My 340 getting 6 mpg

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Vinny said:
Is your speedo accurate? Is it possible that your actual miles driven is not correct? Just
MOPAROFFICIAL said:
Speedo no worky. I think he's using the navigation mileage and fills the tank deduced by how much he can put in the tank bringing it to full again.
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Speedo not working doesnt mean anything. Im getting bad MPG bottom line ! Lol
Vinny said:
Is your speedo accurate? Is it possible that your actual miles driven is not correct? Just a thought.
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Speedo not working means nothing lol. I know the miles om traveling. Are you fuys serious ??
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MOPAROFFICIAL said:
I think the carburetor is to be replaced with a brawler or std holley 3310.
Someone's been into it. Ootb it's supposed to be 71/82. Bleeds have been fiddled and the jets were 65/75, could correct it.. but the plate is **** and it's not a good look so far.
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I agree. I decided on the holley brawler 680 vacuum secondary.

The plate i have now is ****. I dont recommened FST carbs to anybody lol
 
junkyardhero said:
well, there's part of the problem.
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Miles are miles guys. My speedo not working is 100% irrelevant

You dont have to be a mathematician to know that I drove 81 miles combined. 62 yesterday and 19 today and my brand new fuel tank that is 18 gallons, (which i filled up completely full before my day out yesterday) my brand new fuel guage is saying my tank needs a refill speedo or not now im getting 5 miles to the gallon

Did i get on it a couple times. Well duh ! But i dont believe 1 or 2 pulls to 4500rpm can really affect your mpg that drastically

But we did suceed in getting the motor to make more power
 
Newbomb Turk said:
I didn’t mention it because if it takes that long to seat the rings the engine will never produce the power it should.

A properly finished bore will seat the rings in 5 minutes. You can see it on a blow by meter.

The idea that it takes XXXX miles for an engine to seat the rings is another one of those things that need to get corrected.

If it does take that long they need to be looking for a different machine shop.

At the outside limit the rings should be in by the time a FT cam is broken in.
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Its broken in officially with yesterdays drive. Motor is running super strong.
 
Nat said:
6 mg ? no idea what is wrong but that is bad stock my 340 got 15 to 17 mpg 323 gears now getting 20+ with gear vendor and 3:55 gears . Even my 426 stroker got 9 mpg . Those 3:73 gears don't help much . With the right parts and tune you should get 15 mpg
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Yea. I at this point i might just go the route of getting it dyno tuned. And see what that does
 
Dan Georgino said:
Wooo, 750cfm is way to much carb. 600cfm is plenty. Then play with your jets till you get 18 to 20 vacuum inches at idle. What is your vacuum at idle now?
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Yea im going to buy either the 600 or 680 Cfm Brawley vacuum secondary.

Vacuum was at 5 if i recall correctly
 
Last of the Fast said:
I have a 340 Duster running a Holey 750 double pumper primary jets 72, secondary jets 76 power valve 45 Edlebrock LD340 intke. Your jets are too small so you have to push the throttle so far that you are always into the power valve. What is your elevation? I'm in southern California pretty close to sea level. Your cam sounds like it is a little too hot for the street (Sig Erson hyFlow 2?) The Sig hyflow 1 is a better choice. My Duster gets 12mpg with 3.55 gears with posi and the stock cam (hydraulic). I have a 4 speed. It turns low 12's/high 11's depending on the track.
If you have an automatic, what is the stall speed on the converter? My V-8 Vega has a 700R4 2200 rpm stall, 3.08 gears with posi and Sig Erson HyFlow1 cam with Fuel Injection. Both cars have headers. You can write to me for more details at barry91765 at g male
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Im in Torrance CA.
 
str12-340 said:
So I'll chime in on carb choice. The carb you have has mechanical secondaries. When you floor it the secondaries are wide open no matter what, plus a double shot from the accelerator pumps (they did that to cover the massive sudden vacuum drop in the carb). You could use a Holley with vacuum secondaries, or an Edelbrock AFB or AVS2. All those carbs have secondaries that are controlled by the demand for air/fuel by the engine. With them you can guess high on the size of the carb and the secondaries might not be all the way open until the top end rpms. I'd try an 800 AVS2.

If I can get 15 mpg out of my 340 with 2 Eddie AFB 500s, you can do better than you are doing! By the way I absolutely love your car!
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Yea. Im about to order the Holley 680 Vacumm secondary.
 
Newbomb Turk said:
I didn’t mention it because if it takes that long to seat the rings the engine will never produce the power it should.

A properly finished bore will seat the rings in 5 minutes. You can see it on a blow by meter.

The idea that it takes XXXX miles for an engine to seat the rings is another one of those things that need to get corrected.

If it does take that long they need to be looking for a different machine shop.

