The detonation under load was bothering me, so I decided to pay a little more attention to the distributor. The mechanical advance springs had relaxed over time, so I decided to try and buy some replacements. I had no luck finding advance springs for this distributor, but what I did find was a kit for a later 1974 onwards GM HEI distributor. It came with three different strength springs so you can tune the curve. Sounded good to me so I bought one on eBay for A$14 including postage from the US. It arrived today, and I couldn’t wait to see if it would work.
I turned the packet over, and there was a graph which showed how strong each spring is. The weights are different compared to my distributor, but the springs were similar. I decided to go with the strongest springs (black). You can see in the picture above the three springs. The black ones are what I removed from the dizzy – it is clearly visible that the springs have relaxed because there is a gap between each coil. Anyway, I changed the springs over and re-assembled the dizzy.
Next step was to adjust the timing. I started the engine and disconnected the vacuum advance in order to see how the mechanical advance had changed. It was clear that the mechanical advance was coming in later, and was increasing at a slower rate as the RPM’s rise. Sweet. I decided to take a chance and set the timing back to 10 degrees BTDC (I had retarded it to 5 degrees). After this was done, I readjusted the mixtures and idle speed.
The engine was purring. Took it for a quick drive, and the throttle response is noticeably better, and the engine feels smoother. More importantly, it does not detonate any more under load.
AWESOME!!
Sometimes it is the smallest things that make a difference!
1960 Cadillac Eldorado Brougham 