Thirsty 400!!!??!!

[Pickupmanx2]

Evening all, so I did a full Cap/Rotor/Points/Plugs/Wires on my new friend Elmer,,,, He still drinks gas like it is ladies nite and he is drinkin for free!! I mean he will get up and go,,, but it is at a cost, MAYBE 6 or 7 mpg I am guessing,,,, would that be the reason for the 3 tanks??!!??!! I have some 265/70/15 Hercules Terra-tracs coming, hopefully that will lengthen his legs a bit from the P225/75s or are the BB 400's just thirsty? I have to say I enjoy driving him, and once I get new shocks and tires I am sure he will ride even better, but as a DD he is not gonna make it!

[Longhorn Man]

you need to figure out what your actual MPG is before we can say anything... 6 to 7 is pretty low. I generally get twice that in my big block burb.

[dan42]

Originally Posted by Longhorn Man

you need to figure out what your actual MPG is before we can say anything... 6 to 7 is pretty low. I generally get twice that in my big block burb.

X2 - if you're going off the stock odometer, it could be off a ways - find something you know the distance on, and figure it out.

[Pickupmanx2]

Well, it is 6 miles to work,,,, make it 7 with a stop at NAPA, and then 6 home, I am down to 1/2 tank after a few days of driving back and forth,,, I keep track of the miles in a check register, I will check Wed. when I drive him in, but my rough, in the brain calcs were about 6-8, and the odo is very close, within a few 10ths... I am guessing a carb rebuild is in the future right now for starters.
As a side note, will an Edelbrock 1405 bolt on in place of the stock quadrajet?? I have one in my collection of things... has 2 sets of bolt holes...
Thanks

[Longhorn Man]

You'll need an adaptor plate if it is a stock intake. If you go this route, get the more expensice edelbrock piece, not the Mr Gasket el-crappo unit. The latter one has to go on upside down and is restrictive.

[capev86]

stock q-jets are excellent for fuel mileage if in good shape and tuned properly. i am currently building up an 84 q-jet with edelbrock guts to throw on my 72 burb. big blocks are heavy and naturally thirsty due to displacement and you may also be fighting the gearing, etc that the truck is set up with. you need an overdrive and a 4.10 or softer rear for modern driving. also, if you are driving in traffic or at low or high speeds you are not realizing the best economy the vehicle is capable of. that vehicle is gonna get best mileage at a steady 40 to 50mph. even modern trucks with fuel injection and overdrive get drastically better mileage if you keep the speed at 60 or under. trucks push a lot of air out of the way as well as move a lot of weight so gentler speeds are a must for gas savings. my 72 burb 3/4 ton 2wd has a measly 307 with 3 on the tree and 4.57 rear gear and the best mileage i have gotten is 10-11mpg on mixed driving. i try hard to keep it to around 50 on the highway, especially since the engine is screaming. i have a 4.10 rear gear ready to go in and eventually plan to add the factory OD to the saginaw and that should get me closer to the 18mpg that my dad's 87 burb gets.

[into69z]

Last year ,I built a 454 for my 69-everything is new, about 8:5 compression, a xe262 cam, roller tip rockers,Edelbrock performer and a 750 Edelbrock carb,rebuilt 400 trans,swapped the rearend to 3:23 gears(speedo is still off about 15mph)and drive at "about" 60mph and I'm at 8 mpg. Its a c-20 with 2008 hd tire/wheel take-offs 245/16. I was thinkingn maybe a 600cfm carb might help with mileage but not sure. Gear vendors would help but you can buy lots of gas for $2500......any other ideas?

[capev86]

the engine will only pull as much air as it needs, so a big carb 'can't be hurting you, especially if it is a spread bore type. you can get an overdrive much cheaper. a good condition 700-r4 will survive behind a stock big block if you don't abuse it (use synthetic aft). my dad has an '84 700 that ran fine behind a built up 400 small block and it never missed a beat on the month long trip out west and back with our 6,000 pound trailer. i don't think a gear vendors is worth the money when you can build a factory 4 speed trans to do the same job for cheaper.

[dwaite72lnghrn]

Rebuild the qjet or buy a rebuilt unit. If fuel economy is your goal do not put a square bore carb on it. I think part of your problem might be the choke is never coming off with a short run like that. Add a vacuum gauge to the truck and when you run down the road try and keep the highest vacuum on the gauge possible. My truck runs about 20" of vacuum running down the road at cruise speed when I am in town it is about 12" or less and my fuel mileage drops off quick. If you are running stop lights the whole drive to and from 8mpg might be all you can get out of it.
If I run with a light gas peddle and keep the rpms at 2500 I can get about 13.5-14mpg. That is with the c30 longhorn that weighs 5300lbs and 4.10 gears and a turbo400.

[Ticker]

I'd tend to check the A/F ratio before looking any further - if you've got a performance shop nearby or even an emissions/tune-up place they can probably get some readings for you. The assumption here is that the timing is done right - the first thing to check.

Q-Jets are great street carbs if properly rebuilt, but it's not unusual for them to be gas hogs due to wear - or leaking out the bottom plugs. It's worth spending the money to have them professionally rebuilt by one of the few shops that do it right (see my .sig) as typically at this point they need a fair bit of TLC to work as designed. Do no - do not - buy a rebuilt from NAPA, Schucks, etc. - they'll be complete garbage.

[Pickupmanx2]

Thanks Gents!! I have a few things on the to-do list, 1) TACH,,,a must have, 2) timing/dwell 3) new carb base gaskets,,,possibly rebuild it while it is off,,, most likely rebuild it Drive drive and drive!