My 85' has a high performance 360. It has a holley 600 on it. For some reason, at sustained higher Rpm's, it back fires. Most days i can't go over 60mph (about 2500rpm) for very long before it starts back firing. Will a spacer help this? I'm not sure if the secondaries open up, but the fastest I've ever been able to go is 70mph. I have stock gears (3.21's I think) and 33 inch tires. I assume this is a carberator issue. is there sum way to test the secondaries?