[klem]

Well, I spent a few hours in the Tangmere library today but did not find any factual data or reports, just some observations from former Spit/Hurri Mk1 pilots, Alex Henshaw (Spit test pilot) and the pilots notes. It paints a picture which seems to be "negative G is quick to affect the engine but not instantaneous and it recovers in a couple of seconds" but here's what I found so you can form your own opinions:

From the cockpit: Spitfire by Wing Cdr T.F. Neill DFC AFC
"it caused the carburettor to flood after the briefest period of negative G"
"engine ceased to pull for a second or two"

Spitfire - The Biography by Jonathan Glancey
"When the Spitfire is thrust into a sudden dive the carburettors would flood causing the engine to cut out."
"the Merlin always came back on song in a matter of moments"
"All this took precious seconds"

Pilots Notes Spitfire IIa and IIb
Merlin XII
"Inverted flying. This is Normal"
"A moderately slow roll is best as the engine can be kept running normally...best if slight barrel roll... if engine shows signs of beginning to fade the stick should be brought back a little, almost imperceptibly"

"True Slow Roll* This can be done if high speed is used at the start but the engine will cut out when inverted. If the engine is throttled back** as the roll is started it will be possible to get the engine started again earlier in the final stages of the roll."

*In opening sequence of the film "Battle of Britain" (is that a "True Slow Roll"?) if you look carefully the roll is begun with an upward pitch and a slight barrel element iaw the Pilots Notes. From the moment the lift vector ceases (inverted) there is about one second before engine response and about two seconds after rolling out before it picks up again.

** Presumable reduces flooding

Sigh for a Merlin by Alex Henshaw
"I would open the engine flat out in a vertical climb and at approximately 1200 feet push the nose over forward and with the engine closed complete the half of an outside loop... usually round off to a few feet above the ground *** ... push the machine into an almost vertical climb.... then pull the control gently over to form a half loop, hoping as I did that the engine would burst into life"

***(klem:inverted)

There are frequent references to diving in pilots notes, Jeffrey Quill's and Alex Henshaw's books etc with no mention of engine problems. I expect the severity of the dive would have had some influence, perhaps a low -G or reduced G pushover to a sustained dive would allow the Carburettors to keep up? Some of these dives achieved very high speed and were quite steep.

There is a time element to onset but it's hard to quantify and almost certainly related to the severity of the pushover as the floats and fuel rose at various rates vs reducing G value. A sudden severe pushover would presumably have had the floats and fuel wanging up in the float chambers.

The recovery or catchup appears to be a matter of only a couple of seconds once more normal G values are recovered.