Holley Carb Jetting Help, Lousy Fuel Mileage

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Reddartowner

Reddartowner

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My son's '68 Dart has a 600 cfm vac sec Holley recently installed on an Edelbrock Performer manifold on a 318. Engine is otherwise stock, but has HEI ignition upgrade. Gas mileage is dismal at 8-9 mpg in town stop & go driving. Inspection of the plugs would suggest that we're running more or less OK ratio-wise, as plugs are off-white to light tan. Compression is 150 psig across the board. Timing is 18 deg BTDC with no vacuum advance. Primary jets are #66's, with a stock secondary metering plate. Total advance is approx. 38-40 deg. Idle vacuum is a rock-steady 20 inches Hg. As we can't seem to find anything else to suspect, I wonder if the primary jetting is off. The car runs fine, just the mileage sucks.......

Any suggestions....???

Thanks,

Mark.
 
Well for one thing determine what the advance curve actually IS if you have not, and then HOOK UP the vacuum advance. Just about any car "tootin down the road" at speed will have in the vicinity of 55* or so total, that's initial + mechanical + vacuum.

I assume this is a square bore (not a spread bore)? In my opinion, not the best for mileage. You want little bitty primaries, which is why TQ and Quadrajets did so well. In fact, I ran a QJ on a 340 for quite some time. "Back in the day" when I had a stock 71 hi compression 340 in my 70 Road Runner, with headers, Dana 3.54, and 4 speed (had been a 440-6) that thing would get 17.5 on the highway. This was with "hang on" A/C and I was never all that carful around town, or on freeway ramps. That car weighed in the neighborhood of 3850 empty and most of the time had another 200 lbs in the trunk.

Frankly if he has any kind of "right foot" that might be all you can expect in town.

What do you have for cam/ exhaust?
 
All your numbers look great. The #66 jet is a as delivered primary on just about all 4 digit list # 600's. It's hard to jet down Holley's more then 1 number in most applications. They just don't like going that lean. Not sure it would make that much difference. I agree that mileage sucks though. I'll think about it and see if i come up with anything.
 
Install an A/F meter so you know EXACTLY what's going on.

Anything else is just guessing.

Could be any number of things, but, that's horrible mileage for a 318 unless it's got 4.88's.
 
Seeing 18 initial,assuming the distributor has been curved.What total @2500-3000? X2,on the A/F meter.
 
67Dart273 said:
Frankly if he has any kind of "right foot" that might be all you can expect in town.
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Yeah, I've considered this..... He's mellow when I'm in the car, but when he's with his friends, who knows... All I can say is I haven't noticed any rubber chunks on the rear fenders....

An update on the advance issue. We're not running vac advance at the moment because we can't find a vacuum advance can with less than an 11 stamped on it. Trying to run the current can (which is unmarked), the engine shuts off when we hook up the vac line. Idle advance shot to over 40 deg with a 10 deg base setting (checked by hooking up a hand vacuum pump and timing light). So for now we're just running more base timing until we can find the right advance can.

The exhaust is stock manifolds, 1 3/4 head pipes down to 2 1/2 duals starting at the torsion bar crossmember, with Flowmaster 40's. The driver side headpipe has a couple of sizeable "reliefs" dinged into it to fit around the torsion bar on that side (done by a PO). We're considering replacing the headpipes with Accurate Exhaust's 2 inch headpipes, but I'm not sure how much of a difference that will make.

Cam is unknown. Engine was rebuilt by a PO. Car idles smoothly in gear at 700 rpm with no noticeable lope, so if its not stock its close.
 
Reddartowner said:
we can't find a vacuum advance can with less than an 11 stamped on it. .
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Been awhile since I tried to get a vacuum can from the usual Chinese suppliers, yo know, NAPA, O'Reallys

If you are running a Mopar breakerless dist. try asking for an advance for an EARLY SB, like a 67 273. My book shows they are around 8* distributor degrees, which would be 16 at the crank.

Even the 69 book shows the 69 340 is as much as 10* (dist) so 20 at the crank.

You either need to run ported vacuum. Some guys say run full vacuum at idle, but if you do, you'll have to completely rework the curve.

