spark plugs are dark tan. It's definitely still rich, which is a good thing, since that means I have room to improve. They're not sooty or anything, just very dark. I've been driving it like a grandma for the last few weeks, trying to get the best mileage out of it, so I don't even think I've cracked the secondaries open for a while. Lastly, the best setting for the idle mixture screws is about 1/4 turn out each. Barely anything.

Another thing I noticed is my exhaust still pops. I did the dollar bill over the exhaust pipe trick and when it would pop, the dollar bill would snap suck in, which is usually indicative of a vacuum leak. I sprayed around the intake and carb and found that the vacuum and engine would falter if I sprayed on the throttle shaft for the secondaries.

I contacted Holley about it and they told me something I never knew, which is that throttle shafts are NOT sealed setups. They know they leak a little air and tune the entire system with that leak in mind. BUT, if it leaks too much and there's an issue from it, it's definitely under warranty.

I'm wondering if my rich condition is from the secondary throttle shafts leaking too much and since secondaries are designed to be richer, it's getting too much fuel. But I'm not sure how to tell.

 

On a Holley, as far as I understand, the nozzle size affects how much fuel is allowed to spray out at once. The cam size changes how quickly it happens. The cams have different curves to them that either bring most of the fuel in at the beginning, delay it a while, spread it out evenly, etc. etc. The nozzle size lets more or less out at once, and if you're accelerating a lot, it can affect gas mileage with a larger size since that squirt can be quite a bit of fuel.
Then, if you have a large size AND an aggressive cam that squirts a lot of fuel over the whole duration, the accel pump well can run dry, so they have a kit to upgrade from a 30cc setup to a 50cc setup.

Right now, I have a size 40 in there, which is what it came with. That's REALLY big, considering most other Holley carbs come with a 25. I'm not sure why it comes with a 40, but that's the stock setup. I'd try some others, but as I was stating earlier, it uses a different type nozzle, so I don't have any to switch in at the moment, and they're a bit more expensive, so I haven't purchased any other sizes. It does seem to like the 40 though, so maybe it's supposed to be that big on this one with how high the velocity is through the primaries.

I'll let you know on the O2 sensor. I know that came in really handy last time. Thanks.

 

Well, in my discussions with Holley, they've deemed that my carb is defective and they're sending me another one. The fact that the idle mixture screws are only turned out 1/4 turn for highest vacuum coupled with the fact that I'm running really rich with jets that are leaner than stock were a tip off that something wasn't right.
First time I've had a warranty on a carb, so I'm going to use it!

Really cool of them too, because they're sending me a new one so I can swap them, and then they'll pay for the return shipping on the old one.

I love the way it drives, I'll at least give it that, but the 2 - 4 mpg drop going from a squarebore to spreadbore really isn't my thing. Considering they're both Holleys and are comparable size, it should be the other way around. Heck, I'd take the exact same mileage, as long as I get the beautiful drivability of this carb. I simply can't say enough about how awesome it drives.

I also think unmetered fuel is passing through, since I dropped 3 jet sizes and my gas mileage is still the same. About 10.5 - 11mpg city.

 

Good on Holley for doing you right, that's great news. Well, the satisfying resolution is great news.

 

That's great, AB. I'm glad to see them coming through.

 

Me too. Probably the most helpful guy I've dealt with there. Usually I'm a little irritated by how difficult it is to motivate them to really dive into an issue.