Carb Swap 32 ADFA2 to 32 / 36 DFEV

[davidbruce]

Further to what Bryan said. One nice thing about this carb is the idle jets are accessible from the outside under a couple of screws so experimenting is pretty painless and quick. This chart shows what should be installed from the factory.

http://www.redlineweber.com/html/Tech/c ... _best_.htm

[18Fiatsandcounting]

Thanks for that David, I don't think I've seen that info from Redline. In any event, since the various idle "screws" often get confused, here is a summary. I think all of us know all this, but for posterity's sake, here it is again:

Idle jet: Located just below the top carb cover, the jet itself is a small friction fit tube inside the outer brass screw. This outer screw should be turned in reasonably tight at all times. Sometimes the screw has a rubber o-ring where it seats. The idle jet itself is very easy to plug up with a piece of dirt or the like, and this will cause havoc with the idle. Note that there are most usually idle jets on both the primary and secondary sides of the carb. Idle jets are usually 45, 50, 55, and 60 in size, and I think this is the fractional diameter of the opening in millimeters (0.45, 0.50, 0.55, 0.60 mm).

Idle speed screw: This screw, most often with a spring behind it, pushes on a cam on the primary throttle shaft. Usually this screw faces way from the engine. There is no equivalent on the secondary throttle shaft. Screw it in or out to get the idle speed you want.

Idle mixture screw: Usually located in the middle of the flange on the bottom of the carb, facing away from the engine, sometimes with a locknut and sometimes with a spring. Screw it in until the idle starts to falter, then back it out 1/4 or 1/2 turn. When all else with the carb is right, this screw should be roughly 1 and 1/2 turns from its fully seated position. Do not continue to turn this screw in once it bottoms, as you may damage the tip.

Choke idle adjustment: Depending on the choke design, this screw has a long tip that extends into the choke housing where it contacts a cam. When the engine is cold and the choke is engaged, this screw should be adjusted so that the idle speed is 1500 to 1800 rpm or so. Your choice, depending on what seems best for the engine as it warms up.

Hope this helps. -Bryan

[davidbruce]

Bryan, that all sounds about right. The only thing I would add is the importance to adhere to the maximum the idle screw may be turned in. If screwed in too far you will be idling on the high speed circuit and never get it adjusted properly.

[geospider]

standard idle jet: prim 60, sec 50
I put a 50 in my primary and ran better/ less rich and broadened the adjustment. I have read where folks run a 47 in primary.

Mains: P 137/ S 140
Air Cor P 165/ S 160
Emul Tub P F66/ S F50

I am running around sea level, so your jetting would end up different. I believe you can order idle jets down to the 47 at Pierce Manifolds apparently even down to a 40.
https://www.piercemanifolds.com/product_p/74409.htm

[18Fiatsandcounting]

The only thing I would add is the importance to adhere to the maximum the idle screw may be turned in. If screwed in too far you will be idling on the high speed circuit and never get it adjusted properly.

Good point Dave. If everything is working correctly, and you look down the barrels with a flashlight while the engine is running, you should NOT see any gas coming from anywhere. The hole for the idle circuit is below the throttle butterfly valve, so you can't see it with the butterfly closed (i.e., when idling). I've seen some cases where the idling circuit is plugged up somewhere, and so folks will open the throttle so the main circuit starts to spray fuel. Like you describe. It will idle, sort of, but the main circuit shouldn't come on until 2000 rpm or so. In between idle and 2000 rpm, there is a progression circuit which I'll leave for another day to discuss.

standard idle jet: prim 60, sec 50
I put a 50 in my primary and ran better/ less rich and broadened the adjustment. I have read where folks run a 47 in primary.

Mains: P 137/ S 140
Air Cor P 165/ S 160
Emul Tub P F66/ S F50

I am running around sea level, so your jetting would end up different. I believe you can order idle jets down to the 47 at Pierce Manifolds apparently even down to a 40.
https://www.piercemanifolds.com/product_p/74409.htm

That sounds good Geo. I need to replace the 28/36 DHSA that I have in my '71 with a 32/36 DFEV that I bought, so I will probably start with your jetting and go from there. This DHSA with the goofy '71 manifold isn't very performance oriented...

-Bryan

[jon8christine]

Link sharing turned on so the pictures will work now

[18Fiatsandcounting]

Yes, the pictures work now, thanks. I think we sorted out the carb issues (yes?), but just a note on the distributor: Bobina made the distributor cap, but your distributor is almost certainly a Marelli unit. It will have a designation like S144C or the like. Usually stamped in a round circle in the narrow neck of the distributor, between the flange that fits onto the engine and the base where the cap fits onto.

