Thread: Gas Milage Fix on my Chevy 350 and cold stall issue

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So i've been using my 48 buick as a daily driver and i finally got arround to doing a bit of math and determine my gas milage. Turns out i'm getting just over 7 MPG. This is not what i was used to with my 66 mustang with the fuel injected HO 5.0. I get it. It's a much bigger car (about 1,000 pounds heavier!) and the carburated 350 it a bit more aggresive. But what are my options.

I CAN'T FIND THE DATE STAMP ON THE ENGINE!!!! The block number is 3970010 so it's 69-79. But i have no idea what year the block is. I know it's got the smaller spark plugs "peanut plugs" so that means it must be post 1970? Doesn't really narrow it down.

I have it attached to a TH350 tranmission. And a 63 rear end with traction lock. How can i tell what gears it is without tearing everything apart?

The engine has an edelbrock performance intake and a quadrajet carb i tuned myself (ugh). Should i just expect single digit gas mileage or is there a hope to push my big pig into double digits?


Also while i'm at it the engines been acting a bit wierd. When i cold start it i need to keep my foot on the gas and feed it up to 800-1000 rmps for a min or two or else it stalls........anybody have any ideas? I just put new plugs in (did it myslef! AC Delco)

An easy way to figure out your rear gears is to use the formula (mph x trans ratio x axle ratio x 336) / tire diameter in inches = rpm, or to convert to axle ratio make it (rpm x tire diameter") / (mph x trans ratio x 336). First measure the diameter of your tire, or better yet make a chalk mark at the bottom and on the ground at that mark, then roll the car forward or backward until the chalk mark is on the bottom again and measure the distance traveled in inches. Divide that number by 3.14 (pi) and you have your actual diameter in inches. Your TH350 is 1:1 in high gear, so that drops out of the multiplication. Now go for a drive, get it in high gear at any given speed and note your mph and rpm. For talking purposes lets say you drive 45mph and your rpm is 1950rpm, and let's assume you calculated your tire diameter at 27.5". We multiply 27.5 x 1950 yielding 53,625, and we divide that by 45mph x 336, or 15,120 which gives us 3.546 or 3.55 for the rear gears. Plug in your numbers and you know your gears.

On the carb, it sounds like your choke is not working right, and your statement about tuning (ugh) says that you're not too confident in the carb. The quadrajet is one of the best spread bore carbs out there, but when they get worn they need specific attention by someone who knows what they're doing. I'd suggest that you might consider finding a good carb shop that knows the Q-jet and have it rebuilt, then have them walk you through the tuning process to get it nailed.


Read more: All About Axle Gears - How To Calculate Your Cruise RPM - Car Craft Magazine Page 3

Ditto what Roger said. Q-jets are really pretty good on mileage as long as you keep your foot out of those giant secondaries. What is your normal drive? If it's a lot of stoplight-to-stoplight the mileage is not going to be great; however, unless you've got some really aggressive gearing I'd think that 15MPG on the highway should be attainable. Do you have a tach? if so, what does it read at 60MPH? if you can keep the cruising RPM down below 2,000 the mileage should certainly be better than what you're experiencing.

My 2 cents,
Glenn

I agree with Roger, the Quadrajet needs to be rebuilt. I rebuilt the one on my Vette, 350, and if I keep my foot out of it'll get anywhere from 12 to 15 mpg.

If your rear was not a posi, it would be simple. Jack one rear tire off the ground, rotate tire 2 times and count driveshaft turns. The amount the driveshaft turns will be the ratio. Won't happen with a traction lock.

Originally Posted by Scooting

If your rear was not a posi, it would be simple. Jack one rear tire off the ground, rotate tire 2 times and count driveshaft turns. The amount the driveshaft turns will be the ratio. Won't happen with a traction lock.

Yeah, that's why I gave him the speed/rpm/tire diameter method, Ray. It works no matter if it's open, posi or locked.

