That lower vacuum reading at cruise points toward air leaking in somewhere. My Bronco had the same symptoms - 18+" hg at idle, fits tuning the carb, lots of foot in the gas on the highway. Turned out to have leaks in the A/C (at the plastic plug/lines going into the plenum at the firewall), at the brake booster, and the vacuum advance diaphragm.

I'd use one of those hand pumps to draw a vacuum on every single line attached to the carb/manifold first.

Then I'd get an Edelbrock. You knew that was coming, right?

 

Oh, and AB, since you seem like a pretty stand-up guy...

I've got a wideband A/F I could probably be convinced to lend you for a week or two if you really want to see what you're doing with that Holley of yours. Assuming you don't have a sensor bung in your exhaust you'd need to get one installed, but that's such a little worry for something that's so helpful in carb tuning. PM if you're interested.

 

Hey AB

sorry cant help ya much but look forward to see'n what ya sort as I know this must just be doing your head in by now.... got some trips lined up here and gonna post my MPG up again now that I have few k's on the moter

look forward to see'n ya get'n this sorted

 

In the end it could be that some engines just don't play well. Take two identical engines 1 SN apart in identical vehicles and one may run different and get different mileage than the other...for no known reason and/or excuse. Also, 1550 rpm at cruise is too slow and inefficient...unless it is a big truck diesel. JMO

 

Yeah, the speedo is right on. I've re-adjusted the gear to match the tire size and double checked with my GPS.
And if I was closer to Iowa, I'd take you up on that offer!


It would sorta make sense for the powervalve to kick in from time to time since that's what it's for. Use less gas while cruising, and then a burst of fuel only when you need it. But for it to need it to do any kind of accelerating seems a bit much.

I've thought about a Q-Jet before, but from your story it sounds like they're a lot of work!

Both the '81 and the '84 have a (nearly) identical ignition with the Duraspark II setup. The only difference being the '81 has a high performance coil, as well as a timing re-curve.

I remember discussing that before. Even using the mathematical equation for cfm requirements would attest to this. I don't know how accurate this is under real world conditions, but still...

CFM requirements = (CI * RPM) / 3456
CFM = (300 * 2100) / 3456
CFM = 182

That's almost 100% of the 390s primary side air flow (195cfm), so the primaries would have to be nearly straight up and down to maintain 65mph @ 2100 RPMs. Any extra effort is going to immediately either use the secondaries, use the power valve, or both. If I were in overdrive, they'd theoretically be closed much more (about 65% open) and get better mileage, but the extra strength necessary to turn the gears would negate that.
On the '84, with the 600, it's 215cfm @ 2500 RPMs, which means the throttle only needs to be opened around 72% of the way. (215 / 300cfm)

You'd *think* the 390 would be enough though, since the 1bbl was able to keep up with it. Then again, it didn't have the extra gas to dump in when the engine demanded it. It just lost speed.

I don't use the OD anymore. Most all these results and tests are in 3rd 1:1 gear. However, for much of the duration I had the 4bbl on there, I was running 215 75R15s, which are around 28". With the 1bbl, I used OD all the time and used to go 75 - 80mph on the interstate (sucked going up any hills though), but I keep it at 65 and don't shift out of 3rd with the 4bbl due to the gas mileage. The 31s are a relatively recent acquisition, and gas mileage didn't change much when I got them.

(The switching of transmissions is something I've considered, but the '84 is a hydraulic clutch and the '81 is mechanical, so it makes it a little difficult.)

I've started to really consider this lately. It's a bit of work, but really not much more than I've done a dozen times now. It'd answer a lot of questions.



Hrmm.... That's something to think about. There's no A/C and all of the vacuum lines are less than a few years old, but that's not to say the brake booster itself isn't bad. I've never changed it out. I wonder if it has an internal leak or something. I may have to try disconnecting it and capping it off and go for a test drive.
The '84 didn't have a vacuum line to the brake booster when I got it so I know how they stop without it. Certainly not ideal, but not bad if you're expecting it. Be a quick way to eliminate that.

And yes, I've considered an Edelbrock! I have nothing against them and would love to give them a try. I just don't have one, access to one, or money for one at the moment. I've been keeping my eye on craigslist for a while, but nothing but intakes or big 750cfm ones have come up in quite a while.


I'll definitely be curious to hear how you do, since you have a Holley 390 as well.

I've thought this for a while except that there's been two engines in it now and both respond the same. Makes me think it's something bolted to it.
As said though, 1550 is what it would be at if I used overdrive, which I don't anymore due to it just being too low of an RPM. (It even pulls more vacuum in overdrive going DOWNHILL than it does in 3rd.) It's been a while since I've even shifted into OD.
The 10hg of vacuum at cruise was at 2100 RPMs, which should be plenty for normal power I would think.

Out of curiosity, what RPMs & vacuum readings are you at at cruise? I know you've always been successful with mpg in your truck.




Thanks again for the responses everyone. I've been mulling them all over in my head and seeing what I can do next. Hopefully something comes out of it.
In the meantime, I think I'll swap the 600 over and see how they each respond.

 

Hey AB: I was just joking about selling the Qjet. I've put too much time and $ into it to sell. Besides, I'm very curious to see how it does. I'm just waiting for the oem m.rods to arrive. The ones in both the carbs I have look like a dog chewed them up, and the side that rubs on the jet is worn flat!

Yes, they are a lot of work.