-Bryan

[jon8christine]

After the first go at an autocross race I'm finally back to tuning this carb. Where is the best (cheapest) place to get jets? None available here local.

The carb as it arrived:

Idle - P: 60 S: 50
Main - P: 137 S: 140
Air - P: 165 S: 160

In my research, idle jets need to drop a size or two, main jets drop one size, and air jets increase 4-6 sizes (all increments of 5) from sea level to get to 6,000' where I am.

As I go about tuning I think I understand that you start with the idle jets so the idle screw is no more that 1-1/2 turns in, then move to performance testing by feel on the main jets and then the air jets. My question is about the primary and secondary circuits. Do you tune the primary at lower rpms and then tune the secondary when mashing the throttle?

[18Fiatsandcounting]

You're right, in that I'd start with the idle and then move to the primary and then secondary circuits. A few thoughts:

1) The idle mixture screw can compensate for just about any idle jet, even if that jet isn't quite the right size. But, what you do want is for the idle jet to be sized right for the transition from idle to the main circuit. The main jets don't kick in until around 2000 rpm under most conditions, while the idle jet really only works around 1000 rpm or so. Between those two speeds, the transition circuit is operating, and it also gets its fuel from the idle jet. So, if you have a momentary engine stumble as you increase from idle on up, then you may need to increase the idle jet size. You just have to experiment. And yes, usually when the idle jet size is correct, the best idle will be when the idle mixture screw is turned out between 1 and 2 turns.

2) Main and Air Corrector jets for the primary and secondaries are hard. Without putting the engine on a dyno, or having an accurate air/fuel ratio measuring device, there are really only two options: A) see what feels best in terms of smoothness, power, lack of stumbles or missing, etc. or B) check your spark plug colors. For the latter, white is too lean, and dark brown or black is too rich. However, spark plugs don't change color instantly, so you have to run it for a while (an hour?) and then check them. What I do is take a trip at highway speeds for an hour or so, with just normal driving (i.e., without engaging the secondary barrel). The plugs should be light grey or tan colored. If not, adjust the primary main jets accordingly. The secondary is a bit harder, so you just have to drive it hard for a while and look for stumbles, hesitancy, unwillingness to rev past 4000, etc.

3) Air corrector jets are somewhat of an art. They do two things: A) they introduce air into the fuel stream from the main jets which helps the fuel to vaporize more readily and evenly as it enters the engine, and B) they act as sort of a "leaky straw". For the latter, imagine sucking your Slurpee through a straw. If the straw has a leak in it, you have to suck harder to get at that Slurpee. The bigger the leak, the more you have to suck. Anyway, the air corrector jets have the same function, in that the larger the jet (the leak), the less fuel will be sucked through the main jets from the vacuum created by the engine. Where this matters is mostly in the smoothness of the power delivery throughout the rpm range, and the right air corrector jets will make the power delivery smoother throughout 2000 to 7000 rpm or so.

4) Lacking access to a dyno or an A/F ratio device, I've usually just set the jets based on what seems to "feel" best, all while keeping an eye on the spark plug colors.

More than you wanted to know perhaps, but I hope this helps.

-Bryan

[jon8christine]

Excellent info. Thanks. I tried swapping the 60 for a 50 in the primary idle jet and couldn't keep it idling at less than 1-1/2 turns in (was closer to two and a quarter), so back to the 60 it went. I'll look at spark plugs color, I imagine you only need to pull one?

[18Fiatsandcounting]

If a 60 idle jet works and the mixture screw is between 1 and 2 screws out for best idle, you're probably just fine. As for spark plugs, I'd pull two of them, say #1 and #4, just because there can be differences in mixture from front to back. Since the #4 spark plug can be hard to reach, #3 would also work. But, if you just wanted to pull a single plug (#1 or #2), I wouldn't argue with that approach.

-Bryan

[SteinOnkel]

Get yourself some tools.

https://www.amazon.com/Gunson-G4074-Col ... B0012MB4VE

Or a wideband a/f gauge (about $200).

At the very least, a vacuum gauge.

Everything else is just guesswork. It can work, but only if you have experience with jetting carbs. Also, with the cocktail of additives in modern fuels, the spark plug color has become woefully inaccurate. It will only show you extremes.