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View Full Version : Tuning my carb for gas mileage and Performance



gchandler
08-17-2004, 06:48 PM
I am in the middle of a long term effort to tech myself how to really get a good tune on my carbed small block.

Take a look at what I have so far:
http://www.hotrodhomepage.com/articles/carb_tune.htm

69boo307
08-18-2004, 06:54 AM
I'm gonna tackle learning the Q-Jet when i get my new motor put together. A small block with a well tuned Q-Jet can get 20+ mpg on the highway.

MarkM66
08-18-2004, 10:19 AM
Get one of these;

http://www.innovatemotorsports.com/home.php

Salt Racer
08-25-2004, 08:15 AM
Wide band A/F meter sure would be handy, but here's a cave-man method I typically go through...

Keep leaning the primary jets until the motor starts surging at crusing speed (35~45mph), then add 2 jet sizes. Then go to a drag strip and play with secondary jetting until you get the highest trap speed.

My Riviera with 425 Nailhead gets a little better mileage than yours. I'm running a 750 Holley vac. w/ Proform main body. 68 primary and 78 secondary jets w/ tall yellow spring. 5.5 inHg PV, and 0.030" on idle feed restriction. These numbers are JFYI and don't mean anything unless air bleed sizes are the same. The altitude in my area is about sea level ~ +500ft. Tuning procedure for idle feed restriction is pretty much the same as primary jets.

You can also open up the power valve restriction channel a little bit, and use leaner primary jets. But this may require PV with numerically higher vac. rate.

Accelerator pump is another area to play with. I think I have a pink cam and 31 shooters. Clearance is set at 0.019". This is as lean as I could go w/o getting stumble or bogging with engine at operating temp. It stumbles big time when motor is cold.

The timing is set at 12 initial and 16 mechanical lead (10.3:1 compression w/ stock iron heads), and stock vacuum can which I don't have a clue how much timing/rate it has.

I once got 21.5 mpg in my '69 Nova when it was running. 377ci SBC, 750 double pumper with same tuning procedures, Trick flow heads, 230/242 @ 0.050 Comp cam, Procharger 600B, 0.62 6th gear and 3.55 axle ratio. The car ran 12.65 @ 116.5 in the very same configuration.

Good fuel economy and good performance can coexist with carburetor (to some extent). It's just tedious and time consuming.

K