My trans am has a 383 stroker, t56 transmission, 3.42 gears, holley 750 double pumper with proform mainbody and 4 corner idle, and runs on 93 octane. I just took a 200 mile trip in this car. I was doing 55-75 95% of the trip. Never took the car on this long of a trip since its been mostly finished. The carb is tuned best i could with a vacuum guage, but everything else is untouched such as jets. I got this carb a while back because it was just a good deal and i needed a carb. I averaged 12 mpg highway and it was running dog rich. I drove it easy almost the whole way, engine braking, and rolling down the hills. There was no city driving in this average.

Im running vacuum advance with total timing at 50 degrees counting the vacuum advance. Keep in mind i do have double overdrives, but does 12 mpg sound right? I was hoping i could at least break 16 mpg but oh well.

I made a post a few weeks ago i think about how to tune this for mpg, but i think ill just sell it and buy another carb. I would definetely come out ahead because the carb has little use on it. What carb would be ideal for the engine? I want a carb that will get me a good mix of performance and economy. Im having things a little rough so i want as many mpgs as possible. Where i live you drive mosly highway so that shouldnt be aproblem. Im going to buy a used carb this time and save as much as possible.

If you need more on the engine specs, ill post them. What size do you think a 383 would need at most? I want to switch to a vacuum secondary now.. Im runnign an hei distributor with vacuum advance and an edelbrock dual plane rpm intake. thanks!

 

Vacuum secondary and a t56 is a no no. Just jet it down some and make sure you check the plugs. I would stick with the 750 dp. Not for nothing but if you want good mpg you should have bought a v6.

 

Look dude, i had a vacuum secondary carb with the t56 before it it wasnt much different. Also, no i shouldnt have bought a v6. It isnt hard to get 20mpg highway and thats what im aiming for.

 

"Dude" Wasnt much different except for the fact that the car bogged if you really ever got on it cause the secondarys cant keep up. No its isnt hard to get 20 mpg in any car the question is why did you get a 383 when your gonna be down on horsepower big time trying to get your 20 mpg. I would start thinking about efi if i were you.

 

 

Better gas mileage should be attainable. I routinely averaged 12MPG with my 1975 Nova, 406, 750DP, no overdrive, 3.73 gears. You stated above that the car was running rich while cruising. My guess is that your idle and transition circuits need some help.

 

i agree with pancherj. you also didn't mention what cam or heads you are using. what rpms are you seeing at freeway speed? 4 corner idle can be tricky to set up, i think there are posts here to help. if you have a long cam your idle transition slot could be uncovered too far causing the rich idle. that carb should be just about right for your engine. you said you have double overdriive? could you be turning the engine at too low rpm to be in the cam's efficency range? is the vacuum advance adjustable? did you set total timing with v/a disconnected at the distributor and plugged? what rpm is tour mechanical advance all in at?
lots of things need to be dialed in on these engines. start at square one and only change one thing at a time. if you get real frustrated, i'd swap you my vac sec and 700r4 for your d/p and t56!