Adjusting Weber 40 IDA 3C carbs and getting very cranky!!!

[don hopkins]

I am having a heck of a time getting the Weber carbs I installed recently to run right. The car does two things.
First, it backfires on deceleration. Second, it stumbles in the mid-range between 2-3k. Sometimes I seem to get it to run OK and then the next day it runs terrible.
One guy that looked at it said it was the timing and/or the distributor needs recurving. I have not set the advance timing yet b/c I am too chicken to do it with the engine running at 6k (35 degrees BTDC). I have set the timing for the idle setting (5 degrees BTDC). The 911 Performance Handbook says that as an alternative to recurving that 78-79 distributors will work. Others have told me that the stock distributor is fine and that I should just have it tuned at a shop with a dyno.
Also, when I set the mixture screws the guy at my garage says to start at 1 1/2 turns and then adjust each individually.
Should I set all the screws the same number of turns or do each one individually?

[BURN-BROS]

How are you syncronizing the carbs? What engine? Give us the lowdown on what you got. There are abunch of good guys on the board to help out.

[don hopkins]

Its a 1981 SC 3.0. I balanced the air flow with a synchrometer. Any help would be appreciated.

[A Quiet Boom]

Sounds like the idle is way to lean, adjust the idle mixture screws out until the stumble goes away. If it doesn't check you idle jets for obstructions. My car with Zeniths did the same thing, I found a partially clogged jet on one cylinder

[BURN-BROS]

I would verify what the one guy said about your timing. If you can map the curve along with the rpm and post it here. Check for loose/worn advance parts(irratic readings at idle). binding that would not allow full advance or does not return repetitively. If you have a spare dizzy pop it in or borrow a known good one.

[BURN-BROS]

There can be poo from the tank thats getting in carbs too.

[araine901]

Your valves and timing must be dialed in first before the carbs will be able let themselves get adjusted. Once you have the air flow all figred out ie: bypass screws adjusted to even out the airflow. you can start with the idle mixture. I like BA's advice to adjust at 1800 RPM's It is much easier to get them good at this RPM compared to 900. Adjust the mixture screws in and out until they find thier happy spot (highest smoothest idle). But if the idle jets are too much or too littile your adjustment will harder.

[don hopkins]

Well the problem seems to be pretty much fixed. I adjusted the mixture screws way way rich up to where they almost are loose they are so far open. The car idles rough but it no longer stumbles around in the mid rpm range. I had followed the advice in "101 Projects" to turn the screws in (lean) until the idle drops and then back off a little. That seems to be the wrong approach for my carbs. If it continues to run this way I will be very happy.

[Nitrometano]

First of all you must equally both carburators (left and right) using a Uny Syn or a bubble type flow meter. When you equal the carbs. Set the Webers screwing the mixture screw complety to the lean position closing the gasoline pass. Then loose it slowly until the engine rise the RPMs. When the RPMs not rise any more. Turn the screw 1/4" more out and leave it.

[RoninLB]

Hey guys.. I'm adjusting some new PMOs right now for the 1st time.
I picked up some good info on this thread, thanks.

[A Quiet Boom]

If you had to go that far with the mixture screws chances are your jets or idle circuit are partially clogged. If they aren't clogged I'd go one-two sizes up on the idle jets and see if you can't get those screws more toward the middle of adjustment. What a lot of people don't realize about carbs is that the idle circuit plays a big role in part throttle drivability. Driving down the highway at a steady speed your butterflies are barely open and your're pulling very little fuel from the main circuit most of it comes from the idle and transition circuit and on many carbs these run off the same jet. Hope this helps!

[RoninLB]

my situation is that I'm not familiar with tweaking them.
6 IR is a bit different than the common plenum. I know what the book says, but the feel hasn't developed yet.

[A Quiet Boom]

Yeah IR isn't nearly as forgiving as a common plenum. Still I like to be a little on the rich side with my idle mixture no matter what setup I run. Big cammed V8's for example always seem to run best for me when the idle's a little rich. I drove my car half the summer putting up with a midrange stumble and popping from the exhaust until I finally got sick of it and adjusted the carbs, problem was with the screws almost all the way out it was better but not fixed, that's when I pulled the idle jets and found a partial obstruction. I cleaned them, re adjusted the carbs and accelerator pumps and the car runs fantastic. The throttle response is instant, it idles at 500rpm and can pull from 1000rpm no problem, of course I'ce got short gears so that helps!

[RoninLB]

Originally posted by A Quiet Boom
. Still I like to be a little on the rich side

Big cammed V8's for example always seem to run best for me when the idle's a little rich.

me too.
I always has less trouble with a 600 double pumper running rich than a 650 on a small block. Big block had a vac secondaries.

[don hopkins]

Where exactly are the idle jets?

don hopkins
___________
philly, pa
1981 911SC
1976 914