I have about 150 new springs in stock. I have checked about 50 of them in many ways. They are all so close to one another that I quit testing. I compressed a brand new spring to its solid height and it lost tension to the point that it was no longer better than a spring with 14 races on it. I stretched a spring, and it went up in tension, but after compressing it at full cam lift, it was right back to where an untouched spring was. All the fresh springs measured 10.8 lbs at .850”. My scale is only accurate to .2 lbs, but my measurements are accurate to .001”
So here’s the kicker. If you are running the stock air box and filter and the stock muffler, heavy springs are actually bad because the engine makes more power and is faster at about 5200 RPM, which is well below the 5800-5900 RPM valve float RPM. Added tension is just a waste of energy. With the little header and muffler along with an air filter adapter and bigger jet, the power band moves up slightly in the RPM range, but still not enough to turn the engine to the point of floating valves. So basically, if the intake and exhaust is left stock, worn out springs are better, and with an aftermarket intake and exhaust you will want to freshen your springs after a few races. Changing springs is very simple and can be done with your bare hands without removing the head from the engine. I'll do a picture story on that and post it here this weekend.