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Old 05-30-2010, 07:22 PM
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holley carb tuning

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 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.
-Tune then the carb with a vacuum gauge to get the highest RPM.

Thanks guys!

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Old 05-30-2010, 07:30 PM
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Do not retard the timing when hooking up the vacuum advance,at wot there is little vacuum so the advance is not working,but under cruise it will advance the timing to help backing out of the throttle. Is the dist set up for advance curve,how much timing are you running ,what engine specs,what cam, what gears,more info
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Old 05-30-2010, 07:59 PM
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Don't have a clue about the advance I have ( not the stock damper as the damper from the LG4 has been put back on this 350..( got the car this way), gear is 3.08 and tranny is stock th700r4 with stock converter as well.
Engine is forged internal, sportsman 2 heads, comp cam XE284, dual plane intake world, holley 750, hedders , free flow exaust etc..
car works flawlessly jut burns to much in city.
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Old 05-30-2010, 09:40 PM
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NO problem that combo. is just right, I do see a problem with your right foot thou!!

There is a VERY fine line between performance and gas mileage.
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Old 05-30-2010, 10:23 PM
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the only way you are going to improve on the carbs idle and off idle circuit is to precision fine tune it using a afr meter/gauge.

When ever you have your foot in it enough to open the secondaries, you are going to burn a lot of gas.

Let that simple concept sink in.

The cam is way too big for mileage. If mileage is a priority.
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Old 05-30-2010, 11:01 PM
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The brown or black secondary spring will all but prevent the secondaries from opening, so if this will help matters, use one of them. Or just go easier on the throttle.

Using manifold vacuum will not increase the total advance while under heavy load/throttle. If the timing is correct now and you change from ported to manifold, all you will do is lower the idle speed to compensate for the increase in idle speed this causes.

If you hear pinging on throttle tip-in, you will need to physically limit the vac can's travel w/a vacuum advance limiter plate: Crane #99619-1, instructions for installing and setting it: #99619-1.
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