Long story… the subject is a ’70 911T 2.2L engine with Weber 40IDA3C carbs. For several weeks I have read and re-read these posts but now I'm throwing in the towel and submitting my questions. Before going further, let me express my thanks to all the contributors to this forum as it is very helpful!
Here's the background: I don’t have any history on this engine as the previous owner had it sitting in his garage for many years. I cleaned it up and set it up on a “test stand” so I could check it out before installation in my car. It appears to have had a top-end job at some point and had the chain tensioner modification.
Here’s the problem: These Webers continue to spit and pop. When they are cold, that’s expected. After warm up, they will idle without too much shenanigans, but as soon as I give throttle to push the rpms up to 1500 – 3000, I get a lot of spitting and popping on the #1-3 bank. If I really push the throttle to get up on the mains, the engine responds well with no popping… so this just occurs during “transition” or in what I would call the “level cruising” zone, where you would be cruising at 3000 rpm with a light foot on the throttle to maintain a fixed speed.
I have gone through these carbs and they are clean. The shafts are not loose as I had them zinc plated to build them up a few thousandths. The butterflies seat nice and there is no ridge in the barrels. The accelerator jets all squirt into the barrels OK. If I release the cross-linkage, I can rev up bank 4-6 without any popping and the engine sounds good. If I do the same with bank 1-3, I get bad popping, most especially on #1. I swapped the carbs and the problem follows the “bad”carburetor, making bank 4-6 have the same problem, while the “good” carb fixes bank 1-3. Using a uni-sync, the balance between each throat and side-to-side looks reasonably close. I have tried precision balancing using the air bypass screws and even using a manometer. This doesn’t help. It just seems like #1 is always too lean during transition. So I disassembled that carb again today and carefully verified that all the idle circuit and progression passages are clean and clear.
I have also verified the following:
(1) Compression: All cylinders at 145-155 psi.
(2) Checked valve clearance cold … all are .004”.
(3) Installed new spark plugs and set them at .028”
(4) Points are gapped correctly and set static timing at TDC.
(5) Verified distributor is advancing and returning properly.
(6) Timing around 5 degrees at idle and at high rpms around 33 degrees.
(7) Plug wires all measure good.
(8) Fuel pump pressure is 3 psi.
It seems to me that the “bad” carburetor runs leaner (especially on #1) even though all the jetting matches the other (“good”) carburetor. From all my readings on this forum, it seems that these values
should be suitable for this stock 2.2T engine:
Mains 125
Idle 55
Air Correction 180
Emulsion Tube F26
Venturi 30
Spark Plug read: shows #1 looks like it is burning leaner than the others, which are mostly black as I have been spending a considerable amount of time idling this engine.
I tried installing a #60 idle jet in #1 and this decreased the popping. I installed #60 in #2 and #3 and I could see a small improvement. It just seems to me that this is wrong. Why would this particular carb need larger idle jets than the other one? Perhaps I could just bite the bullet and install #60 idle jets in both carbs, but it seems like this is an overkill or might be ‘masking’ some other problem.
I suppose it is possible that these Webers are just not well-suited for this engine. Perhaps their previous life was spent on a 2.0L engine. They do have fairly low serial numbers ... (1358 I believe.) I also understand that the 2.2T engine typically arrived in the U.S. with Zenith’s. My goal is to get this engine set up for good driveability. I am not necessarily looking for maximum power. I saw one post that stated the "pecking order" is PMO, Zenith, then Weber as a last choice in this regard.
Any ideas about how to resolve this problem? Should I just bite the bullet and set up the idle circuits in an over-rich condition? --Bob
