2.4 with Webers missing and popping

First of all, thanks again to those of you offering assistance on my brake rebuild dilemma last weekend. I’ll tell the whole story another time, but I have good brakes now…used ATE rebuild parts and all is well.

This week’s question
Vehicle: 1972 911T w/ 2.4 engine with headers and 40 IDA 3C Webers. (same one from last week’s brake questions). I just acquired this car and know little about it’s history.

Problem: missing/skipping during mild acceleration and at even speed around 2,500-3,500 RPM. Great at WOT

What’s been done: I rebuilt the Webers and re-installed them 3 weeks ago. These carbs were nasty inside and out…appearing to never have been rebuilt before. They are spotless inside and out now, with new gaskets, accel. pumps, and needle valves. All jets, emul. tubes, and pump jets are clean and free flowing. I did reset the idle mix screws and the air bleed screws near the original position when re-assembling the carbs. Upon starting the engine, it popped and misfired at idle and was pretty rough through the RPM range, so I assumed the throttle shafts needed boring and bushings. A couple of days ago, I started the engine again and let it run till warm. I then adjusted the idle mix screws (outward on most ventruis) and balanced the idle speed screws and the engine actually smoothed out and sped up noticeably (to about 1,100 RPM’s which I slowed to around 900 RPM’s). Took it for a short test ride and it felt really good…no popping, no missing from idle to about 4,500 which is all I dared under the circumstances. Smooth acceleration and good response. Parked it for a couple of days.

I had purchased a Unisyn and studied the Pat Braden - Weber book on setting these carbs and thought it was time to fine-tune the carburetors. Probably should have left well enough alone… First of all, I wasn’t getting enough draft through the carbs to get the Unisyn to react at idle (as prescribed by the book). I sped up the idle till I could get the Unisyn to register and adjusted the idle mix and air bleed screws so all 6 were pretty close in volume. Test ride was disappointing…surging and missing in the middle, ok-but rough idle, great at wide-open throttle. At even speeds between 45-55 MPH, I can feel it missing. I tried re-setting the mix and bleed screws to the previous settings and disassembled the carbs wondering if I had sucked some trash into a jet or e-tube. All was clean.

Other info: I haven’t yet adjusted the valves. I have some exhaust leaks (new headers and mufflers will be purchased soon).

Any thoughts?

Thanks

A couple of comments:

I have never seen air flow too low to register on a Uni-Syn ... Did you screw the center restriction 'dial' [in the middle of the Uni-Syn] in further to make the Uni-Syn more sensitive?

You SHOULD HAVE adjusted your valves and set your distributor points/timing/etc. BEFORE trying to do any adjusting on the carbs!!!

After you have done all of the preliminaries, you may not have any real problems with the carbs!

[This message has been edited by Early_S_Man (edited 08-05-2000).]

Early "S" man is right... get all the rest correct, then work on the carbs. One intake valve too tight and it will not draw correctly through that throat and everything is off. The same with your plugs, wires and electrics... and please make sure that your fuel level in the float bowls are even.

My experience with the Webers was much the same then I found that there was some rust somewhere in the system and putting bits of it in the carbs. Put a inline filter in each side and it helped.

What most people do not realize is that the idle circuit provides fuel up to 2-3000 RPM where the main circuit takes over. My carbs were getting stopped up in the idle vents and jets and would not idle smoothly... and then when I tried to accelerate it would pop and spit due to a lean mixture. When I would blow out every one of the idle jets and vents it would do fine for a while then start again. Clean gas cured this...

Screw down the sync tool and it will register the airflow. If you cannot get any suction from one cylinder on the meter, put your hand on one venturi for a sec, then compare it with another. If one is way off when compared to the others, then look elsewhere... carbs will not cause or cure this unless the butterfly is totally closed.

Getting a set of Webers or Zeniths working right is black magic.. took me years to learn how to do it and lots of it is trial and error.

Good luck,

JoeA