Something weird happened when swapping carbs

[Paul G]

This happened on my 49 Plymouth with a 318 and 4 speed.

It had a good running Holley 600 dual feed carb on it. Problem with the carb was flooding out due to sticking secondary floats. So I swapped my 670SA on it till I get the 600 rebuilt.

With the 670, it had a bad stumble just off idle. Made sure the accelerator pump lever was set correctly, stock 31 squirter working well. The SA carbs are shipped lean OOTB. This one is jetted up from 65 main jets to 68's.

Base timing was at 8 degrees btdc with the 600, had no stumble at all, actually was very easy to break traction just releasing the clutch quickly with a small amount of throttle.

Trying to get rid of the stumble I moved the base timing up to 12 degrees btdc, and the stumble is completely gone now with the 670.

Why it wanted more timing with the bigger carb is the question?   

[Kern Dog]

The 670 may have an accelerator pump that has more volume, more fuel. The added advance helped burn it correctly.

[LaOtto70Charger]

I had similar issue changing to a pertronix electronic distributor with my edelbrock carb on a 318.  With the original points distributor very little hesitation off idle at timing at 10.  With the pertronix bad hesitiation and stumble at anything less than 14 when cold.  Sometimes even backfires through the carb.  Runs fine after a mile of driving.  So far I haven't changed any settings on the carb.