[Dean Feiock]

If you have fire in the intake, you have only one of two problems:
1) Timing of spark is incorrect (firing order, ignition timing, cam timing)
2) An intake valve that is not sealing.

I would check your valve springs if you think they are weak. As Alan stated, have you checked it with a vacuum gauge? If you have a valve hang open (sticking guide or weak spring), it will show on a vacuum gauge.

You can have a valve hanging or weak spring and still have good leak down and compression.