I remember reading in the Mopar "Bible" that fuel economy and Horsepower TOGETHER is of no concern...LOL!! With that being said, it does seem that 6 mpg is a little gluttonous. Following to see what you find.
My 340 getting 6 mpg
- Thread starter 64whiteghost
- Start date
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And I get 18-19 with my 273-4 barrel with a 650 AFB and a 3.23 on the highway.
Super-Cuda
MOPAR .. Move Over, Plymouth Approaching Rapidly!
That's a great looking car!!
Bigger Air bleeds . More air less fuel. Easy to change on the road by removing the air filter. If you change over to removable style.
also square the carb so your not into the PV at idle. Idle off the secondaries and leave the primaries square or drill a hole in the venturi valves as pictured
also square the carb so your not into the PV at idle. Idle off the secondaries and leave the primaries square or drill a hole in the venturi valves as pictured
I agree completely about taking a look at the air bleeds first. For me though, I would be too chicken to change them out unless I could stuff a blue towel into the bores before pulling the bleed out to make sure it does not accidently drop into the butterflies... then I somehow jerk the throttle and let it through. Ugh!
Super-Cuda
MOPAR .. Move Over, Plymouth Approaching Rapidly!
Yup! Im in that group!
Me, I'm taking the carb off to do any work like that.
Paid way too much for the motor to risk something falling into it without me noticing and then ruin it for the sake of removing 4 nuts and some linkage.
I agree completely about taking a look at the air bleeds first. For me though, I would be too chicken to change them out unless I could stuff a blue towel into the bores before pulling the bleed out to make sure it does not accidently drop into the butterflies... then I somehow jerk the throttle and let it through. Ugh!
Amen!!!
Yep, and if the secondaries are coming in early that could definitely contribute to some pretty crappy mileage.
SixPaksRule
Well-Known Member
I'm glad you guys are not working on my car
timing curve is wrong try 16 -20 initial, trial by pinging; all in by 2800 rpm
vacuum advance
195 thermostat
carb most likely is overjetted
timing curve is wrong try 16 -20 initial, trial by pinging; all in by 2800 rpm
vacuum advance
195 thermostat
carb most likely is overjetted
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MOPAROFFICIAL
Oogliboogli
TrailBeast
AKA Mopars4us on Youtube
MOPAROFFICIAL
Oogliboogli
I'm pretty sure that when I was there working on the car I dropped the butterflies and only raise them a little each one so that they're probably in the idle circuit just fine. The car was a mystery carb used he bought it already had like 65 in the primaries we didn't even get into the secondaries to find out if it had a second power valve for instance, or what the Jets even were on the secondary. It was just a tune to get th3 cam broke in and a decent idle for the test drive part. It's still fresh.
The thing may have a secondary power valve and now is sitting at 67 primary and 65 secondary with who knows what power valve in the secondary but we don't know cuz we haven't gone into it yet. I gave instructions on what to start doing and he wants a second opinion is why this thread was started. Keep in mind none of the instruction has even been done yet though/yet.
All good.
I'll be driving up to do it myself and nip this in the bud.
The thing may have a secondary power valve and now is sitting at 67 primary and 65 secondary with who knows what power valve in the secondary but we don't know cuz we haven't gone into it yet. I gave instructions on what to start doing and he wants a second opinion is why this thread was started. Keep in mind none of the instruction has even been done yet though/yet.
All good.
I'll be driving up to do it myself and nip this in the bud.
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MOPAROFFICIAL
Oogliboogli
Not the smiling version either. HahaEddie Murphy?
SixPaksRule
Well-Known Member
I just can't help it.....
Every time I think of Eddie Murphy I can't help but think of his banana in the tailpipe stunt....
I will jump on the timing band wagon. I run a 440, 9:1 Hughes cam, ported heads, MSD pro billet ready to run dizzy - 18° initial/36° all in at 2400 - no vacuum advance and tunnel ram with 1:1 linkage 660 center squirters. I get 13mph on average in the current tune and 3.23 gears (287miles each way to Mopars in the Park, Stillwater, MN each year). I am not sure with the 3.91s as that is usually just for play around locally.
