Even before I swapped gears and was still running 2.76s in the back, the Dart was getting downright abysmal mileage. Like 10-11 MPG on the highway. It's worse now with the 3.55s, obviously, but I haven't worked out quite how bad yet. High single-digits, I'm sure. I'm going to be driving the thing a long distance next week, and I'd like to see if there's anything I can do beforehand to make the gas bill hurt less. Of course, saving money on future race expenses would also be nice; we burn something like 6-7 gallons an hour at the track and don't have the horsepower to show for it.

For reference: The motor has a moderate cam swap, a Holley 350 (2300) on top of a super six intake, and a stock exhaust manifold with a single 2.25" pipe. The carb is definitely jetted rich, so that's one thing I'm planning to play with. Back when my AFR gauge was working, it was measuring just over 10:1 under load. Pretty sure I've eliminated any leaks surrounding the carb and adapter plate, but it's possible I have a leak where the intake hits the head. I know I have a minor exhaust leak at the head mounting surface. Eventually I'm going to install a better manifold gasket, but I don't have time to address that right now. The valve lash was set recently: something like .013 intake and .025 exhaust, if I remember correctly.

Any other ideas for common efficiency-related issues to check out? Suggestions on what my base timing should be? I assume that mileage in the low to mid teens wouldn't be an unreasonable expectation even with 3.55 gears.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

10:1 AFR is bad for power and horrible for economy. Is that Holley 350 stock or is it a circle track carb? If it's been modified for circle track use you're better off using the carb as a door stop and buying a new one. You might be able to buy just a stock metering block and get it close to normal that way depending upon what all was changed.

If the carb is standard make sure the float is good, the jets are standard and the power valve is good. Set everything by the book and go from there.

You should be running timing about 5-10° BTDC initial and 30° total timing with the mechanical advance.

_________________
Joshua

I second that...10:1 is not a daily driver ratio...and the timing needs to be right in where Josh has laid it out...

If it is a 0-7448 Holley, can you tell us what jets and PV you have in there now? Confirm the metering block is also stamped with the same number that is on the airhorn...

Just from general Holley experience I would be inclined to set it up with a pair of #56's, and an 8.5 PV if your tuning and timing is all close to where it should be... I would also be very inclined to take a pair of small paperclips and bend them over the air horn and stick the ends into the high speed air bleeds to get the mains to come on line closer to 1900-2000 rpm and eliminate any transistion circuit issues that would lead to a lean out and the PV band-aiding the situation. Another issue is the acc. pump cam...if it was "stock" for the 7448, it is a little "short" (red I think), and you should install either an Orange on the #1 screw. or a Pink on the #1 screw if you need to have fuel through out the throttle shaft stroke including the high end.

Two more items to look for...

If you bought it used and it's pre-1992, you need to go get a PV backfire kit and install it...the Spectre kit works fine, is priced right, and has the same stuff in it as the Holley kit (which is 3x the price)....otherwise once "puff" and the ruptured PV will spill gas into the intake like no tomorrow (and you mileage will be as you describe...which is another item Josh is hinting at)...
The other worry, is the acc. pump shooter... I would jack the rear of the car up, put it on stands, and run the engine up to 2300 rpm and have it in high (with a driver behind the wheel for safety)...look down the airhorn and see if the vacuum signal is strong enough to pull drops of fuel out of the acc. pump shooter nozzles...if so you need to install a needle weight (like in the bigger holleys), or a check ball to prevent this from happening (this is worth some mileage and AFR).


2 cents,

-D.Idiot

Damn, I was typing up a response to Josh before I left work, and apparently I forgot to hit send.

Okay, I bought the carb from Andrew Howard (kidd) a little over a year ago. He told me that it had been recently rebuilt, and I installed it as soon as I got it. This is a front-hanging bowl, FYI. The last time I had the bowl off, the float seemed to be in good shape. I'll check the numbers on the metering block vs. the airhorn.

