WEBER 34 ADF carb problems, Need Help!

[1978len]

by 1978Len on Thur Aug 05, 2010 5:27 pm
Hi All;
I'm kinda new to the Forum, but have been reading for awhile, and consider it the best source of Fiat info., bar none.
I recently replaced my 32 ADFA Weber with a new 34 ADF Weber on my stock 1978 Fiat Spider, and can't seem to get it run right.
Prior to this installation, when I removed the stock 32 ADFA, I removed a lot of smog stuff except the Air pump, which will come later. I followed most if not all of the recommendations from the Forum, and removed the electrovalves and vacuum lines, capped off the EGR, and all the unused vacuum ports. I only left the manifold vacuum lines for the brake booster, and the Gulp valve. I connected the crankcase vent line from the carb to the vapor canister. This was the only vacuum line on the 34 ADF. I did not need a vacuum advance for the dizzy since I have the centrifugal type.
My problem is , upon cold starting, the high ldle only goes to about 1000-1100 rpm. On the original 32 ADFA, the fast idle was around 1500-1600 rpm until warm-up and then kicked down to idle speed. Maybe I'm not doing the fast idle adjustment screw right.
About half or three quarters through the warm-up, the idle starts to climb to about 2000-2500 rpm. When I kick it down, the throttle response is choppy, and it frequently backfires through the carb. If I feather it, I can smooth it out, but only by revving up to about 1500-2000 rpm. At low speeds, it wants to die.
When the engine is warmed up, throttle plate fully open, it idles at about 1000 rpm, no lower, and the idle speed adjustment screw is backed out all the way.
I have not tried to adjust the mixture screw yet, I guess I didn't feel it would reslove the problem.
Also I was thinking that maybe this is too much carb for a stock engine, or the cams need to be upgraded.
In any case, I'm baffled, and need help to figure this one out.

[manoa matt]

Aloha Len, welcome to the forum. When you get a chance, drop by the new members/introduction section and tell us about yourself and your car. We love pictures too.

Any carb is not going to run perfectly right out of the box, they need minor adjustments. The 34ADF should not be too much carb for a stock engine. There are three things you need to adjust in the following order: Automatic choke housing indexed to the carb body, Air/fuel mixture screw, and fast idle diaphram.

There is a small casting mark on the automatic choke housing and a corresponding mark on the carb body. Those marks must be lined up, and more importantly the bi-metal spring inside the housing must be engaged with the choke unloader cam. If the car is cold, the choke flap should be closed, and when you press it down with your finger it should spring back.

The idle mixture screw should be screwed in till the screw lighlty seats, than backed out 1.5--2.5 turns out. Here you are looking for the best, fastest, smoothest idle. This takes some rolling road adjustments to get right. If its popping in the exhaust on decelerations and going down hills, back the screw out more.

The fast idle diaphram is adjusted via the test button on the driver's side fender. Pull the linkage to get the carb above 2500 rpm, push in the button and let off the linkage. The idle speed should be about 1550-1650 with the button pushed in. After you release the button idle speed should drop back down to the normal 850rpms.

I have several diagrams and a supplement to the factory service manual that cover the 32ADFA. The carbs are basically the same with the exception of the throttle bores, and minus some emissions parts. I can't say for sure the "on the bench adjustments" specifications would be the same for the 32 and 34 carbs, but should be close. Send me an email for copies of the files. matt.scarton@gmail.com

[So Cal Mark]

good advice Matt, but that is assuming the jetting is correct on the new carb. Also, you suggest making the idle mixture richer if there is popping in the exhaust on decel. That popping is caused by an overly rich mixture, so turning the idle screw out will make the problem worse. One other item to look at concerning the idle speed is the stop screw for the secondary throttle. If it's adjusted to hold the throttle open even slightly, you won't be able to get the speed down

the rule of thumb on Weber low speed jets is to lightly seat the mixture screw. Then turn in out 3/4 of a turn. From there if you have to turn the screw out more than 1/2 turn to get best idle, the jet is too small. If you have to turn it in from there for best idle, the low speed jet is too large. About 80% of normal driving is on the low speed jet, so it's critical to get it right

[manoa matt]

The specification sheet that comes with a 32 ADFA rebuild kit states 2 turns out. Clymer manual states 3 turns, Chiltons states 2 to 3.5. The rule of thumb is: 2 turns out for progressive carbs, 1 turn out for synchronous.

My 78 is at 2.25 turns out and running well. A few weeks ago it was popping slightly. I noticed the spring was not under the mix screw, it was loose, and 1.5 turns out. Re-installed the spring, turned it to 2.25 and the popping stopped. I should probably go from a 50 to a 55 idle jet.

The secondary throttle gap on a 32 ADFA is 0.04 to 0.05mm (0.0015 to 0.0019) or the thickness of tracing paper.

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