I think if you switch carbs the milage will follow the carbs,...slightly.

 

The Overdrive on the SROD is a 0.71:1, and yeah it's pretty useless.

The reason I brought it up before is because I used to drive in it all the time when I had a 1bbl and would easily get 17mpg at 75mph. With the same tires, that mileage dropped to 12 @ 65 with the 4bbl. I started driving it in 3rd and was able to bring it up a little bit, but not much.
When I swapped on 31" tires, nothing really changed in the mpg department.

That's why I'm confused. For a moderately sized carb, 2100 RPMs should be pretty ideal for mpg and it's just not happening at all. 15 - 17 I could understand, but 12? It's like it's dumping out the bottom.

On that note, Howard, my RPMs in 3rd are nearly identical to yours. Not sure what size tires you have, but 31s aren't necessarily big or anything.



I'll be swapping carbs sometime soon. I port and polished the head I had and just got it cleaned up at the shop, so I'll be swapping it in soon. The carb's will be coming off at that point. I guess we'll see if there's any change.


PS: Kevin, I got a kick out of your transmission story. But yes, I agree. If you have synchros going out, you're looking at a full rebuild. And ZFs are not really a transmission you rebuilt yourself. They require a professional.

 

235s. I think the OEMs were 215s. I've always up-sized the tires on every vehicle I've ever owned by one or two over factory issue but no more than that. I don't like heavy loads on and handling of dink sized tires nor the looks, noise, and handling of inordinately over sized tires. But, each to his own.

 

WOW You guys rock! I am a complete newbie and am going to be able to pick up my motor in 11 days. I have filled 2 notebooks with the invaluable information such as above. Thank you all for all the help and info you have given me! I have so much confidence because of this site
Thanks again!
David

 

That's how it's done! Way to go.

There's so much valuable information on this site.

 

David: Paypal works for all of us. lol

 

[quote=F-250 restorer;10554779]David: Paypal works for all of us. lol[/
LMAO!!!!!! Bet you guys ain't cheap either! LOL

 

3.55 gears
granny 4 speed
31 inch crushers
@65 on highway

16 mpg

 

Is that yours?
If so, pretty commendable! What did you get before your upgrade?

 

yep just toped it off from the first trip bro, before I was around 10-12 MPG before any upgrades with the over size tires and crap smog stuff on it.

Thats driving with an egg under the gas peddle granny style

 

That's pretty good. I'm seriously considering swapping from my 2.75 rear to 3.50. I'm also running 31's on the rear.
Currently @ 65, I'm sitting at 1956 RPM.
Swapping to 3.50 will put it @ 2490 RPM.
I seldom ever drive much over 55, especially towing a trailer, as it takes too much throttle input to maintain speed. This has a negative effect on the MPG. Holding 55, I can get close to 16. Trying to run 65, it drops to 12-13
I'm betting the hyw MPG will go up after the gear swap.

 

Alright. Back on this.

So, since last time, I've swapped the 390 and the 600 carbs. I haven't done any actual calculations, but just from "feel" it seems whichever Bronco has the 600 does better on gas mileage, and whichever has the 390 does worse.

I've been able to run some tests with a Wideband O2 (thanks Baron!).
This has mostly been on the '84 Bronco (3.55 rear, 31" tires, NP-435). Roughly 2500 RPMs at 65mph.

My idle is around 12.5 (19hg vacuum). A little on the rich side, but it runs rough when I bring it up to 14.5 and the vacuum drops to around 18.

With the size 50 jets I had in the carb, it had a good cruise (65mph) AFR of 14.5 - 15.0. Right around stoich at cruise.
I put in some 49s and it was 15.0 - 15.5.
48s resulted in 15.5 - 16.0.

At 45 mph, most all of those numbers dropped by a whole point. So the 48s were around 14.5 - 15, and 50s were around 13.5 - 14. The faster I go, the more it leans out.

So, I know that my cruise AFR is good. It's not doing something mysterious like dumping gas, flooding, etc. etc. If I was getting like 12 or something, I know that it would be puking rich and something was wrong with the thing (I was actually really hoping for that!)


Here's two things that I did notice though. The first has been an issue I've always noticed whenever I try to lean it out. I even notice it some with stock jetting. Any type of acceleration is super lean. No amount of accelerator pump finagling will cure it. Reason being is, it's the type of acceleration where the gas pedal isn't moving, so the pump isn't being activated. For example, if I'm holding the gas pedal at 25% at cruise, and then move it to 40% and hold it steady the vehicle will slowly increase in speed. Since I'm holding it steady, there's no pump shot.
The problem is, during this acceleration period, the engine is SUPER SUPER lean. The more the vacuum drops, the leaner it gets. It's to the point where you can audibly hear the engine starting to beg for fuel. However, if you press it hard enough, it drops into the powervalve and boom. 12.5 AFR, power restored, acceleration is strong.

It just seems that this super lean spot is what's killing my gas mileage. Sure, it's doing fine on gas while you're maintaining exact speed, but that's rarely the case. So, any type of acceleration is either powervalve territory, or screeching lean. Neither are good for mileage.



The second oddity I noticed is that my secondaries weren't opening until 4500 RPMs! I always assumed they were opening until this week when I tied a twisty tie around the secondary shaft and after a few powerful romps down the road it hadn't moved. I put the lightest spring available (the short yellow), and completely floored it in first gear and suddenly, at 4000 RPMs, there was a big surge of power. Wanting to be sure it wasn't just a load thing, I went out on the interstate and floored it in second gear. Around 55mph (4000 RPMs) I could suddenly feel the secondaries kicking in. And by kicking in, I mean kicking in. It felt like a second engine turned on.
As far as I know, they've never been opening.

What would cause them to open so late? As said, I have the lightest spring available. They should be opening around 2000 RPMs or even sooner. This is at the last moment. Mathematically, the 390 should be completely out of breath by 4500 RPMs and they're not even opening until 4000.

Is there an internal vacuum leak? Could this be causing me lean surge on acceleration?


Thoughts?

 

On the acceleration issue, what are the vacuum readings as it's leaning out and which PV is in the carb now? I know Holley sells two-stage PVs just so you don't have to stand on it going up hills and whatnot. Seems like that might be the ticket to keep leaner jetting for cruise but still have a safe afr at part throttle acceleration.

Are the secondary plates sticking in the bore? On the Autolite core I've got the secondary plates feel like they hang a bit unless the adjusting screw is turned in pretty far - almost like setting the idle on primaries.

Are you running a 4-hole spacer on that C intake? That helped a ton on mine.

 

That would be my guess. Seems like booster signal vacuum is getting leaked somewhere. Maybe past the secondary diaphragm. I don' know any other explanation why a 300 cube motor would need to spin that fast to open up the secondaries on a 390 cfm carb with it's lightest spring in it. A small tear in the diaphragm?

 

...don't know much about Holley carbs, but from what I have read they are highly adjustable? Isn't there an adjustment to determine when the s. open?

AB--if my Q.jet works out, I know someone who will be getting rid of a nice Eddy 500, with calibration kit. But, I imagine, you'll be looking for something with a blend of hp and mpg?