Frankly, I'd be looking for a small spreadbore, even if it means using a spread/ to square adaptor.

Hell, even my old 440 did better than that around town!!
 
Does the dipstick smell like it's got fuel in your oil?

Do all the basics with a Holley. Float level would be the first item to check. Maybe lower it an 1/8 turn from where it is if it's set OK now.

If that has a decent tune up on it, there's no reason it shouldn't be getting in the teens for mileage around town.

Is the speedo correct?

The fuel is going somewhere?
 
Reddartowner said:
My son's '68 Dart has a 600 cfm vac sec Holley recently installed on an Edelbrock Performer manifold on a 318. Engine is otherwise stock, but has HEI ignition upgrade. Gas mileage is dismal at 8-9 mpg in town stop & go driving. Inspection of the plugs would suggest that we're running more or less OK ratio-wise, as plugs are off-white to light tan. Compression is 150 psig across the board. Timing is 18 deg BTDC with no vacuum advance. Primary jets are #66's, with a stock secondary metering plate. Total advance is approx. 38-40 deg. Idle vacuum is a rock-steady 20 inches Hg. As we can't seem to find anything else to suspect, I wonder if the primary jetting is off. The car runs fine, just the mileage sucks.......

Any suggestions....???

Thanks,

Mark.
Click to expand...

Hook up the vacuum advance(one that is not leaking), mileage will improve. I have never had a mild small block with a 600 cfm Holley perform very well with #66 jets. I run the 600 on two small block chevy's and my Duster 360. Try jetting up two sizes, you can run more advance without pinging and may actually pick up mileage, definitely pick up power. Holley's are a mid fifties design, they were not designed for economy.

For a base line, I run a 600 on my 360 with an edelbrock performer, headers and 3:55 gear with 2400 stall. It has #69's for jets. Car runs on regular without pinging, has a performance vacuum can with adustable spring, hooked up to ported vacuum. Car gets 12-14 in town, throttle response is impressive.
 
Reddartowner said:
My son's '68 Dart has a 600 cfm vac sec Holley recently installed on an Edelbrock Performer manifold on a 318. Engine is otherwise stock, but has HEI ignition upgrade. Gas mileage is dismal at 8-9 mpg in town stop & go driving. Inspection of the plugs would suggest that we're running more or less OK ratio-wise, as plugs are off-white to light tan. Compression is 150 psig across the board. Timing is 18 deg BTDC with no vacuum advance. Primary jets are #66's, with a stock secondary metering plate. Total advance is approx. 38-40 deg. Idle vacuum is a rock-steady 20 inches Hg. As we can't seem to find anything else to suspect, I wonder if the primary jetting is off. The car runs fine, just the mileage sucks.......

Any suggestions....???

Thanks,

Mark.
Click to expand...

Off white to light tan with todays fuels on A Holley is LEAN. Plugs should be a darker tan, no white.
 
crackedback said:
Does the dipstick smell like it's got fuel in your oil?

Do all the basics with a Holley. Float level would be the first item to check. Maybe lower it an 1/8 turn from where it is if it's set OK now.

If that has a decent tune up on it, there's no reason it shouldn't be getting in the teens for mileage around town.

Is the speedo correct?

The fuel is going somewhere?
Click to expand...

AND check manifold vacuum and see what power valve is in the thing
 
We're going to bump the jetting up to 68's over the weekend. The power valve is a 65... might install an 85 to get the enrichment a little sooner. Also discovered we have some fuel percolation issues, so a heat isloator is coming to keep the carb cooler.
 
Found out something.... turns out the reluctor wires were reversed, putting the distributor out of phase. So we switched the wires and re-timed the engine. We'll see how that improves mileage....
 
Maybe the primer squirter is a high volume setup, so every time he touches the gas it get a big squirt of the good stuff & if it's stop start stuff, this won't help. Otherwise itis hard to get good milage when you are doing the whole stop start thing.
 
Mupoo said:
Maybe the primer squirter is a high volume setup, so every time he touches the gas it get a big squirt of the good stuff & if it's stop start stuff, this won't help. Otherwise itis hard to get good milage when you are doing the whole stop start thing.
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Quite possibly.....this is a 17 year old teenager, and all his driving is around town..... But we'll see if the re-phased distributor improves things and take it from there.
 