When I got it, the two fuel jets that were installed were stamped 70 and 61H. At some point after I first tested the AFR, I changed the 70 out for a 66 that I had handy. I wasn't able to verify the AFR after that, because my sensor had previously been fried. It didn't seem to make much difference to the MPG, though. I do have some more jets that I've bought since then, so I'll step them down some more and see what happens.

I don't know what power valve is in it, but I will check. Where is it located, and how is it identified? Is there a number stamped on it?

The engine seems to run pretty smoothly, aside from a lumpy idle that I think it's always had since the cam swap many years ago. I've never experienced backfires with this carb, but I do sometimes have difficulty with hot restarts if it's been sitting for a bit. I have performed the over-the-valve-cover fuel line mod, so it's not that.

Thanks for the all the info, guys. I'll check as much of that as I can this weekend. I've been dealing with daily driver and other car issues for a while, so I've been putting off getting this figured out. After this "road trip" I'll have more time for fine tuning before the next LeMons race.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

70 would be great if you had a secondaries on a 650...and 61H would be fine for a Holley 1945, or the primaries on a 600 cfm holley or a 500 cfm 2300 Holley... Both are so rich you are washing down the cylinders on anything save an all out race motor...This isn't a Holley 2305 staged 2 barrel is it?

If not:

Put in a set of 58's as this sounds like a carb that isn't a "street" calibrated carb, and adjust from there...all the other settings since they will be out of whack to the new tuning...

Still need the number that is stamped on the front of the air horn to ID the base settings on the carb and it's purpose (they are on the middle right hand side of the horn/where the choke plate is housed...)

The power valve, is a big "plug" looking item in the metering block that is screwed in between the jets....you use a 1" wrench to remove and install it and the vacuum number is stamped on the side or "code" (like 'E9') is stamped there...if it has no markings it's a generic 6.5"Hg PV that comes in all the rebuild kits...if yours just has a plug in that spot and no PV, it is set up for race and the jetting needs to be 5 jet sizes richer to make up for the lack of the power valve...this also will tank your economy since the carb runs rich all the time, instead of relying on the power valve to open when 'demand' for extra fuel and lack of vacuum requires a nice heavy stream of fuel to band aid the situation....

You have the Holley book by Dave Emmanuel on Holley Super Tuning, right? (If not, get one coming...it will be more first hand help than us trying to remote the issues via the website).

A bit of TV viewing may help.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Okay, I've got two #58 jets in there now. They're the smallest I had. Engine runs fine, will report back on mileage difference. Here are some picture I took of all the numbers I could find.

On the base, front right (passenger side): looks like 12R-5174B


On the top: 6R 1919 B


On the front of the horn: 7448-1 and 1610 below it


The number on the top of the metering block is 6392. I forgot to take a full picture of the front with the bowl off, but you can just barely see the two jets down on the bottom left. I also added a vent screen a while back just in case we were having issues with fuel slosh; it was easier to install than the whistle and I'm lazy. Is the power valve the deal with the spring on it, just above and between the jets? I didn't see any numbers on this side. I suppose I'll have to remove the metering block to get to it.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Yes it is, and yes you will have to remove the block to see the edge that has the numbers if any are stamped.
It's a 7448 Holley standard 350cfm aftermarket carb.

It's stock configuration is #61 jets, an 8.5PV, 031 shooter. Which I've found to be a little rich for most slant sixes, so i start at #56 or #58 and work up from there. The 8.5 and #31 are OK for it's current use, acc. pump cam color would be helpful as well. (I typically like a longer stronger pump shot instead of a quick "dump", so you may need to change the pump cam to a slightly better profile and the shooter to a #28).

Do you know what the in gear idle vacuum is?

I’m betting this cam is causing low vacuum and perhaps a wagging needle at in gear idle which make for a bad vacuum signal.

Big cam will also cause low vacuum at cruse, and when one steps on the gas will dip into the power valve too soon and often if power valve is not properly sized. See Holley TV for PV sizing tips.

If exhaust stinks when at idle it is too rich.

