IDF Tuning.

[lanciahf]

Hi all,

I have a question regarding balancing IDF carbs. I have a nice synch tool but I'm not sure on the correct balancing steps. The engine is a 2L with a 40/80 intake cam, 4mm pistons and stock 4-2-1 exhaust manifold.

The car idles well and has great top end performance but coming off idle the car stumbles and does not sound smooth. I know the idfs run mostly off the idle jet circuit up until 3000 rpms?? Current idle jet size is 55 which should be enough??

Also should the air bleed screws be turned all the way in before balancing between the two carbs.

Sorry for the mish mash of questions just not sure where to begin.


Thanks!
Ralph

[carl55]

Hi
I have a copy of Brad Artigues guide I can e-mail you. I can not find the link.

Carl

[So Cal Mark]

the air bleeds should be lightly seated, the mixture screws turned out from seated about 1 turn. What size IDF carbs and chokes are you using?

[lanciahf]

Thanks Mark.

The Carbs are 40mm IDF Series 70
Jets are:
Air Correct 200
Emulsion Tube F11
Main 140
Idle 55
Chokes 32mm

So you are saying to
1. Lightly seat all 4 air bleed screws.
2. Turn the mixtures screws 1 turn out from close.
3. Then balance the carbs using the synch tool.

[So Cal Mark]

yes, and make sure the idle speed is low when you get around to adjusting the idle mixture

[manoa matt]

I found this video the other week while looking for Fiat IDF porn on youtube. Seams like a practical technique. http://www.youtube.com/watch?v=9wmAhIn8Q1A

[thechadzone]

Thanks for the link, Matt. BTW, were you able to revive those 40mm Solex carbs I sent you last year? They're rare enough that a thread about them would be cool to see (if it doesn't exist already).

[manoa matt]

Chad,

Yes, with the help of those carbs you sent I was able to fill in a few missing pieces, thereby making one good set that I believe will run reasonably well. Unfortunately the person who owned the carbs before you FUBAR'ed them, and they are really only good for some of the parts.

Here are the carbs in their current state: http://www.flickr.com/photos/bigtimes_fiat/4434064705/ http://www.flickr.com/photos/bigtimes_fiat/4434064709/

I've been thinking about writing a technical article regarding the Solex C40 P11-6 carburetors. I've searched all over the internet for information regarding them. Most of which pulls up links to posts on various forums where someone is trying to figure them out, eventually they turn to Webers despite the fact the Solex carbs are exact copies and tuned the same way. I have come across some technical publications that list specifications, found rebuild kits and jets at reasonable prices. The jets that are in the carbs now should work with my stock 1.8 motor.

If I were to write an article it would detail how they came into existence (Weber went on strike), the features they have, what to look for in a used set, where to get parts, and how to tune them. Due to their rarity, the lack of available information, peoples misunderstanding of their operations, many have disregarded them as a viable option.

Their story and peoples' perceptions are similar to Fiat's in America, misunderstood and underrated.

Call me crazy, call me a glutton for punishment, but I like the underdog Solex carbs. I feel that with the good set I have and the knowledge I've gained about them, I can get them running correctly. But like everyone else I got a set of Weber 40 IDF's waiting on the sidelines, for when the underdogs just can't win.

[lanciahf]

Still tinkering. I have a nice idle and top end pull.
The car idles well and pull nicely at high rpm. I balanced the carbs using a SK meter and the throats are pulling in the same amount of air

But off the throttle and up until 3000 -3500 rpm it feels really lean.
When the car is sitting at rest I can run the engine from idle to red line without any hesitation. But while driving the car the engine hesitates until the higher rpms are reached.
I verified that the pump jets are squirting fuel and I'm pretty sure IDFs run off the idle jets up untill 3500 rpm?

Do you think a richer idle jet would help?

My specs are below
The Carbs are 40mm IDF Series 70
Jets are:
Air Correct 200
Emulsion Tube F11
Main 140
Idle Jet 55
Chokes 32mm
Needle valve 1.75
Pump Jet .50

Thanks,
Ralph

[So Cal Mark]

how many turns out are the idle mixture screws to get a good idle?

[sptcoupe]

Is the engine stock except for the IDFs? If so, 140 mains are a bit much for an 1800. Try 130s. Check your back bleed jets to make sure they aren't the 80s that typically come in the 70 series IDFs. If they are, try a 50 or a 35 (if the engine is stock). The 50 pumps might work, but I would try a 55 as well. 200 airs over 140 manis in an otherwise stock engine is likely too rich. I would go with a 215s over 130s unless you have cams, HC pistons and a good header and bigger valves.

I don't know what you did to balance and scynch the carbs, but just because they are all drawing the same doesn't necessarily mean they are balanced properly. You might also want to be sure you don't have a partial blockage in one of the idles, or some dirt under one of the mains at the bottom of the emulsion tube well.

Also be sure your floats are correctly set. 10.5mm closed, 32.5mm open (with gasket). And I would change the 175 needles to 200s. Finally, make sure you are limiting fuel pressure to 2.5 or 3.0 lbs of pressure.

Keep us posted.

[lanciahf]

Mark,
About 1.0 to 1.5 turns out to get a good idle.


SPTCOUPE
The engine is a 2L with a 40/80 intake cam, 4mm HC pistons and stock 4-2-1 exhaust manifold. Floats are correct and the bleed back valves were 55 but have been sealed closed which did not make a difference.

Thanks

[sptcoupe]

You should be pretty close. The last 2.0L carb'd engine I built had all the above, plus 5.5mm domes, big valves and an aftermarket 4-2-1 header (not Mark's). I checked the log, and this is what I had for jetting:

F11emulsions
137 mains
210 airs
58 idles
55 pumps
35 bleed backs
200 needles

I also had the accel pump adjustment rod nuts (only on the 70 series) turned in 8 turns from contact (this is an often overlooked, but important adjustment. I don't know what the perfect setting is because each motor will differ, but they must be equally set so that the same amount of gas is pumped by the accel pump at the same rate. The symptom of poorly balanced accel pumps is a constant stumble on acceleration that doesn't change no matter what you do to the pump jets or the bleed backs. Be sure you see where you are on each of the adjustment rods before you start!).

I'm assuming you have a decent ignition in the motor. I never got it to a dyno, but it was very stong, very smooth engine. The 5.5mm domes were a bit too much compression for 91 octane gas (had mild detonation that was manegable with your right foot, or with a gallon or two of 100 octane gas added to eeach tank), so the 4mm domes sound about right.

What are the symptoms in the mid-range?

[So Cal Mark]

the rule of thumb is if the idle screws are more than 3/4 turn out for good idle, the idle jets are too small. That will show up in transition

[lanciahf]

Thanks for the help guys, just got back from driving the car after having breakfast with George.

The car definitely hesitates in the mid-range. I can feather it up to 3500 RPMs then I can jump on the throttle and the car goes with out any hesitation or hiccups. Off idle to around 3500 I have to be gentle and smooth with the throttle otherwise the car will bog down.

Thanks for the jettings specs I'll try a larger needle valve, idle jets.

Also are the bleed back jets necessary? Some people have told me to seal them up. Presently mine are sealed.

Each accel pumps rods have 7mm of threads showing.

The distributor is a stock 81 dist with MSD Blaster coil. The dist has been rebuilt by Terry Terezakis and converted to mechanical advance only. Gas is Premium Shell.


Ralph