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Old 10-13-2010, 01:25 PM
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Timing Is Everything

I Have A Two Part Question. I Just Bought A New No Name Brand Hei Dist For My Sbc 350 . It Has An Adjustable Vaccum Advance But It Has No Instructions On How To Adjust The Advance. The Dealer Cant Help Says No Info Is Available From Mfg. I Have Instructions From A Pertronix Dist I Used To Have, Think They Are The Same?
Part 2 - How To I Adjust The Timing I Know Base Timing But How Do I Get Total Timing? I Know Total Is Both Base And Adv. But Do I Adjust Advance At Idle? Thanks Gary

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Old 10-19-2010, 09:05 AM
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If you disconnect and plug the line to the vacuum advance and set the idle down to a normal level (usually about 800 rpm), you will get base timing. That could be anywhere in the range of 4-16 degrees BTDC for an SBC. The factory setting is probably 4-8 BTDC.

Continue to measure timing while you rev the engine up, and watch it advance. At about 1200-1800 rpm it will start to rise, and it should peak at about 2500-3000 rpm. This mechanical advance will probably add 18-20 degrees to the base timing.

For example:
Base timing 12 degrees BTDC
Mechanical - adds 20 degrees
Total = 32 degrees

Now you can reconnect the vacuum advance and measure again. If you have manifold vacuum it will pull the advance at idle and add up to 20 degrees - you will have to set the idle speed down to compensate. If you have ported vacuum you will have no vacuum at idle (so you have to leave the idle higher), but will get vacuum at moderate/cruise throttle. The cruise vacuum advance helps fuel economy, but won't have much effect on acceleration. You could run without vacuum advance, but gas mileage will suffer.

If you get too crazy with vacuum advance you will get part throttle pinging because at light throttle cruise the vacuum advance is adding as much as 20 degrees to the total timing you already measured. You may have to play with it to see how far you can go without any pinging.

Bruce
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Old 10-19-2010, 10:15 AM
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thanks for the info i am trying to get the info all together. i gotta figure how many degrees each turn makes when i turn the set screw inside the can. im gonna try goin by an old set of instructions from a pertronix. thanks for the help
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Old 10-19-2010, 10:53 AM
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The ebay 40 buck distributors have a poor timming curve. There is no stop in the mechanical side . It will increase degrees with rpms. My sw type ebay distributor had 28 degrees at 4500rpm. In the old days the weights we left and gave em the black and gold spring from the summit racing timing package. Then you could dial in over 8 degree inital mechaniacal.

My ancient mopar ways allowed me to tune with a locking point for all in mechanical. like 12 inital and 21 more for 33 @2400 rpm.

4secondsflat has kits to do timing this way.
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Old 10-19-2010, 11:13 AM
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thats what i got- a 40 buck e-bay. no instructions on how to adjust. i guess you get what you pay for. any ides on how to check the adjustment?
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Old 10-20-2010, 10:32 PM
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Check it with a timing light.

My distributor had no limiting bushing, the degrees of advance kept going up following rpms. At 14 degrees initial 20 mechanical and 12 in the (nine turns in) vacuum can at 2800rpms . Nice, however. At 5000 rpms the mechanical added 28 to the curve, and on. They should have a lock bushing to control the amount of advance. Let the springs control the rate.
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