Is my 340 Overcarbed? Avg 5-6 MPG

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You said your running a 6.5" power valve, but only pulling 7-8"hg at idle. Power valve should be half of vacuum so 3.5" to 4". It's definitely opening too soon based on those numbers.
 
It's s mechanical speedo from Autometer. Maybe it's no **** off on speed too?!? Engine was dyno'd on an engine stand and not in the car. I drove it again this morning and putting around in second and third gear was 11.5-12.1 on the wideband afr. It's definitely running rich. Warmup is worse, lots of smoke and wideband reads 10.7.

BigBlockMopar said:
If speedo- and odometer-readings don't match up than your speedogauge is off/failing.
On long trips I always recalculate the actual miles travelled by plotting the entire route in Google Maps.

Lean out the primaries for better AFRs.

Your dynorun was mostlikely made with an open hood.
Any chance the airflow into the engine gets smothered by a clogged airfilter or hoodclearance?
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Dubob said:
You said your running a 6.5" power valve, but only pulling 7-8"hg at idle. Power valve should be half of vacuum so 3.5" to 4". It's definitely opening too soon based on those numbers.
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Correct, vacuum is 7-8hg at idle. What do I adjust to correct that?
 
I just figured your poor mileage was due to the heavy foot &7 the Sh** eating grin when slamming through those gears!! Lawrence
 
Dubob said:
You said your running a 6.5" power valve, but only pulling 7-8"hg at idle. Power valve should be half of vacuum so 3.5" to 4". It's definitely opening too soon based on those numbers.
Click to expand...

This is your problem. Replace the power valve with one with a lower vacuum rating.
 
1....Sounds to me your speedo is AFU. I've bought at least 3 "used" GPS off CL and thrift stores from 17--40 bucks. These make GREAT speedos and odometers (they have a trip odo) So that's one thing.......getting accurate measurements

2....Fill up at the same station if possible, with the car in the same pump IE to keep the tank "level" as some pumps are slanted left / right / front / rear

3....Way too fat at cruise

4....You need work on the ignition curve. You probably want a shorter curve, and you NEED to get something with a vacuum advance.

5....That combo is NEVER gonna do that great with mileage. If you want mileage, pull that cam.

This is one of my cheap used Garmin "Nuvi". I've been a bad boy, LOL

attachment.php
 
Just FYI:

Bowling's Vehicle RPM Calculatorhttp://www.bgsoflex.com/rpmmph.html

Input Parameters Are the Following:
  • Differential Gear = 3.23 to 1
  • Transmission Gear = 1.00 to 1
  • Tire Diameter (Inches) = 25.04
  • Vechicle MPH = 60
Computation Results: Computed Engine RPM is 2602
 
Before Al Gore invented the internet, and before we had this fancy GPS stuff, I used to just fiddle until I had 60 seconds per mile on the marker posts. "Correct" the speedo from there However speedo error and odometer error are not necessarily the same thing in all cases

Before we had these fancy online tire charts and speed / gear/ mph calculators, I've been known to actually measure around my tires and "do the math"
 
". A/f is steady at 12.4 at cruise. I agree carb needs to be retuned. It was replaced from the original 770 due to internal leakage and was not the same tuned carb that was on that Dyno run.."

yep, I thought you were running fat but the old A/F table threw me. Cruise can be up to 17 as you require little of the motors peak power to maintain speed. Easier said than done on a carb, a few mouse clicks on an EFI.
 
What ph23vo says is exactly right. I've set many distributors up with
only one light spring so that timing goes to full advance when engine
starts. I'm not saying that is best scenerio for your stick shift, but you
get the idea.
 
Ok I had someone follow me and my speedometer is off by 4 miles an hour. I topped off the tank from the drive in last night and it took 1.5 gallons for a 19mi drive.
 
There are plenty of examples to prove the power valve must be below idle vacuum rating is not so. It's not the issue while cruising on the road.

19/1.5 is more than 5-6... :)

I'd turn up the initial timing to the 22-24 range, get a bushing that limits mechanical to 10* and see how it changes the idle quality and cruise. Find the point where the engine kicks back when hot, that's your limit on initial timing. I would think that set up would pull closer to 10" maybe 11 if initial was raised.
 
Once you get your timing dialed in, you still have to dial in your jetting. Drop
primary jets a few sizes and drive around easy and check A/F ratio. You want it
to be approx 14/1. Jet accordingly. Once you get that, you need to tune power
valve RESTRICTIONS for wide open throttle(wot). Your powe valve restrictions
are the two holes behind the power valve when removed. They need to be opened
up a little at a time until you achieve desired wot A/F ratio in in short bursts. I would
add a couple jet sizes to secondary side before dialing in primaries.
 
One thing nobody has mentioned - get that carb leaned out ASAP or you'll wash down the cylinders. Check your oil for gas smell.
 
Sorry for the long delay on the progress. Momma has been out of town so I have been busy with the kiddos. Today I pulled the primary bowl and took out the 73 and went down to a 72 jet. I did not mess with the secondary side yet since WOT seems to be good. I also pulled the spark plug from the #1 cylinder and it looks VERY rich! I did not have time to tune or drive it yet tonight so I will work on that more this week and update you all from there. What do you think?
 

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Advancing the timing helps fuel economy. As stated, you should probably have more initial advance, and a vacuum advance over and above the initial and mechanical will improve fuel economy. This, as well as getting your A/F ratio where it should be and you will see some gains.
 
smartken22 said:
OK, now that I have my Duster fresh out of restoration, I am trying to figure out MPG and I am eating gas likes its going out of style! My last two tanks made it 81 miles from full to empty. Not good in my opinion. It is a factory 16 gallon tank. Here are the specs on the motor and the dyno sheets. I am looking at the Air1+2 SCFM column and the avg is 573. I am running a Holley 770 Street Avenger so am I over carbed? Thanks all. - 340 Small Block Blueprinted and Balanced - Forged stock crank - SFI Billet Steel Flywheel - Centerforce D/F clutch - Keith Black 0.030 over cast pistons cut to 9.3CR - Forged rods with ARP Waveloc bolts - Kevko oil pan - Melling HV oil pump - Comp Cams XR286HR-10 Hyd Roller Cam at 105 degree C/L 13 degree @ 050 .544 lift / 286 Duration - Comp Cams Double Roller Timing Chain - Comp Cams 8920-16 retro hyd roller lifters - Comp Cams Double valve springs (140/385 spring pressures) - Hughes 1502 1.5 ratio roller rocker kit with holddowns - Hughes custom built pushrods: 5/16" x .083" 4130 HT 7.900" - Single groove valve stems - Ported J Heads @ 68cc w/ 2.02" intake and 1.60 exhaust valves - Edelbrock 7576 Dual Plane air gap intake - Holley Ultra Street Avenger 770 double Pumper - MSD Pro Billet Distributor - MSD Blaster 2 Coil - MSD 6AL-2 Ignition box
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It's a parts mismatch. The cam's too big, and the carb is WAY too big. Event he Comp catalog says a minimum of 9.5:1 and you're below that. On the carb - Compared apples to apples with an older carb you have more than 880cfm with that carb. The dyno shows it's a pig - no offesnse but 390/390 out of a 340 is terrible given the work done. Hence the inability to be efficient and poor mileage. Stick a cam in it that has 225 - like the Comp XR274HR10 - and put on the Avenger 570 and you'll pick up a shitload more torque and horsepower, and mileage.
 
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