Hrmmm... Possibly. I haven't opened it up in a while since I put on the quick change secondary kit. I'll pull it off and take a look. That'd make sense though.
I just did a run in my other Bronco that has the 600 cfm Holley in it, and the secondaries started to open at around 4000 and that's with a heavier spring than the 390 has in it. That makes a lot more sense since it's 600cfm but also shows the 390 isn't opening when it's supposed to.


Yeah, the secondaries are adjusted by changing the stiffness of the spring and currently I have the lightest one they make in there! They should just flop open without even trying.

Been interested in trying an Edelbrock! But Baron beat ya to it. He sent me a 600 one he had. It's missing some pieces and needs to be reworked before it'll run again, but a good solid core. I'm thinking of tackling that one next.
Aren't 500 and 600 Edelbrocks basically the same thing aside from some factory adjustments?

 

I've been going through similar experimentation on my 302, fun isn't it?

Regardless, you should be getting better mpg with a 300. I get the same out of my 302 that i'm messing with. (Still haven't done a recurve yet, plus i'm gonna put a dual stage power valve in it)

The only thing that I haven't really experimented with as far as vacuum is concerned (my vac drops off quick too) is the PCV valve.

 

AB: Is this the same carb you obtained from someone that used it for racing? If so, it's possible that some re-engineering was done to it like using drill bits on some of the bleeds not actually knowing what he was doing. Also, a 390 is a 4160 model carb BUT not all 4160s are equal and the kits may have different internal gaskets for the different metering blocks, etc. The metering blocks/plates might have been changed too.

And it could be that you have a lemon carb with some unknown imperfection built in.

 

I have a two stage PV on hand that I've tried before, and it seems to do pretty well. Just threw in a 9.5 and it seems to really help.

You were, however, correct on the secondaries!! They weren't opening at all. (Not sure what I was feeling that made me think they were). They were stuck closed so bad that I had to tap them with a hammer to unstick them. I couldn't do it by hand. They were completely stuck in the bore.

I adjusted the secondary idle screw (that tiny one you have to access from underneath) just a hare to make sure they never could close far enough to get stuck again. Completely fixed the problem. They came open pretty easily after that (and way too soon with the light yellow spring). Apparently, I've been driving around with a 2BBL carb for quite some time.

 

Far out, nice to see an easy fix every now and again. Is that on the 390cfm carb? Or should I say 195cfm - with the secondaries stuck shut? Do you know if they've ever worked? Did you get the carb used?

You should clean up the bores with some fine steel wool and WD-40. Maybe the edges of the throttle plates, too, then lube your shaft with some motor oil.

Of course the bad news is you'll have to go driving around logging data again now that it's running all four bores. I'll bet it's more fun testing WOT this time around. I'd bet the rich condition just got a lot better too.

 

Yeah, I'll be checking it again soon. The O2 is over on the bronco with the 600 on it at the moment.

The throttle plate was replaced a year or so ago because it was really warped, so they probably worked before that. It's been a while though.

I'll be curious what difference it'll make.

 

Sorry, I missed this one.
Yeah, it's the same carb. Although, upon speaking with the guy later, I think he used the term "raced" loosely, as in 'raced around in it'. Don't know if there was anything more than slapping it on and driving it.
Then again, he only had it for a year or so, and when I checked the date code on the carb, it looks like it was built in '93 or prior, so it's at least 18 years old.

That's another thing. I never understood that about Holley's date stamp. They give you the exact day the carb was built, and then the last number of the year, but completely fail to mention what decade. Mine's 2143, so I know it was made on the 214th day of... '73? '83? '93? '03?
The only reason I know it's older than '93 is because it wasn't drilled for powervalve blowout protection, which was started to be factory installed in around '94 or '95.

So yeah, I have no clue of the history on the carb.
I usually double and triple check what the holes in the gaskets are covering (or not covering) instead of just slapping them on. Again, though, I may have missed something, and just copied the previous owners gasket and maybe something's being blocked off?


Also, in the end, I'm just starting to think it's a bum carb.

 

Well..... Finally some good news!

I currently have the Holley 390 on my wife's '84 Bronco.
Stock ignition
3.55 differential
31" tires
NP-435 trans (no OD)
2500ish RPMs @ 65 mph.

Took it out for a gas mileage run this morning and did a 100 mile round trip. The whole way back was into a head wind. Pulled around 14 - 15hg the whole time and was consistently around 15.5 AFR. Dropped to around 12.5 or so when I'd really get on it, (which I didn't have to all that much).
It ran really well, and didn't seem to lean out like it was when I'd accelerate. Maybe I finally cleared it out? Overall, I was pleased with the performance.

Got back and topped it off and it came out to 17.5 mpg!!
I'm very pleased. I could probably get 18 if I wasn't driving into the wind for half of it.

This calls for a .



Now I need to see if I can finally do the same with the 81.

 

That ain't bad for a full-sized Bronco with no OD. Good on you. What does the '81 turn at highway speed, about 1,500 rpm? That should be pretty gas friendly.

What advance spring is in the carb? I wonder bringing it in a step sooner might lean it out at throttle. Maybe the PV is dumping so much gas that diverting some air through the secondaries would lean it out a bit?

I'm a recent convert so I'm preaching it all the time but switching the vacuum advance from a ported to manifold source made my truck a lot happier. It doesn't change anything at cruise, just at idle/off-idle around town. You might give it a go and see how it runs with your combo.

 

What is "stock ignition" on an '84? This is the kind of mpg I'm looking for out of my van. And a little extra power for uphill grades would be nice, too. I have 3:54 gears but I have the C6 tranny. Do you think I could achieve these numbers with a 3/4 ton van? What kind of compression readings do you have? I'm getting 115-120 and I'm wondering if I need a ring job. I've read some guys have much higher readings. Sorry for all the questions but I'm trying to learn all I can and you guys really know your stuff!

 

It wouldn't suprise me if you would get just as good or better mpg by not even touching overdrive.

Maybe the new 4bbl needs too much throttle opening to get adequate booster signal at low rpms.

 

The '81 is around 2150 at 65. It would be at around 1500 if I was in Overdrive, but I never use it anymore since it usually just rides in the powervalve.

I'm starting to think there's something wrong with the 81 itself and not the carb. Maybe my recurve is screwed up? Brake calipers sticking? I took it in and had it checked and they said 'no' but who knows.

By advance spring, what are you referring to? The secondary spring? The whole trip I don't think it got into the secondaries at all, and maybe into the powervalve once or twice (which is a 9.5, which is really big).

The vacuum advance is currently plugged into direct manifold vacuum, which I like a lot better.

I guess by "stock ignition" I should clarify. It does not have the stock '84 TFI ignition. I did a Duraspark II conversion on it when I first got it. But by stock, I mean, it's a stock DSII ignition. No hotter coil, ignition module. Distributor is stock with no recurve, etc...

The c6 + 3.55 gears should give you exactly what my '84 has, except that a manual transmission will always give a stronger engine-to-trans connection than an auto, so you'll lose just a little bit there.

The compression is also around 145 - 150. 115 - 120 sounds like it's wearing a little thin.

That's what I'm thinking. I'm thinking the 4bbls like a higher RPM than the 1bbl. Low RPMs don't seem to give good mpg.

 

Most engines should have 140 to 160 lbs of cranking compression with no more than 10% difference between any of the cylinders unless its was built to handle higher amounts of boost in which case anything about 140 is usually a bad sign.