TT5.9mag
Two atmospheres are better than one
The difference between cruising at 13:1 AFR and 15:1 isn’t that drastic with regard to MPGs. But if you’re really interested in getting mileage out of it you’ll end up cruising at 15:1 (or maybe even leaner). But you can’t do that without timing, you have to stick a bunch of timing in it at your cruise rpm and vacuum. That lets you close the butterfly’s down drastically and gets your cruise just off the transfer slots and tickling the booster. Then you can jet the main down to get the cruise afr you want. But you have to put the timing in it to do that. For wot you get the fuel back with the power valve channel restrictors on the primary side. If it has a secondary power valve, plug it and jet the secondary up 10 sizes (starting point) from the primary. Then once you have a basic mpg tune up in it, you can start worrying about dialing in the fuel curve with air bleeds and stuff like that.
AJ/FormS
68 Formua-S fastback clone 367/A833/GVod/3.55s
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Not a lot to go one, but
I can guess with the best of them.
My questions are;
>at what speed were you driving to get that 6mpg, or by tach, what rpm? or was it combined city/hiway?
Then
>At what elevation are you operating at; cuz 67s at sealevel are way too lean.
>What's your Cylinder Pressure? Cuz that combo wants at least 10/1 true Scr, /or 160/165psi..
At 140 psi, the fuel-economy will tank.
>iron heads?
>Who told you the power peak on that cam was 6800? My guess is more like 5300.
>With 3.73s, a manual trans, and 24" tires; 65mph calculates to 3400rpm. For cruising and to get optimum fuel economy your timing is gonna want to be about 56* advanced.
with an auto-trans your rpm
might be/should be, within +/- 3 %.
What I imagine; and/or am afraid of;
> is that cylinder pressure is lower than optimum, and the
> for cruising;
> the cruise timing is way short of the mark, and
the idle-mixture screws are set way too rich, and 67 MJs are way too lean
and the the throttles are way too far open, in an attempt to compensate for all those short-comings.
> and I'll bet you are trying to run a 160* thermostat, with the carb sucking hot underhood air.
I mean your cam is not that far from mine, and my car comes in at 3650/me in it, runs 3.55s, with a 360 even!, burns 87E10 with PMJs in the 70/72 window, and in my wildest dreams I could never get it down to 6mpgs on the hiway. notta chance. If it wasn't at least 3 or more times that, I'd worry.
Oh sorry, I run at 65=2240 rpm in overdrive lol.
And BTW
Your car requires a certain amount of fuel to make a specific amount of cruising horsepower. Lets say your particular car requires 40 hp to maintain 65 mph. Doing the math on that, I get 16.7mpg. Because your engine is spinning at about double the rpm it would require to make 40 hp, a bunch of fuel is getting burned up and/or wasted in just making the rpm, and the cylinder pressure, and fighting ring-drag, etc. So while I agree with everyone that 6mpg is insane, lol, Yur combo is never gonna be that great, and I can tell you, that one of the reasons, is the very short extraction period of that cam.
By my guess, that comes to around 106*@ zero lash, probably less, which means that your pistons are down just a tic over halfway, when the exhaust valve opens, allowing the still expanding hot exhaust gasses to escape the cylinders and heat up whatever they come into contact with. Are you having overheat issues? this could be part of it.
And another thing;
By my guess, that cam of yours has around 67* of overlap. Which, with headers, and screaming down the hiway at 3400 rpm, is gonna put a major yank on the plenum, pulling raw gas right across the piston-tops and into the headers. Imagine what that does to your fuel-economy.
BTW-2
The 750 double-pumper is the right carb for that combo.
1) the accelerator pumps only operate while the throttle valves are moving, and after the shot is delivered, you won't get another shot until you close the throttle valves first. So; at a steady speed, neither of the pumps are doing anything.
And you can adjust those pumps about three ways, to work on your combo. Not every combo needs them to run at max.
BTW-3
I did not read every post, so by now, surely somebody talked about synchronizing the Transfer slots to the Idle-mixture screws.
If that carb does NOT have a 4-corner idle system,
then IMO, the primary throttle valves will need to be drilled, to give the engine the air that it wants without jamming extra gas into it. If this is not done, then the mixture screws will end up wrong, usually too rich at off-idle but low throttle openings. ie a rich cruise condition.
BTW-4
>you are running a PCV right? this is NOT an option.
> I've never had an unheated O2-sensor read right at idle.
Crap, I'm falling asleep, I gotta go lie down
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Gotcha. I don’t know how to tune a carb…
Tony set timing 16 initial and 36 all in. Don’t remember my jet sizes but car ran good.
Maybe I’ll buy a new carb this year and have someone tune it.
64whiteghost
Well-Known Member
At what rpm do you do 65 on the highway ?
64whiteghost
Well-Known Member
Thank you !
I always thought 1 mile to the gallon was pretty good, less than a quart per 1/4.
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