Stoopid Idyot Q: Diff in carb and FI cold start

[RSBob]

When starting the 2.7 RS Spec MFI car cold, even on a hill, it will pull away smoothly, just like any modern car.

When I cold start the 3.0 with new Webbers, on the same hill (and it's relatively steep) it coughs, lurches, and wants to die.

On the flats both cars start fine and drive normally.

So is this a characteristic of a carbed car - a lot of load when cold going up a good sized hill? And if so, why?

[Jeff Higgins]

Your MFI inducted car has a cold running compensation circuit in the form of the warm up solenoid, which holds the main rack in a rich position until the car warms up. Your carbureted car lacks this.

Most carbureted cars have a "choke" mechanism on the carbs. This is a secondary butterfly, above the main throttle butterflies, that restricts air flow though the venturies. With the throttle butterflies beneath them opening to expose the intermediate and main jet passages to the airstream, and thus increasing fuel flow into a restricted (by the choke butterflies) air stream, the carbs provide an overly rich mix until the chokes fully open.

Since Webbers lack these choke butterflies (or choke plates) they cannot enrichen the charge to the degree required to make a cold engine run well. Some of this is masked by accelerator pumps, but if they are set to deliver enough fuel to make it run well cold, it will be too rich when warm.

A lot of us have learned to kind of "dweedle" the gas pedal on a carbureted motor when it's cold, making multiple stabs at it to keep those accelerator pumps pumping to make it stay rich enough to accelerate without bogging when it's cold. It's a fine art to keep the car moving without lurching or bogging through this warm up cycle. You have to hold your mouth just right while your right foot goes into a somewhat controlled epileptic fit...

[RSBob]

Thank you Jeff for that most informative yet funny as h*ll reply. Dweedle - that facts of car life dad never explained to me.... At least I got the back seat thing down in HS.