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Old 08-04-2010, 10:20 AM
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Most performance cams will benefit from more initial timing at idle your setting of 12 with this cam is OK for now, the higher the duration the more this applies and on top of the base timing the vacuum advance will add another 20+ or so at idle with a stock HEI, IF you put a timing light on it at idle with the vacuum advance connected to manifold vacuum you would have seen this, it would be around 34-36 BTDC at idle. I was trying to determine if your vacuum advance was adding anything at idle. Since you are on ported vacuum there will be very little or no added advance at idle due to vacuum cause the ported side is above the cab throttle plates which only see's engine vacuum when you increase the throttle opening. So as far as the ruff idle goes if you switch to manifold vacuum you will most likely take care of this issue cause you will be adding in another 20 degree's give or take, give this a try, when you switch to manifold vac your idle rpm will increase once connected to the vac can, then disconnect it and insure that the timing drops back to 12, this will verify that the vacuum advance can is working. Set your idle to about 800 rpm with the vacuum advance connected, drop it in gear and see if your idle quality improves, it should drop to about 650 rpm or so and smooth out. Next is to determine the mechanical advance curve added by the HEI dist. You can do this first if you want. Disconnect the vacuum advance plug the hose, use the light to check the initial timing at idle it should be back to 12 right, then rev the motor up while watching the timing light you should see the timing advance increase, keep giving it throttle till the timing stops advancing and note the RPM at which this happens, most likely about 3500 rpm and also note what the timing is, you want about 34-36 degree's BTDC. Some balancers will not afford this as they only show 20 degree's BTDC, if this is the case then you need to make a timing tape or get an light with a turn back pot. Some HEI dist will add more than the numbers I listed but give this a try to see what you have got, it may be necessary to limit both the mech and vacuum advance ,,,we'll see,,, if you run 18-20 initial as I do this limiting is a must but your cam does not need that much initial.
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