At the outside limit the rings should be in by the time a FT cam is broken in.
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From what we’re seeing by using this secret ring seating compound, (that the top fuel guys are using) the rings are set before the motor is even started.
 
This is just a consideration, Stop n go so. Cal traffic is a terrible way to average hwy mileage.
On average it takes 1.5 hrs to drive 68 miles at off n on 80mph in stop n go traffic. My impreza 4 banger shows 6.5 -9 mpg when accelerating to speed. ..then it's 29-37 mpg at cruise. Dbl pump 340 with a cam is going to see less.
Just food for thought.
That carb isn't right and imo is main problem. I didn't build this motor, but ime my motors don't need break in time to make decent fuel mileage. Things do not get or are not that tight from a re build.
Driving style, carb, tune , open headers...those will hurt the mpg.

I love when people who aren't well versed speculate...then latch onto one detail they do understand and start pushing back with it. Get the fkn carburetor changed then comment. You don't need a dyno tune...unless you can't read a book n tune a carburetor or get me to come up that day. This combo isn't tricky to tune, Trust me.Tried cranking comp test but the gauge he has pissed the squeeze out. Junk. It DID push into the 150's before spinning back down 'old snap on' . The motor , if it had any sealing issues, would not pull or perform very well if it did...would be popping ex and coughing if the valves weren't sealing... it would push oil out the dip stick tube and breathers if the rings weren't sealing.


It's the carburetor.
 
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With manual trans I would go Holley double pumper. Maybe compression test and hook an air/fuel ratio gauge up see whats going on. Lastly go new carb not some old unknown carb.
 
340sFastback said:
With manual trans I would go Holley double pumper. Maybe compression test and hook an air/fuel ratio gauge up see whats going on. Lastly go new carb not some old unknown carb.
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I've been using the older version of this one for years (love having the choke on a street car, but I do live in MN :lol:)
Holley 0-4779C Holley 4150 Double Pumper Carburetors | Summit Racing
Quick Fuel BR-67213 Quick Fuel Brawler Street Series Carburetors | Summit Racing (cheaper but only comes with electric choke)

Nice car! Best of luck on whatever carb you choose.
 
mpg is only relevant for comparison when cruising at a constant speed on the highway for a decent distance, say 100 miles. Too many variables in town or stop and go traffic.
 
64whiteghost said:
Miles are miles guys. My speedo not working is 100% irrelevant

You dont have to be a mathematician to know that I drove 81 miles combined. 62 yesterday and 19 today and my brand new fuel tank that is 18 gallons, (which i filled up completely full before my day out yesterday) my brand new fuel guage is saying my tank needs a refill speedo or not now im getting 5 miles to the gallon

Did i get on it a couple times. Well duh ! But i dont believe 1 or 2 pulls to 4500rpm can really affect your mpg that drastically

But we did suceed in getting the motor to make more power
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Other than the carb and tune that you are already addressing, the cam overlap is where I would look for fuel consumption related to the engine.
 
MOPAROFFICIAL said:
750 dbl or vac should get the same if only driven in the primaries.
Mine got 15 mpg.
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My opinion is basically mute (I've never messed with vacuum secondary carbs) I've just always read that manual trans cars should use mechanical secondary. Cary on, enjoy seeing a tune improve!
 
Is there a fuel leak somewhere? How is mileage being calculated (i.e., how is distance being determined - gps, stock gauge, google maps, etc)?
 
DentalDart said:
I don’t think I have ever checked my MPGs in my 340…
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My van with a 340 get 10 (3.55 gear) and my 69 Dart gets around 12-14 (3.23 gear), depending on the fun I want. Cold running and just around town, maybe brakes dragging a little, could be a couple of issues. Should get better than 6, unless you are having a lot of fun. Also maybe a little clutch slippage, as in a high stall converter or weak springs or a little oil on the flywheel or clutch plate.
 
Scody21 said:
Have you done a leak down test or a cylinder pressure test to check?
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No I have not. But the motor is running super strong no signs of bad cylinder pressure.

My compression checker is an old napa one my dad gave me. I need new one ro check my compression. Will update on that
 
Vinny said:
Is your speedo accurate? Is it possible that your actual miles driven is not correct? Just a thought.
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Very highly unlikely. Last I checked Google maps is super accurate. Im taking the roads its mapping to the locations ive been going. So im sure speedo or not its accurate.
Im entering actual location from and actual location too. no way im not tracking mileage correctly
 
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64whiteghost said:
Very highland unlikely. Last I checked Google maps is super accurate. Im taking the roads its mapping to the locations ive been going. So im sure speedo or not its acurate
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So, unless you are using Google Maps from specific address to specific address, then it will take you to city center, which it can be miles different from where you are going. Just looking to help with actual miles recorded.
 
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