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Old 04-09-2012, 08:46 AM
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vacuum or lack of

On my sbc 360 rebuild I have a major issue.My car idles at 1100 rpm until I drop it in gear where it won't idle at all unless I keeps the revs up.I have a 2.8k stall and a 284h cam .I am running a 750 holley al.heads w/roller tips and al. intake.Tremendous burnouts on power braking with no engine misses but when I jump on the brakes they are gone and the engine dies because I lose vacuum so quickly when I get off the gas.Suggestions?

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Old 04-09-2012, 01:55 PM
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Steve, you did not mention what compression you are running. What type of intake, weather it's dual or single. Is the Holley VS or DP. How did you set the timing? Is it vacuum or mechanical advance. Do you have power brakes? Do you know the name of the cam and specs for the 284H? First thing I noticed is the converter. It's too low for the cam you are running.

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Old 04-09-2012, 02:24 PM
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Comp Cam 12-250-3(duration 284/296,lift507/510), Holley750 dual line with vacuum secondaries and a Weiand square bore intake.Initial timing set at 10 deg. at idle and after hooking up vacuum advance on dist set at 30 total. At full warm up cylinder running at 460-600 deg. F.Checked vacuum at PCV, 12 at idle.Checked for vacuum leaks and found none.1982 Z-28 with vacuum booster on original brakes.Turbo 350 trans. I realize the cam was bigger than what I needed for the stall I have but still figure there is a way to make it work .botred .60 over,flat top 4 relief pistons and was told +10-1 compression
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Old 04-09-2012, 03:03 PM
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Does the vacuum drops as soon you like go the gas pedal? That does not sound right. Usually the manifold vacuum would jump up at first as soon when you let go the gas pedal and it will then settle down once you slow down. I think you just get too much cam for that setup. I'm suprised that your power brakes still works.

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Old 04-10-2012, 12:08 AM
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The distributor advance curve needs to be modified.
a stock curve will not work.

lock out the distributor mechanical advance curve. Set the locked out timing at 34-36deg BTDC. swap the carbs power valve for a 4.5" rated.

flip the carb over and look at the throttle blades and fuel transfer slot exposure at idle ( pri and sec. reset for .020 to .030" exposure at idle and reinstall the carb.

reset the carb idle speed and idle mix screws.
Then connect the vac adv hose to the ported vac source on the carb.

This will maximize the idle manifold vacuum clean up the idle and sharpen the throttle response.

It will idle in gear about 800 rpm and 900rpm in neutral/park.

The comp XE284H-10 cam likes locked out timing.

If its hard to start when hot install a ignition power interupt switch.

For max performance with that cam get a 10" 3500+ stall converter

OEM GM starter motor: make sure the rear starter motor support bracket/brace is installed.

Last edited by F-BIRD'88; 04-10-2012 at 12:24 AM.
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