Fuel economy of carburetors

[Topless]

Hi all,
Curious if anyone had a guess as to fuel economies of the different carbs we run on our Spiders.
What drew me to this question particularly was reading up on the fussiness of a 28/32 but wondering if it has better gas mileage than a 32/36 or 34 ADF.

[18Fiatsandcounting]

Curious if anyone had a guess as to fuel economies of the different carbs we run on our Spiders.

My experience is that engine size and driving style/conditions have more of an impact than the specific carburetor, assuming the carburetors are all tuned correctly. That being said, some carburetors will run richer under certain conditions (engine speed and load) than others, so while they offer a "fatter" power band, the mileage will drop. But, the difference is usually slight, unless one is racing or driving flat out most of the time. Most carbureted spiders get around mid 20s mpg, although some of the very early ones did better due to smaller engines. My '69 with the original 1438 engine could go for a little over 400 miles on its 11 gallon tank, which works out to over 36 mpg. That was driving on the highway at a constant 60-65 mph, and city driving dropped to the mid 20s.

Your mileage may vary, of course. You were of course waiting for me to say that, right?

-Bryan

[Fiatlanta]

I yanked out all the emissions stuff when I got my '79 a few years ago. Put in an 1800 intake manifold and slightly upsized the carb. 80/20 split of city/highway driving gives me about 20ish mpg...get about 180 miles on a fill up.

[18Fiatsandcounting]

80/20 split of city/highway driving gives me about 20ish mpg...get about 180 miles on a fill up.

That sounds low, as I would have expected at least low 20s. What carb are you running? Do the plugs look OK? Should be light grey or tan colored. Sooty or oily isn't good.

I have to modify my earlier statement that carbs have only a slight effect on gas mileage. Since I wrote that, I put a 34DMSA carb and 1800 manifold in my '71 spider, to replace the original crappy manifold and 28/36 DHSA. While the performance definitely increased, the overall gas mileage dropped from around 30 mpg to around 25. Still, with an 11 gallon tank, that gets me 300 miles although admittedly I am running on fumes at that point.

-Bryan

[Fiatlanta]

80/20 split of city/highway driving gives me about 20ish mpg...get about 180 miles on a fill up.

That sounds low, as I would have expected at least low 20s. What carb are you running? Do the plugs look OK? Should be light grey or tan colored. Sooty or oily isn't good.

I have to modify my earlier statement that carbs have only a slight effect on gas mileage. Since I wrote that, I put a 34DMSA carb and 1800 manifold in my '71 spider, to replace the original crappy manifold and 28/36 DHSA. While the performance definitely increased, the overall gas mileage dropped from around 30 mpg to around 25. Still, with an 11 gallon tank, that gets me 300 miles although admittedly I am running on fumes at that point.

-Bryan

I replaced the plugs not long ago, but admittedly have not driven as much since then. Running a Weber 32/36 DFEV.

The carb definitely needs a tune...I feel like RPMs are higher than they should be at whatever speed I'm driving or even while idling. With electric choke, the thing will rev up to 3000+ while warming up and then idle ar 1000-1200 once warm.

[18Fiatsandcounting]

Sounds like the choke isn't working properly, and if it's sticking "on", that definitely will decrease your gas mileage.

I don't have the specs here in front of me, but my recollection is that the idle with the choke on should be around 1800 rpm, and the idle when the choke is fully off should be 850 rpm.

-Bryan