604 with pop letting off the gas

[Zoom45]

Just bought a new Willy's 604 crate motor carburetor. I put it on and it ran great out of the box. Even before I put this carb on the last couple of races, the motor had a pop in the right exhaust when you get out of the throttle. It only pops one time. It doesn't have any problem under power. It only does it soon as you get off the gas. It has been doing it the last couple of races. I thought I saw a flame out the header when he was on the track going into turn 1 and letting off. What can cause this? Leaking header gasket? Bad valve spring? I'm scared to run it till I can find the problem.
Thanks,
Zoom45

[rusty9cab]

I think you are on the right track. I would check the spring pressures and reseal the headers. I don't like gaskets, personally. They always leaked on me. We use Ultra Grey RTV to seal the headers. Just put a bead on the flange or the head, which ever is easiest for you, loosely bolt the headers on so that rtv sticks to both surfaces and let it set up for a bit before cranking down on the bolts. Also check the valve adjustment while you are checking the springs.

[Racer111]

Also look for a crack in the header..

[kenromingerjr]

change springs and look for the washer under spring as they will break. best to change everything on valve springs