Thanks for all the great advice! It looks like I have some tinkering to do!!!! My neighbor is a mechanic and is going to help me out. Thanks so much

OK so update here! I had my neighbor come over (old mechanic) and he dialed things in MUCH better. Apparently i was running WAYYYY rich (over 6 turns on one idle screw). A little adjusting on the mixture, adjusting the idle screw, and tightening the carb to get a better seal all helped improve things. I'm still getting a stall initially if i don't keep the gas up for 10-15 seconds or so right when i start. but then it runs much better. He said to look into replacing the carb gasket because it's one of those extra thick ones and sometimes those degrade and can leak.

So i have an edelbrock performance intake. If i were to buy a carb and rebuild it so that i always have a spare....what should i buy? What will fit?

Well as far as I'm concerned the Quadrajet is/was the best street carb ever built.

Also on your cold run problem, that intake probably does not have the heat passages under the carb since it's a performance intake and if it does a lot of guys block them off. The heat passages help vaporize the fuel on a cold engine and on cooler days. So it may be something you have to live with. That being said I would
make sure the choke system is adjusted and working properly.

Monitor your fuel pressure at the carb inlet. Quadrajets work best at 4 3/4 psi. More pressure than that can override the needle and seat and allow the fuel pump to blow raw fuel into the intake manifold, which does little for tuning or mileage. Tee off at the carb inlet and run a line up to the cowl, in front of the windshield. Mount a 0-15 psi mechanical fuel pressure gauge on the cowl so you can see it through the windshield while you drive. It will only need to be mounted temporarily while you get the fuel pressure under control, so tie wraps and duct tape are ok. If you find you need to lower pressure, purchase a quality pressure regulator and mount it permanently in a place where it will be protected from engine heat and rocks thrown up from the pavement.

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Well that Buick is pretty heavy but I have always heard mostly good things about the quadrajet. I took one off my 1976 Corvette 350 and replaced it with an Edelbrock 600CFM four barrel on an Edelbrock Performer RPM and most of the time I get 16 mpg but on highway driving I have gotten 17 mpg but my roadster is only 2800 pounds with me in it and the rear is an 8" Ford with 3.55 gears and the trans is a 700R4 with a 4th gear OD. Actually this was disappointint to me but it is almost exactly what the 1977 Z28 got with a four speed in the 1977 reviews.

Don Shillady
Retired Scientist/teen rodder

The question was cost to me and I did know where to get the quadrajet rebuilt. The cast iron intake is also heavy so the Edelbrock carb is new out of the box and the aluminum intake is lighter than the stock intake.

Don Shillady
Retired Scientist/teen rodder

So here's where i'm at with everything. After my neighbor helped me lean the mixture and adjust the idle it worked much much better.....for a while. Now it's back to cold stalling. It's only 60 ish in the morning so it's not even that cold. I can only imagine how bad this will be in the winter (my garage isn't heated).

At first i thought i would just switch fron the divorced choke to a manual choke which i've heard is much better. But i think you are all right and i might have to actually rebuild the carb. So my question now becomes......what replacement?

I will rebuild the quadrajet on my own in the next few weeks. I want a brand new carb i can throw on so i can keep driving the car. I think having the spare rebuilt and ready to go would be good anyway. If i buy a new carb i might go with an electronic choke.

I have 2 that i'm looking at

Demon carb 625
Demon Fuel Systems*-1900*-625 Street Demon® - Ball Burnished Aluminum

or the
Edelbrock 1405 or 1406
Performance Carburetors and Accessories - Thunder Series Carburetors - Edelbrock, LLC.

I've hear people say that electric chokes are TERRIBLE and if your going to move away from a divorced choke you should go to a manual choke. Any advice there?

Also i noticed that there is a lot of different types of carbs with different bowl materials? Suggestions?

Lastly i was looking at Holley carbs and quickly became overwhelmed. There are a lot. Is a double pumper something i should get or is it not worth the money.

Thanks for the help all!!!!!!!!!!!!