My 81 Had A Different Engine Installed The Person I Bought It From Was Not Mechanicaly Inclined And Knows Nothihg About The Motor. To Me It Looks Like A Good Wrench Engine..it Has A Edelbrock 750 Carb And A Edelbrock Manifold..just Tuned It New Plugs, Wires, Rotor , And Rotor Cap..timed At 8 Deg.btdc. And I Tried To Lean Out The Carb As Much As I Could. Still Getting About 8 Mpg. Any Ideas

 

You got 1 mpg on me. What cam, heads, etc., if it's stock all but the carb and manifold, you prob'ly got too much carb. I've got a 383 stroker and I'm dumpin' my 750 for a 670.

 

found out the lock up in the tranny might not be hooked up.. the computer was disconnected..that will effect the milage also

 

I get 7-8 around town, 10-12 on the fwy. sounds like you might be within range given your set up.

 

Set the initial advance to about 12 to 16BTDC. This will help driveability and economy. You will probably be able to back off on the idle speed now.

Find out how far out the idle mixture screws are. Make sure they are even. Ok, starting wherever they were set originally open them evenly until the idle speed stops increasing. Now, turn them in evenly till you get a 50RPM drop in your idle speed. This is about as lean as you can get on the idle circuit and still have some driveability.

As far as leaning the carburetor, these things were calibrated lean from the factory. If you go too lean it could be forcing you to really lay into the throttle to get any performance out of it. Richen up the mains and you may not need so much throttle to move the car around.

BigBlockk

Later.....

 

going to look into a B&M lock up control,that should help the milage a little

 

be carefull I had a 600 double pumper on my 383 and it was not enough, put a 770 and it made a huge difference. mileage stayed about the same 8 in town 10 on highway

 

Good god you guys, I used to get 16 and 17 mpg before I started modifying things (and my rings let go). Sub 10 is big block only territory!

First of all, as Lars is apparently not here to say it, 90% of tuning problems are timing problems. 8 btdc, while factory recommended, is probably not optimal and definitely not what you need to be basing your timing on.

First of all, install lighter mechanical advance springs into your distributor (available at your local autoparts store as a kit). You might've already done this with the rotor? This will advance the timing much sooner which is good

Second, after you have installed this, check your timing at 3000rpm or so when mechanical is all-in. It should be about 34-36, set that and let the idle-timing fall where it may. As long as you are not getting knock (and most won't) that's all you need to do. You should get better mileage AND better performance from that.

Make sure your mechanical advance moves at all, just twist it by hand with the distributor cap off. Mine froze up twice on the original distributor and literally halved my mileage.

Once the timing is sorted out, only then is it time to get the carb to a good baseline. BigBlockk gave you some good advice. Keep in mind that the idle mixture screws and the idle speed settings both affect eachother. I don't know anything about Edelbrock carbs.

Oh, rather important information we need, how does it run?? Idle, cruise, acceleration, etc. Hope the convertor helps but even if that is the main problem you could still use some optimization.

Good luck

Chris

 

Chris, it idles smooth and has good acceleration with a lot of pep on top end..I did notice a gas smell when I put the car in the garage. checked everywere for leaks ,found none.

 

Ever have any trouble starting after it sits? A common problem (for q-jets at least) is gas continuing to pour out after turning off due to heat. If that's the problem, fix it soon, that's why my rings need replacing.

-Chris

 

there is a thread in here

 

I have no idea why your gas milage is so low, or even the other ones.
You could change your fuel filter cheap and easy.

In comparison with 427 and 4.11 I get all around about 14 mpg.

There MUST be something wrong with these with such a low mpg and a highway rearend. Lockup auto just isn't enough to figure the loss.

 

14

 

I take back my bigblock comment, way to go Brute force is great but there's no reason to neglect efficiency.

-Chris

 

you would be surprised what a difference lock up makes on my car with a 2500 stall speed 700r4....its about 4-5 miles per gallon difference

 

What distributor and type of carb is on it? Edelbrock makes more than one kind. If it's the stock distributor, you need to get a vaccume advance one to work properly without the computer. I couldn't even get my vette to move with the old distributor and new carburetor.

You should be getting 16mpg on the highway with that setup, that's what I get with my q-jet and lock-up not working. However I had the dyno shop make my q-jet actually work properly and set my timing to 36 degrees total advance.

 

ther carb is a edelbrock 1407 performer (750) and the distributer was changed to a HEI with a vacuum advance..I 'm running off the ported port on the cxarb.

 

'77 350, Stock Q-Jet just rebuilt, 16.25 MPG Hiway, Starts good when cold but really hard to start when engine is warm or hot. Garage smells of gas. Please tell me more about the "gas continuing to pour out after turning off" ....is this a float level problem?

 

So how do you know when you got it "right" as far as MPG? I have a 77 4spd manual and I get about 15ish MPG, and most of my driving is highway. The car is "very" stock except for a suspension upgrade that I have installed. I would expect a manual should do slightly better than an auto, but Vette's have some peculiars that throw convention a bit.

The carb is a q-jet, and the engine has not been "tuned up" that i know of, but was rebuilt just under 2 years ago (however, mismatched spark plugs make me question everything).

I think the rear-end is stock as well, but I can't seem to find the stamp to locate the ratio.

Brian.

 

I would first ask: What method are you using to compute your fuel economy?