 

AB, the hydraulic vs mechanical clutch isn't an issue. Leave the clutch/bellhousings alone. They both should have the same trans to bellhousing bolt pattern, and the same input shaft size.
Up in the 80-86 section, swapping a NP435 (or BW T-18) in place of the 4OD is talked about as being a direct swap, needing only the trans and matching shifter. Depending on the year of the truck, the floor plate *may* have to be swapped, as the 4OD has 2-3 different shifter locations over the years. The NP435 never relocated the shifter, as far as I am aware.
Both vehicles *should* have a bolt-in plate on the top of the trans tunnel.

 

Heh, I figured.


I thought there was more to it than that. Mounting the slave, replacing the pedals for the clutch linkage, etc.
Plus, there's a few extra steps when you get the 4x4 in there since the low range shifters are on different sides.
Either way though, since I only use 3rd anyway, I'd be going from a 1:1 to a 1:1 as my top gear. It'd give the '81 a granny, and may help the '84 out in mileage more, but it wouldn't solve this particular issue (which is why I haven't really delved into it too much yet).

 

True, it won't help the 81, at least not for this issue.

I still think swapping the carbs will tell a lot. If the MPG doesn't go up on the 81, I'd be surprised. The 84 may or may not change. I just can't get that one figured in my head.

If swapping the carb doesn't bring up the MPG on the 81, it may be time to seriously consider regearing it......

 

My truck pulls anywhere from 10 hg to 16 hg cruising depending on the speed, geography, wind, etc. You know the geography around here...not much in the way of level driving anywhere...anything from 7% grades to slight inclines and declines...sometimes a little level running. It will gain 1 hg between Spokane (2000' mol) and Lewiston (900' mol). Elevation does make a difference. Boise is 2700'+ so your norms might be a little different than mine.

I second the motion on switching carbs between the two engines.

Also, has the transmission and differential lubes been changed in the 81? If not, they could be plenty thick and fresh grease especially synthetic might make some difference.

 

AB,

I'll keep my eye out for you on the ZF... I have passed on a couple of ZF's that would only need forks and syncros. One was $50 and the other was $75. Don't know how much freight would run but I'll ship it to you for my cost if I find another one. Iowa has TONS of these ZF's sitting around in broken farm trucks around here.

Keep us up do date on the mpg situation... I don't have the 390 on my 300 yet (wife says other chores 1st...ugh...) but I expect I should have time this weekend to get around to it. Pretty sure Ol' Girl has 3.55's but I'll have to check the door jam to be sure... Hope I don't have all these mpg problems... Then again you should have this all worked out by the time I'm done! LOL

 

What RPMs do you run at @ 65, out of curiosity?

I was mostly curious about your vacuum to see what it averaged. It sounds like the Holley 600 on my '84 is about like yours. 16 at cruise, maybe down to 10 with some climbs.

Since the 390 seems to run at around 10 or so all the time, I'll switch it and see if it changes. If not, that'll lead me away from the carb.

The bearings have been repacked and the transmission and differentials have all been flushed and changed within the last few years. Probably much more recently than my '84.

 

[quote=nstueve;10545646]AB,

I'll keep my eye out for you on the ZF... I have passed on a couple of ZF's that would only need forks and syncros.

From what I have learned about the ZF is that to replace the synchros means a full rebuild, and that is not cheap by any means. And, if the syncros are going, then, shouldn't the bearings be close behind?

I guess because I have been burned with bad bad bad trany work, I would rather spend the $ to know it is going to last, and not leave me stranded in a God-forsaken Mexican village covered with grease and watching my vacation dwiddle away. If there had not been ice cold beer, it would have been a total waste.

P.S. I forgot to mention, although the ZF is said to be a great mod to old trucks, bronco's, etc., it comes with a hefty price tag. Not only does a person need the expensive trans, there is about $600 worth of other parts needed for the swap, and I mean for a 2x conversion. To buy all the used parts, and the trans, it would cost me around $1,200 to $1,500 by the time it was completed, and probably 6 trips to the j/y, and six to the a.parts, etc.

A.B.: Sorry for the rant in your thread. Yes, I do feel better now.

 

"What RPMs do you run at @ 65, out of curiosity?" It depends...my cheap Tach has an ongoing anomaly of running 200-300 difference at any one speed at different times. Generally runs 1800-2000 rpm at 60 mph.

 

If you know the axle ratio and tire diameter, the following will get you a very accurate figure:

(MPH) X (Axle ratio) X (336) / (Tire OD) = RPM

Example, my truck has a 2.75 ratio axle, and 30.7" OD tires:

(65) X (2.75) X (336) (grand total is 60060) / (30.7) = 1956
I have a C6, so there is a bit of loss in the converter. A manual trans *without* OD would be turning 1956 RPM at 65, while mine sits a bit over 2000, and varies with load due to loss in the TC.

For an OD trans, the formula works the same, but the axle ratio figure is replaced by final drive ratio, which is axle X OD ratio, such as 3.00 X 0.76 = 2.28 final drive.

AB, not sure what that 4OD has for an OD ratio, but that 2.28 Final drive is fairly close to what you have in OD. I think this trans uses a .67 OD, if so the final drive would be (3.00 x 0.67) = 2.01. No wonder you find OD useless. It's somewhere between 2:1 and 2.3:1 final drive. Even the 300 doesn't have enough torque to overcome that, particularly with 31" tires (30.7" actually, according to BFG)......