If idle is set so that the throttle plates are into the transition slots it will idle rich and mixture screws won’t react.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Well, I've seen major improvement on fuel economy. On the 3ish-hour trip I took this weekend, mostly on the highway at 60-70 mph, I averaged a little over 16 mpg. This is with an A904 and 3.55 rear gears, so I'm pretty happy with it. This was just from switching to #58 jets. I will probably need to make some adjustments to the power valve or accelerator pump, because the car definitely bogs down when you hit the gas at low speed. It did this a little before, but it's more noticeable now that the carb isn't running really rich. I also noticed that the car has even more trouble with hot restarts now. It cruises fine, though, and pulls well when you give it gas at speed.

I still haven't checked my power valve or accelerator pump jet, but I'll look into that this week. I'll also check the manifold vacuum if I can get my hands on a gauge. Thanks for the help.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Finally got a chance to pop the metering block off and take a look. I'm guessing by the numbers 8 and 5 on the back side that it's an 8.5 power valve. The letter on the other side is a K.



Here's the accelerator pump disassembled. The check valve (I think that's what it is?) is red, and the pump diaphragm is a bluish green. No other markings found.



Here are the numbers on the outside of the pump housing, if that means anything. Doesn't show up well in this picture, but that third letter looks like an R with a dash partially superimposed on the side. So it would be "34R-21788" with a 2 below.



I'm thinking I need a larger pump shot, due to the car stumbling when you hit the gas hard at low speed. It did this even when the main jets were way too rich, and it's only worse now. Doesn't seem to do it at speed, though.

I'll check out Holley TV, wjajr. Thanks for the link.

_________________
Somehow I ended up owning three 1964 slant six A-bodies. I race one of them.
Escape Velocity Racing

Your acc. pump check valve, spring, and everything looks correct, but what determines how much the pump actually pumps is the plastic cam screwed to the throttle shaft near where the throttle cable hooks up...there will be two holes and the arm is stamped with a 1 or 2 to determine which position the pump cam is in and how much it pumps. I'm thinking you are going to go to your local speed shop and ask for a pink #330 cam and install with the screw in the #1 hole...

Make sure to get the Holley book by Emmanuel coming to your doorstep PDQ as all of this with nice pictures and descriptions along with cam profiles and chapters on tuning are worth the $12 (I've worn mine out and still pull it out for reference).

-D.Idiot

It's interesting that Amazon lists the book, but so far I can't find it on the publisher's page.
This is the book, correct?

_________________
Thom

Cross-threaded is tighter than Lock-tite

That's the newest version. If saving a few pennies you can also find the older version which works fine as well.

Yes, get the book as soon as possible. Where you have a two barrel vs a four barrel you have half the tuning to do which is a plus. But because it is a two barrel with larger throttle plates than a 390’s primary throttle plates, when the throttle plates are quickly fully opened on a small displacement engine such as a slant, it sees a lot of blue sky which causes a lean condition until rpm increase can move more air. A properly adjusted, or in other words, tuned accelerator pump will temporally overcome the lean-out.

One way to help in tuning a Holley is to install an o2 sensor. These devices can give an A/F reading as the carburetor transitions through its four levels of operation; idle, transition to main jet, main jet, and enrichment (power valve).

More on o2 sensor used for carburetor tuning found here.

Disclaimer, I have followed Gerard’s direction and installed an o2 sensor and A/F gage in the Dart equipped with Holley 390 & big cam / low vacuum, and it enabled me to tune my way out of the weeds that had entrapped me for four years.

As others have suggested, a pump cam and its screw hole, and possibly accelerator pump nozzle may have to be changed. Make one change at a time, drive for a while to determine its effect. Keep a note book, and record the change and its result. Keeping good notes on any changes with date and odometer reading will help to prevent replowing the same ground.

Here is how I keep track of repairs, tuning, line ups, other alterations, and any ideas I run across on this and other sites that may be useful to my Dart in these three loose leaf binders. My atrophied brain can’t remember what my last meal was let alone what I did to the car three years ago…



This is how I store all of the Holley parts have been piling up over the years as I blindly tried to tune that thing:

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC