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Weber IDA 40 3C Calibration Issues

I have already read many threads on this board regarding this issue, so I think I have a pretty good handle on what’s going on. I just want to make sure that I am on the right track and get some input on the proper part setup for my Weber’s.

I have a 1970 911T with a 2.2L engine. As far as I know the engine is stock except the exhaust, which is an M&K 2in 2out setup. The engine has been gone over “Except the Carbs” by my Porsche mech guys and they say it’s in good working order. Compression averages 120-125 across all 6 cylinders, ignition is good, etc,etc,etc.

My problem seems to be the same as most Weber owners. The engine pops/spits at startup, pops/spits at 3000+ rpm “while cruising”, pops/backfires on deceleration and will not rev past 5000rpm. Idle to 4000rpm revs up fine but it is nearly imposable to get past 4500rpm. I have tracked the popping to Cyl 1&2. From everything I have read it looks like I have an air leak, the carbs are running lean, and the venture / jetting is not setup right.

Other then having 30mm Venturi I have know idea how the carbs are setup. I am assuming there stock right out of the box and bolted on by the past owner. The only reason I say the Venturi are 30mm, is that when I look down the barrel I can see a “30” stamped into the Venturi.

So, in your guy’s vast amount of experience what is the best setup for my Weber’s?
Here is what I have come up with after reading many, many posts.

Venturi: 32mm
Main Jets: 1.35mm
Air Correction: 1.65mm
Idle Jet: .60mm
Emulsion Tube: F3
Air ByPass: 1.10mm
Carb rebuild kit and all new gaskets.

From what I have come to understand from all my research on this issue, this setup will address the entire popping/spitting/backfiring issue do to leanness and or air leaks, and address the 5000rpm rev limit do to leanness, and do to small venture/main jet setup.


What are your guy’s thoughts???


Thank You: Fantor


Last edited by Fantor; 05-29-2011 at 10:54 AM..
Old 05-29-2011, 10:45 AM
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I would check ignition first. Set dwell and then timing before messing with carbs.
Verify plug wires go to right cylinders.
If that does not help, you may want to adjust valves and verify/adjust cam timing.
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Old 05-29-2011, 11:25 AM
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BK911 :

I already had all that done, as I mentioned in the second paragraph of my post. However, I do thank you for you input.
The engine has been completely gone through and is up to spec other then the carbs.
Old 05-29-2011, 11:49 AM
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Fantor,

I can't help you but I can tell where you can find some answers to your questions.
Paul Abbott is the the 911 Weber man. His web site is Performance Oriented.

Paul just re-built my Webers and he did a superior job. Not only is he extremely talanted mechanically he's a gracious fellow...My engine is alive!
Smooooth as silk!

Hope this helps.

THANKS PAUL ABBOTT @ PERFORMACE ORIENTED!
Old 05-29-2011, 12:21 PM
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Augustus:

Thanks for the Paul Abbott tip. I have already have been to his site, “I have it bookmarked”…:-) I have used his guide to setup the Air, Fuel Mixture, Idle, and air balance. However, I still have the same issues....:-( I am sure my Porsche mech guys here locally can do any additional work. I just need to confirm or modify my setup sizing that I posted.
Old 05-29-2011, 01:08 PM
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Fantor,

Before fussing with a wholesale jetting revision it would be good to know that your carbs are in fact tuned properly. Your engine may be in good working order but if the carbs are not correctly tuned then you will suffer all manners of driving ills, these motors require exacting tuning to perform smoothly. Ignition system (wires, plugs, timing, advance mechanism, etc.), mechanical system (valve settings, compression) and fuel system are interelated. Since each cylinder has its own carburetor to adjust it is important to get each carburetor tuned to that cylinder. There are many ways to get this wrong but rather few ways to get it right.

Of course dirt in the system will disrupt the whole show but even misadjustment of the drop links will ruin performance. I'm sure your shop doesn't need help in setting up Webers but this technology is old and the mechanics who speak Weber are not as plentiful as before. Have a look at my web site for initial setup and tuning for guidance:

Weber Carbs Technical

As for jetting recommendations; the 30mm venturis aren't bad but you could go to 32's. If you do then 130-135 mains, F26 tubes, 55 idles, 170 air correctors. The idle air correctors are not adjustable without modification, IDA carbs have 110's while IDTP's have 140's and a different progression circuit drilling pattern. If you stay with 30's then the mains would be 125-130 with the other items the same. I'd bet you have F26 tubes already.

Popping during deceleration indicates exhaust leak. I'd guess your throttle linkage needs tweeking also; drop links should be the same length and adjusted to slip over 8mm ball studs at idle (use the clearance in the brackets supporting the cross bar to get this adjustment) and the cross bar should be checked that the 8mm ball studs are coplanar with the axis through the cross tube. If the cross bar isn't right then one bank of cylinders will open at a different rate than the other bank which helps explain the difficulty in reving past 4500 RPM.

Hope this helps :-)
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Last edited by 1QuickS; 05-29-2011 at 01:31 PM.. Reason: added jetting recommendations
Old 05-29-2011, 01:14 PM
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1QuickS:

Yes I have been to your site a number of time over the past few months. I have followed your setup guild to the letter, and I just re-read it 2mins ago to make sure everything was set right. Now as far as the .55 idles and F26 tubes. I have read post over post that people are better off with .60 idles and F3 tubes. Any additional thoughts???
Old 05-29-2011, 01:36 PM
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Fantor,

Give me a ring, 530.899.8371
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Old 05-29-2011, 01:54 PM
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Paul Abbott Is A good Man. He helped me with my Webers and rebushed them for me. Thanks Paul. Craig.
Old 05-29-2011, 07:34 PM
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Major update....

After talking to t1Quicks on the phone a few weeks back I was now finally able to install a Holly low pressure fuel regulator and an addition fuel filter on to my Weber’s. I then had my floats checked and set, and WOW what difference that made.
Going form 7psi to 3.5 psi on the carb's made all the difference. All my issues have been cleared up. I have been battling with this car for over a year and to high of fuel pressure and probably over flowing floats were the problem.
The car no longer spits, jerks, or backfires anymore. Even coming down from 110 mph NO backfire..:-) I am now very happy with my first Porsche. It's been a long battle but I think I got it licked..>:-)
Now off to do some body work and a new paint job.

Thank you 1Quicks for helping me figure out what was going with my Weber’s.


Fantor
Old 06-20-2011, 10:20 AM
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I always like a success story, thanks for sharing...

"Well, sometimes the magic works. Sometimes, it doesn't." Chief Dan George in Little Big Man

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Old 06-20-2011, 11:24 AM
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