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Old 05-16-2010, 10:44 AM
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Adjusting vacuum advance?

I put a summit distributor kit on my 350 in my '78 camaro. The motor is a 350 headers Holley 750 duel line, Edelbrock performer intake. The advance is adjustable, but I set the timing and say 15 degrees base, then I hook up the vacuum to the advance it jumps to around 40. I tried turning the screw inside the can either way and I don't seem to get a different result.

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Old 05-16-2010, 02:16 PM
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Slow the timing down to 12 degrees before top dead center with vacuum advance disconnected and plugged. Rotate the inner screw of the vac advance counter clock wise to reduce the amount of vac advance, as you move it you should feel a click or a notch, Back it down 1 whole turn, reconnect the vac advance and check the timing. Your shooting for 20-24 degrees at idle.
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Old 05-17-2010, 10:26 AM
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I also noticed to get it to idle good at around 650-700 rpm in gear, it idles at a little over 1000rpm in park. I was told that was way to high in park. Also the carb is a 750 Holley duel line with the vacuum secoundary disconnected and a nut and bolt on the linkage for mechanical secondaries with no choke. Is this too much carb for this motor I was told yes, and no. With no choke will I have to let it warm up before I drive it? It seems to run better, more power, after I let it run for a little bit and give it some gas.

Also I run the vacuum line to the side of the carb on the idle mixture plate now, it seems to have just as much vacuum as the port under the fuel bowl that I used to run it to. Is this right?
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Old 05-17-2010, 10:43 AM
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If that`s the only mods done to your 350, and everything else is stock then yes, you have too much carb. The stock cam is pathetic, the heads are like corks, and the compression is dirt low. Getting a 600 Holley or better yet, A Quadrajet will fill the bill nicely. It should idle around 800 RPM. When when put in drive it should drop to around 700. You can connect the vac advance line anywhere it`s pulling vacuum full time. If it`s pulling vacuum at idle is the clue it operates full time. I would never place a bolt and nut in the secondary slide and why is because if there isn`t a pump shot for the secondary side when it`s forced in which will cause a bog. Also if the secondary`s aren`t working then it`ll go dead lean and pop back through the carb when the secondary butterflies are forced open without any fuel.
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Old 05-17-2010, 10:51 AM
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The motor is early 70s 350, Edelbrock intake, Hooker Headers, mild cam around .488 lift, the 750 Holley, Flowmasters and an electric fuel pump. I think the bolt was set right, when the secoundaries open the rpms grealty increase and it sounds very mean. I was told to just unhook the advance with the cam I have, I was told the fuel millage would suffer, but with the intial timing set right there would not be any detonation or anything bad. The distributor has a new rotor, cap, harness, capacitor, module, new weights, and medium springs.
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Old 05-17-2010, 11:25 AM
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The .488" lift cam requires a recurved distributor I suggest 24deg BTDC at idle and 34-36deg at max advace. That requires shortening the advance curve to 10-12deg max. (limit the mech advance to 10-12deg)

Then: you can run 10-12deg of vacuum advance for good cruising at part throttle.

The stock timing curve will not work.

That cam will not work with a stock low stall torque converter.
Either change the cam or get a 3500 stall converter for it or it will never idle correctly in gear.

Use a 4.5" power valve in the carb.

If you just use this car as a dayly driver with stock converter and gears , the .488" lift cam is all wrong. Change the cam.
A milder .430"-.454" lift cam (204 to 218" @.050" will suite your purposes much better and work with a stockish distributor curve, gears and torque converter.

The 750vs carb is not too big if you remove the screw in the linkage and repair the vac sec housing so that it is functional. Repair parts and gaskets can be bought from most good auto parts stors to service it.
You will get a severe bog on acceleration if/when using a screw in the sec linkage to for the sec throttles open.
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Old 05-17-2010, 11:26 AM
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I was hoping I didnt need a stall, but believed I probably would. But what about the advance question, need it or not?
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Old 05-17-2010, 11:30 AM
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Quote:
Originally Posted by lizardking7750
I was hoping I didnt need a stall, but believed I probably would. But what about the advance question, need it or not?
if you ever expect this thing to idle and drive correctly you will need the converter, gears and modified distributor curve.
3500 stall 3.73-4.10's 24deg at idle base timing.

Why not change the cam that matches what you really want to do?

Picking the wrong cam for their actual driving purposes is probabily the most likley mistake people make.

Do you know the actual cam in your motor now or are you guessing the specs?
What are the specs?
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Old 05-17-2010, 02:50 PM
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I was told by the builder of it that it was a .488 lift, thats all he told me about the cam. Is the port on the side of the Holley fuel metering block supposed to have vacuum at idle?
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Old 05-17-2010, 08:33 PM
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Quote:
Originally Posted by lizardking7750
I was told by the builder of it that it was a .488 lift, thats all he told me about the cam. Is the port on the side of the Holley fuel metering block supposed to have vacuum at idle?
It will if the throttles are too far open at idle.

If you see a vacuum on this port at idle i means the throttes are open beyond their sweet spot ( idle fuel transfer slot exposure.

The cause is not enough base spark advance at idle. When you give it the spark advance at idle it wants you can close the throttles down to where they work best at idle. Big cams need a lot more idle timing than stock cams.
Recurve the distributor. shortne the advacne curve so you can run more inital at idle without excessive total advance.

your .488" cam wants a short advance curve 10-12deg so that you can run 24deg at idle. and 34-36deg at ma x advance. stock distributors have too much advance travel.

your .488" cam needs a 3500 stall and high 4.10+ gearing.
If this is not your plan, change the cam to something that matches what you got.

You won't get it right by just monkeying with the vacuum advance.

how much intake manifold vacuum do you have while idleing in neutral? how much when you drop it in gear? How much does the timing drop off when you put it in gear? You need about 24deg base stable timing at idle in gear. and a loose converter.

call the builder and get the cam specs, cam grind number or part number.

Last edited by F-BIRD'88; 05-17-2010 at 08:41 PM.
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