Got my new 350 in the rod finally (it's a 1973 350 rebuilt stock). It's running really good, but for some reason am getting a shot of fire coming out of the exhaust. This only happens when it's at idle in park or neutral and I hit the throttle hard. I did notice it happen the other night on the freeway I got her up to about 90mph and when I let off a small flame came out. Is this due to incorrect timing, maybe carb running rich or someone mentioned re-adjusting the valves after the motor was broke in??? What do you think?