Re the vacuum can, I got one for my small block off rockauto termed "high mileage" and was inexpensive (~$10). I recall it has "8.5" stamped on the rod and the rod is bent so I installed the tip in the inner hole of the distributor plate. I haven't started driving the car regularly so haven't gotten into details or diagnostics.
 
BillGrissom said:
Re the vacuum can, I got one for my small block off rockauto termed "high mileage" and was inexpensive (~$10). I recall it has "8.5" stamped on the rod and the rod is bent so I installed the tip in the inner hole of the distributor plate. I haven't started driving the car regularly so haven't gotten into details or diagnostics.
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Do you happen to have the part number for that vacuum can ??
 
Reddartowner said:
Do you happen to have the part number for that vacuum can ??
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Sorry, I don't find it in a quick look. Might have installed it. I recall for my SB, but have also been buying slant parts. Look on rockauto for 73+ cars (electronic ign). Don't know where the "high mileage" designator in their description came from. I haven't read of that elsewhere.
 
I found an 8.5R can in the NAPA/Echlin catalog which I have ordered. We'll get that installed and see how it goes. Incidentally, the entire NAPA/Echlin electrical illustrated catalog is available online in a PDF format, which you can save to your computer. Makes a great reference....

[ame]http://www.napaechlin.com/upload/NAPAEchlin2/Documents/ECH_NA141_HQr.pdf[/ame]
 
How did you make out? If your mileage is still subpar, you might want to try running a leaner idle mixture setting. A significant amount of fuel is still delivered by the idle circuit during part throttle cruise.
 
We're probably going to rejet the carb back to 66's now that the distributor is in phase and the 8.5 vacuum can is installed. We noticed that the insides of the exhaust pipes are black, so its definitely running rich. After rejetting we'll slowly lean out the idle circuit and see how it goes..... Be nice if I could find someone in my area with a chassis dyno and a wideband O2 sensor.....
 
You might start with a simple lean/rich O2 indicator (MSD, Holley, others). I have bought for $25 on ebay. That will get you started and you can later screw a wideband O2 sensor into the same bung. Some give a little more than just an on-off signal. Double the cost if dual exhausts, or else run 1 sensor and assume both banks run the same. Wideband sensor kits are coming way down in price. Used to be $600, now ~$150. One advantage is you can install the sensor in the tailpipe, but I would rather weld in a bung than run such a long cable. I use a lean/rich in my 65 Newport to adjust my Holley Pro-jection. I can run a bit lean by turning the knobs a little more after the rich light goes out, or use it in feedback mode. What I see on the indicator and what I feel in how the engine runs are well correlated. I measured 18 mpg regularly when driving that 4000 lb car w/ 383 on the highway.
 
Wow, the lowly Pro-jection is showing some love, eh? My buddy loves his Pro-jection on his 392ish Cleveland so much that he is buying another for his 451 Mopar. That wideband up the tailpipe needs to be WAY up the tailpipe as any O2 that is sensed should be coming from the cylinders instead of atmosphere. ONly way I can see it working properly is if its as close to the collector as possible, nice and hot. Most all 2008 and newer cars should have a wideband O2. just need a controller to see the ratios.
 
pishta said:
Wow, the lowly Pro-jection is showing some love, eh?
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I certainly don't love mine. It requires regular tweaking of the knobs, even with O2 feedback, and seems worse since I switched to the little "2D" box. I do have ideas to improve Holley's design.

pishta said:
That wideband up the tailpipe needs to be WAY up the tailpipe as any O2 that is sensed should be coming from the cylinders instead of atmosphere. ONly way I can see it working properly is if its as close to the collector as possible, nice and hot.
Click to expand...
Many after-market places sell a tail-pipe mounting kit for permanent installation. It only needs to be far enough up to insure it isn't seeing air from outside. A wideband sensor has its own internal heater that always runs (not just warmup like 3 or 4 wire regular O2 sensors). Indeed, you can ruin the sensor if you run it long disconnected. Without the heater running, the passages can clog permanently with soot.
 
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