Is my 340 Overcarbed? Avg 5-6 MPG

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smartken22

smartken22

U.S. Navy Senior Chief
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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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What's air1 plus 2CFM?
What is the duration @ .050?
I'd say no. Carb CFM looks good.

What's converter and gear and tire size?
 
cam card say 236/242 @ .050

Centerforce D/F clutch

must be a 4 speed..

MSD Pro Billet Distributor
- MSD Blaster 2 Coil
- MSD 6AL-2 Ignition box

Timing?....as usual....
 
Does the distributor your running have a vacuum advance? If it doesn't that is where your loosing some MPG.
 
Id say you were way fat A/F but figures dont lie! All those speed part call outs dont tell us what rear you are running. 4.30 rear in a street car will rack up the fuel costs. I had a 351C 4V in a 57 Ford with a Predator and a 3.70 rear. A/F were close to stoich and that car only turned 8 mpg. Funny how a 372 vette can pull 17/29 mpg and still lay down 455 hp. Incredible.
 
Total timing is at 32° and initially is 16. Yes, it's a 4 speed. Rpms are @ 3000 at 65 mph. Gears are 323 with 15" rims. Dizzy is mech advance not vacuum.
 
Here are some more pics for you. Idle A/F is at 14.2-14.7. At cruising speed the A/F is steady at 12.4. Here are some pics of the A/F at cruising speed, the speedo at 55mph and the A/F at idle.
 

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12.4 at cruise... FAT!

Should be around 14:1

Did someone take the primary power valve out of the carb

3000rpm, 3.23 and 65 mph is a 23.5" tall tire. ?????????????????????????
 
Did you put in or use a reproduction fuel pickup? Mine will show empty at only 12 gallons or so. And it will go dry around there too. The pickup is not at the same factory location.

So, how many gallons are actually refilling in the tank?

Have you only driven it 162 miles so far?

How much fun are you having driving around with this new motor?
 
crackedback said:
12.4 at cruise... FAT! Should be around 14:1 Did someone take the primary power valve out of the carb 3000rpm, 3.23 and 65 mph is a 23.5" tall tire. ?????????????????????????
Click to expand...

crackedback said:
3000rpm, 3.23 and 65 mph is a 23.5" tall tire. ?????????????????????????
Click to expand...

^^THIS^^ is what I was gonna jump on. My 67 (a'fore I tore it down) was 28.X tire, 3.7x Ford 9 and it runs about 32-3300 at SEVENTY

Transmission? Converter? Vacuum advance?
 
Sorry see you have a 4 speed. YOU NEED vacuum advance if you are gonna get "great" mileage.
 
I think your speedometer, and of course odometer, are off quite a bit. So when you compute your MPGs that's off too.
 
Yes there is only 162 miles on the complete drivetrain since the rebuild. As you can see in that picture I only had 26 miles on thetrip meter and I was down to half a tank. It is a reproduction tank and sending unit. When I filled up tonight the tank took 12 gallons. Speedo is dead on because I used a GPS app on my phone and the speed matched.
 
If that tire is 25" tall, the car should be rolling about 69mph at 3K rpm.

What I would do.

Get the car on level highway, run the car in high gear, stable throttle position at 1500, 2000, 2500, 3000, 3500 and note the spot where A/F ratio goes down. This is the point where your IFR's are no longer feeding the engine and main jetting takes over.

Something is way off, whether it be something simple like float level because it's very fat on cruise. If the numbers are a constant 12.5 range with no reduction, that carb needs a ton of work.
 
smartken22 said:
Yes there is only 162 miles on the complete drivetrain since the rebuild. ---o< snip o<--- When I filled up tonight the tank took 12 gallons.
Click to expand...

If you have gone 162 miles on 12 gallons, you're getting 13.5 mpg - not bad for that setup
 
Ok will do. Yes Rpms are around 3k at 69mph. 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. It is the same 770 and all the jetting, squirters and power valves were transferred from the old carb. 4 corner tuning is close, but not how it was with the old carb.

I also just noticed that the odometer reads 12mi from my last destination, but I know it is actually 19miles away. Why the difference from the speedo and odometer. Speed is accurate as it was verified with gps and an iPhone app. Do I need to adjust the trans gear?

crackedback said:
If that tire is 25" tall, the car should be rolling about 69mph at 3K rpm.

What I would do.

Get the car on level highway, run the car in high gear, stable throttle position at 1500, 2000, 2500, 3000, 3500 and note the spot where A/F ratio goes down. This is the point where your IFR's are no longer feeding the engine and main jetting takes over.

Something is way off, whether it be something simple like float level because it's very fat on cruise. If the numbers are a constant 12.5 range with no reduction, that carb needs a ton of work.
Click to expand...
 
Chief, from what I see, your w.o.t. air/ fuel ratio is too lean and your part
throttle ratio is too rich. You need to put smaller jets in primary and open
up power valve restrictions to make up for smaller jets at w.o.t. I believe
you would need in the area of 14/1 for part throttle and 12/1 full throttle.
Also I believe you could use more timing at both idle and total. Try twisting
that distributor up to about 36-38°. Was your motor tuned on the dyno? I
think you have a lot more power waiting to be tuned in.
 
I have a 340/4-speed/3.23's and 225/60-15 tires on a car that made 470hp with it's 340. I have a 750 Quick Fuel Annular Discharge double pumper carburetor. I'm getting 12.5-13.5 mpg.

My carb has 68 front and 78 rear jets in it. 9.5 power valve in the front. Since I have replaceable idle feed restrictors (not sure if you do), I have 0.032s (1 size up from stock). It still runs a little fat (13.4:1) at high speed cruise.

If you drive around with a vacuum gauge plumed into the car, find what the vacuum is when the engine tries to pull a hill vs what it is at cruise. You want the power valve to come in under load when load is needed. I'll bet you'll want more of an 8.5 or 7.5 power valve. After that is figured out, i'd try jetting down two sizes and see what that does for you.
 
Well it looks like I have some testing and tuning to do. I will mess around with jetting, etc and see what happens. I did throw a vacuum gauge on it this morning and it was pulling 7-8" at 800rpm.
 
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?
 
that combo prod needs 30 DEGREES INITIAL!.... if not more... my 340 wanted 38 degrees.. [296 -557cam] just locked out dist... keep kicking intial into it until it pings or kicks back hard when starting hot...i bet your mileage and power go way way up!@
 
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