Once you've got your slots welded up so you have the correct amount of mechanical advance just install the one light spring and leave the heavy one out. Don't worry about the empty spring slot. With just the one light spring in place you should have full advance by 2000-2400rpm. You may want to increase the total timing to 38 degrees or more on a low compression motor. I've seen as much as 42 degrees mechanical advance work real nice on bb with little or no compression. You can also run up to 52 degrees advance with the vacuum advance hooked up. Dave