Throwing A CurveR.L. Hutchison, San Angelo, TX: I've got all my pieces together for my '76 Comet but I'm having motor setup problems. At 4,000 rpm at WOT in any gear, the motor seems to lose power. The motor is a 0.060-over 302 with 9.2:1 compression, Trickflow Twisted Wedge heads and roller rockers, Comp Cams 268H cam, Edelbrock Performer RPM intake, Hooker 151/48-inch long-tube headers, ACCEL distributor and 300+ CD ignition, and an Edelbrock 750-cfm carb. I know the carb is a little large, but it's what I had. The power runs through a T5 five-speed trans to a Currie Enterprises 3.89:1 TSD differential and out to the 245/60R14 BFGs. The car weighs about 2,800 pounds. The problem only happens under load, which is why I suspect ignition timing. Timing is set at 21 BTDC, which I think is too much, but the motor runs well and makes good manifold vacuum (about 15 inches) at idle. The distributor mechanical advance is the way ACCEL shipped it to me (all in by 3,600 rpm), and the vacuum advance is disconnected because even 2 degrees of additional advance creates a misfire under light load. I guess I don't understand enough about timing curves. Please help.
Jeff Smith: It sounds like you've put way too much initial timing in the engine in search of a smoother idle. The Comp 268 cam isn't that radical, but its additional overlap will pull the idle vacuum down to about 10 or 12 inches of vacuum at around 900 rpm or so. In your case, with 21 degrees of initial timing, you have too much total mechanical advance at higher engine speeds, which is why the engine lays down. While we don't know the exact model of the distributor you're using, ACCEL's John Hrinsin tells us that if it's a relatively new distributor, it probably has around 24 degrees of total mechanical advance. Add in your 21 degrees of initial, and that's 45 degrees of total timing at or above 3,600 rpm. This means you're starting the combustion process very early, so at higher engine speeds, the engine has to work against greater cylinder pressure that's building before the piston gets to top dead center (TDC).
So here's the fix: First of all, use a dial-back timing light or a timing tape so you can measure total mechanical timing. Let's assume you're at 45 degrees above 3,600 rpm. At idle, set your initial timing at 12 degrees BTDC and recheck-you should then have a total mechanical advance of 36 degrees: 12 + 24 = 36 degrees. The idle vacuum will drop slightly, but it should still be reasonable to run accessories like power brakes. Now, you will be able to reconnect your vacuum advance and adjust it to add in perhaps as much as 6 to 8 degrees of additional timing at part-throttle. Be sure to hook the vacuum line to the distributor from a ported vacuum source on the carburetor, not manifold vacuum. Ported vacuum on any carb is a source where there is no vacuum present until the throttle is opened slightly. This eliminates vacuum advance at idle, which isn't necessary. Remember when checking total mechanical advance to disconnect the vacuum line to the distributor, otherwise you will be reading total advance including the vacuum advance.
Should you prefer more initial timing than our suggested 12 degrees, you can modify the distributor to reduce the total mechanical advance. For example, if you desired 18 degrees initial but found that 36 degrees total was optimal for best power and the distributor had 24 degrees total mechanical advance, this would require modifying the distributor to limit mechanical advance to 18 degrees (18 initial + 18 mechanical = 36 degrees total). Some Ford ACCEL distributors use a GM-style mechanical-advance system placing the weights and springs just under the rotor. If this is the case, look for a pin with a bushing under the advance weights. Using a larger bushing that limits travel in the slot will reduce the total mechanical advance.