Hi all,

I run a chevy 355ci all old school in an 85 transam with stock th700r4 tranny and stock converter. I'm getting now 25l/100kms in city driving being very gentle. I wanna try to tune the holley 750 4160 vacuum secondaries I have on this engine to get a better gas mileage so here is what I was thinking to do if you could tell me if that would help or be worse etc...:

-Move the distributor vacuum advance from ported to manifold vacuum ( should I then back the distributor to avoid to much advance??
-Put a 50 power-valve as my car idles in gear ( th700r4 tranny)at 10'
-Put a spring in the vacuum secondaries housing that opens at the highest rpm ( maybe the black spring)
-Put a wire in the IFR (Idle feed restrictors)to limit the opening.
-Leave the 67 actual main jets (70 originals).
-Tune then the carb with a vacuum gauge to get the highest RPM.

Thanks guys!

 

-Move the distributor vacuum advance from ported to manifold vacuum ( should I then back the distributor to avoid to much advance??
-Put a 50 power-valve as my car idles in gear ( th700r4 tranny)at 10'
-Put a spring in the vacuum secondaries housing that opens at the highest rpm ( maybe the black spring)
-Put a wire in the IFR (Idle feed restrictors)to limit the opening.
-Leave the 67 actual main jets (70 originals).
-Tune then the carb with a vacuum gauge to get the highest RPM.

some might jump on me for saying this but,

leave your vacuum advance alone, tune the carb for proper eficiency, then install the heaviest secondary spring, and drive with a light foot.
You still may not see much of a mileage increase, your pretty close to what most 355's can get with a perfomance cam.

 

1 If your only making 10" at idle put the advance on full vacuum. It will idle better and make a little more vacuum.

2 The power valve can be anywhere between 2" below to 1/2. If you are going for fuel economy try 1/2 but if your primary jets are set up for economy then you need 2" below otherwise you will get a lean condition on light acceleration with 1/2 below.

3 If you are driving the car easy the spring will not effect anything except make less power when you get on it. Your doing 99% of your driving on the primarys. If you have a heavy foot then a stiffer spring will help but in the end is just counter productive because you will just give it more pedal to go faster anyway.

4 Read The Holley Tuning sticky
https://www.thirdgen.org/forums/carb...ey-tuning.html