My 340 getting 6 mpg

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MOPAROFFICIAL said:
The carb is decent, except for the throttle plate... its cheese. The accelerator pump arm screws wobbly pos's and you watch the throttle raise then dip while turning the speed screw in. Trash. The idle changes speed after driving it, raises.. lowers. When yoy come back from a test drive it's gone from 850 to 1100... then you push close the throttle and it's back to normal. It took enough return springs to necessitate standing on the throttle to open it.. I said take those springs off and buy a new throttle plate for this shitter. The thing is nice.. except for that part.
It got dark... so that's why we stopped with the tuning, for now. Going to drive it a little now and check plugs, check milage etc .
it pulls much harder, acceleration is smooth...and it has balls up top now it's not starving for fuel.
I like newbombs guesstimate of timing because that about as high as you would want it while still maintaining an advance curve. 8 to 10 advance is nice. His is all in by 2800 fwiw.
I normally will spend most of an afternoon test n tune with jetting and playing with pump shot.. but I got there a little later in the afternoon. It'll probably pick up 5 mpg immediately but expect it to get closer to 15 mpg. The higher initial means less fuel/pedal to get moving aka more power, less stink being cleaned up idle... not feeding from the mains... lowered throttle position. The timing shows a nice right before the bend initial and total burn mark.
It initially surged with the 65 jets, so immediately I went to 68's. Going to jet up depending on the plugs and feel... then he has a wide band he wants to get working..so that can aid his final jetting
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I can’t remember is his carb a true 850 or some other venturi/throttle size they call an 850?

If it is an 850 that sounds like a pretty small MJ still.
 
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
 
LO23M8B said:
Even 100% stock 71 & up 340's used a 750 TQ. Maybe Mopar had it wrong.
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800 cfm was the smaller TQ. (maybe a Carter fib on the size) A properly tuned air valve on the secondaries will give all the air the engine demands. Some 360's came with an 850cfm. IIRC the Mopar Q-jets were 725 to 750. Triple boosters on the primaries. I pulled a nice one off a 318 van. It did better on the MPG than my TQ. They have air valve controlled secondaries too.

Properly set up you could probably run a 850 TQ slant if you tweaked it enough.

I have a bunch of TQ's. I collect the jets and metering rods out of the emission ones. Several '77400HP auto models. I have a Strip Kit too. (no, I don't want to sell it!)

I have a "Doc Fiberglass" Electric Banana secondary spring adjuster tool too.

Every used TQ, BBD, AVS, AFB, and Qjet I have need to have the throttle shafts bushed.
 
There you go......
Direct from Holley......
Since they make the carbs, maybe, maybe just maybe, they know how their carbs work.....
Air bleeds: 'DECREASNG the size of the main air bleeds....will pull MORE fuel creating a richer fuel/air mixture'. A smaller MAB also starts the system earlier [ lower rpm ]. And vice versa for increasing the MAB size.

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Bewy said:
There you go......
Direct from Holley......
Since they make the carbs, maybe, maybe just maybe, they know how their carbs work.....
Air bleeds: 'DECREASNG the size of the main air bleeds....will pull MORE fuel creating a richer fuel/air mixture'. A smaller MAB also starts the system earlier [ lower rpm ]. And vice versa for increasing the MAB size.

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Go read Taylor’s book. You know, a book that counts.

He says what I say. My testing says I’m right.

That’s wrong. You’re wrong because you don’t test.

Any low IQ moron with an O2 sensor can see exactly how it works.

This is the same Holley that to this day is telling people to incorrectly tune their own carbs.

That paper is garbage. Like your tuning knowledge and skill.

Not testing what other people write makes them worship clowns like DV and believe error like this.
 
6 pages and no one mentioned the engine only has 500 miles on it? It it were me, I wouldn't worry about mileage until there is at least a few thousand miles on the engine.
 
toolmanmike said:
6 pages and no one mentioned the engine only has 500 miles on it? It it were me, I wouldn't worry about mileage until there is at least a few thousand miles on the engine.
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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.
 
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.
 
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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Yeah, when you get too many hands in the pie it's time for a new pie!!!
 
MPG is tough. My right foot dictates. 16 MPG is the best I can get on very modest highway driving. Stockish 440 4-speed B-body running Dana 3.54:1 with talker 28" rear tire. But just a couple fun thumps on the pedal and suddenly drops to high single digits. Or worse if nonsense continues. Lol.

My 440 is running single pumper 750 Holley. I found for best performance and economy? The smallest carb that still gives you the needed CFMs at WOT. Faster throttle response with better fuel economy.

I would think the 340 as described? A 650 spread bore may do that job?
 
cojohnso said:
MPG is tough. My right foot dictates. 16 MPG is the best I can get on very modest highway driving. Stockish 440 4-speed B-body running Dana 3.54:1 with talker 28" rear tire. But just a couple fun thumps on the pedal and suddenly drops to high single digits. Or worse if nonsense continues. Lol.

My 440 is running single pumper 750 Holley. I found for best performance and economy? The smallest carb that still gives you the needed CFMs at WOT. Faster throttle response with better fuel economy.

I would think the 340 as described? A 650 spread bore may do that job?
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16 for a 440 is pretty good. Too bad the OP isn't getting that. Speaking of the op, he needs to forget about mileage until things get broken in and loosened up. In the meantime, look at some simple things like make sure the choke is open and pull a few plugs for a look see. That bad of mileage, you should see some black. Another question, how are you testing your mileage? Fill the tank, write down the miles on the odometer. Drive around how ever you want. Town driving with a hot rod and mileage sucks! I would put 50 -100 miles on it of combined driving. Drive it easy not like a crazy man. Refill the tank and record the gallons used. Divide that int the miles traveled and there's your mileage. I worked at a dealer for 13 years and you can't believe how many people don't know how to check mileage. If there is an excessively rich issue, itt should be corrected now before you do some engine damage.
A friend had a stock fuel pump putting out 8+ pounds and the carb was bleeding over. That could be an issue. I think I read some discussion about timing. SOmething else that needs to be right. (including the advance)
 
harrisonm said:
Modified 340, 3.73 gears, 750 double pumper; what were you expecting?
I get about 10 MPG in town with 3.55s, a 340 that is at about 375HP, headers and vac secondary carb. I have gotten 15 MPG or so on the highway at 60 MPH. Any faster and the gas milage drops. I am fine with my MPG. I did not build my car for gas mileage. Anybody who says they are getting great gas mileage with a 340 has not built it for any level of performance. In my opinion, you can have performance or gas mileage.
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I guess it depends on what someone defines as "performance".

It is a sliding scale, not just an either or choice.
 
64whiteghost said:
Whata up guys !

A little update on the 64whiteghost aka my 1964 plymouth valiant.

Got my 340 running and broken in i havs about 150 miles left to hit the 500 miles. But i have one problem. Im getting 6 mpg

Im running an FST double pumper 750 with 67 jets. Idk what power valve yet. But im assuming it has the stock 6.5.

Motor: 340 .30 over

Cam specs: See photo. It is a custom cam and peak power is at 680p rpm

Cylinder heads: flow bench showed 235cfm

Rear: 8 3/4 3:73s with a posi

Im wondering if my 750 is too much. Power is crazy good and im happy with that. But my mpg is insane lol And im wondering if i need to go down to a 650 and if that would help.

Thoughts?



In wondering

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Is your speedo accurate? Is it possible that your actual miles driven is not correct? Just a thought.
 
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 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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