34 DMSA jetting

[GarageRat]

I realize that most people opt for the 32/36 dfev when upgrading their carburetor but I thought I would post what I found changing my 1976 Spider from the 32 ADFA to the 34 DMSA. I have 8.9:1 pistons, 4-2-1 exhaust and use a crane electronic ignition with stock distributor, 10° static advance. All emissions equipment has been removed. I found that due to the location of the manual choke arm on the side of the carb that it was right on top of one of the intake bolts. I made an aluminum 1 inch spacer rather than replace the bolts with shorter ones because 1. I wasn't sure I would be happy with the carb change and 2. The choke lever is within 10 mm of the base of the carb so i would have to use a spacer with the shorter commercially available bolts. I have a standard Weber 34 DMSA with the idles at 50/60, main jets 145/145, F61 emulsion tubes, and 180, 165 air correctors. When I first ran it like this it was running well at and off idle on primary barrel but hesitated on secondary. The plugs looked like I was lean. I tried changing the idle jets, especially the secondary and that seemed like it helped a little but the plugs were indicating too rich and there was still some hesitation on hard acceleration. I had read that one step up or down on main jet was equal to 3 steps on air corrector. Since 180 is 3 steps higher than 165 found on US spec 1974 Spider DMSA, I figured primary was ok since main jet is 145 rather than 140. On secondary, I changed the 165 to 155, replaced stock idle jets, and that fixed everything. Sorry for the long post but after the time I spent I figured if it helps anyone I would be happy.

[18Fiatsandcounting]

Thank you, Rat, good to know and you can be happy now. I had a 34 DMSA carb on my '69 spider many moons ago, and my experience was similar to yours.

-Bryan

[wetminkey]

Cool, GarageRat! I find the very same info on Weber changes, and your logic is excellent,...thanks for posting this.
I don't find a lot of information on re-jetting carbs, and I think the info is becoming lost over time,...
I only picked the 32/36 DFEV so that I could motor on the 32 mm venturi for mpg (and perhaps reduced emissions, for my home state of Colorado!),...it has the same effective cfm as the 34. The electric choke is also more convenient in a '79 Spider,...
Glad your's is running well,...what altitude are you at?
Todd Compton.

[GarageRat]

I am in coastal Virginia, so usually no more than 200 ft above sea level. Haven't headed toward our Blue Ridge Mountains yet!

[GarageRat]

I will also say that the electric choke is definitely more convenient, I haven't had a manual choke in many years and I keep forgetting to adjust, though the weather is still 80s here right now, will become a pain I am sure in a month.

[wetminkey]

I understand that your sea-level tuned Weber should perform well up to about 2500 ft. So you'll be OK,...
I'm really old-school and much prefer a manual EVERYTHING! But, the '79 Spider was designed for a thermal choke, and I don't want to run a cable for manual,...Convenient, but not as efficient (if you're used to one!).
I was home-ported out of Norfolk 1976 - 1980. Your state is beautiful!

[GarageRat]

Wetminkey,

We went out to Colorado for a family trip this summer and we loved it, talk about a beautiful state. We loved the topography, almost went to Ault as we went down through Colorado Springs. I wanted to get to the Great Fiat Yard Sale in Denver at Aspen Auto Clinic but couldn't get them on the phone!

[wetminkey]

Sorry you couldn't get to the sale at Tom's Aspen Import Auto,...but I think it probably got picked over pretty quick. I hope the huge racks of used parts have been offered up for sale! RIP Tom.
Glad your trip was enjoyable,...front range and mountains are our best!
Cool that your carb is tuned properly. I hope to change air correction jets in mine tomorrow,...