The arrow points to the slot in the HEI distributor that determines the amount of mechanical advance. Reducing the length of the slot will reduce the amount of total mechanical advance delivered by the distributor.
Rocket Racket
Joe Moskowitz via Car Craft.com: I have a '72 Olds Cutlass with the original Rocket 350. It didn't run when I got it, so I yanked the engine and rebuilt it. After assembling it, I got the timing to where it will run pretty well, but it diesels once in a while, and there is some off-idle hesitation. My problem is that I don't know exactly where to time it. It has stock compression (8.1:1, I believe), a Lunati Bracket Master II cam with 0.496/0.520-inch lift, an Edelbrock Performer RPM intake, and a Speed Demon 650 carb. Any advice as to where I should keep the timing would be greatly appreciated.
Jeff Smith: We've got a couple of suggestions, Joe, but some of it you may not want to hear! The bad news is that a combination of 8:1 compression with a long-duration camshaft is not good. First of all, according to our Motor's Manual for a '72 Olds, the static compression for the 350 is 8.5:1, so it's a little better than you thought. The '71 350s were rated at 8.1:1.
Regardless, your problem stems from a lack of cylinder pressure. Static compression ratios are computed based on the comparison of the volume of the cylinder with the piston at bottom dead center (BDC) versus the volume at top dead center (TDC). In a dynamic application, cylinder pressure is dictated by the position of the piston at intake closing. We researched your cam in the Lunati catalog and found the duration to be 224/234 degrees at 0.050 inch tappet lift with a 112-degree lobe separation angle. All longer-than-stock-duration camshafts close the intake valve later than on a stock cam. This means that at low speeds, the engine is squeezing less air and fuel than it would if the cam were stock. The cylinders are squeezing less air because the piston position was higher when the intake valve finally closed. One good thing with your camshaft is that the lobe separation angle is wider at 112 degrees. This reduces the amount of overlap that also bleeds off cylinder pressure. Overlap is when both the intake and exhaust valves are open at the same time.
Your cam is not huge, but it still has a later intake closing point than stock. That means less torque. You can get an idea of how far off your engine is by performing a compression test. Ideally, a good performance engine should have between 180 and 195 psi of cranking compression. Your engine could be down as far as 170 psi or less. If it's up around 180 psi, consider yourself lucky. Another point worth mentioning is that the word on the street with Olds engines is they tend to run better in the car with a single-pattern cam. Single-pattern cams use the same duration and lift figures on both the intake and exhaust lobes, while a dual-pattern cam like yours uses a more aggressive lobe on the exhaust side. The single-pattern cams tend to enhance torque, which is what the Olds is famous for.
This is a bit of a long way around to answer your questions, but it's still relevant. Low-cylinder-pressure engines will require more ignition timing because there is less densely packed air and fuel in the cylinders. For initial timing, we'd suggest a minimum of 14 to perhaps as much as 18 degrees with a total of around 36 degrees. With 18 degrees of initial timing, this means the mechanical advance in the distributor will need to be 18 degrees to produce a total of 36 degrees. The stock initial timing for your engine is probably 8 degrees initial, with the total somewhere around 30 to 32 degrees of total advance. This means the distributor will have more than 18 degrees of total advance. So when you dial in the 18 degrees initial timing with over 20-plus degrees of mechanical advance, the total will be more than 36 degrees. This means you will have to braze or epoxy a portion of the mechanical advance slot in the distributor to reduce the mechanical advance of the distributor. If this is not something you want to do, you can take your distributor to a specialist with a distributor machine, and he can do it for you.
With less initial timing, this may also explain the slight hesitation you're experiencing, since there's insufficient timing combined with low cylinder pressure. You might also try adding a bit more accelerator pump shot, but I think you have more of a chance of making the engine run much better with more initial timing and by reworking the distributor to create the 36 to 38 degrees of total timing. Also, retain the vacuum advance and use it. This will help driveability and also make the engine a bit crisper at part-throttle. But remember, when testing for total mechanical advance, always disconnect the vacuum advance. Otherwise you're actually reading both mechanical and vacuum advance on your timing light. This is a common oversight that can make the numbers very confusing.