'71 911T with Zenith 40s - carb tuning - one cyl way low flow

[Optimusglen]

Trying to get my new motor running well and really struggling with the carb tuning.

Zeniths were gone through by a professional, throttle shaft bushings replaced, 100% certainty in the butterfly setup and alignment.

Following the procedure:
-mix screw 2.5 turns out, air screws all the way in, linkage disconnected

Starts and idles, timing good, went through and checked valve adjustment on the intake side and all are good at .004

Cyl 4 is much lower flow. Even with the air screw backed all the way out its only flowing about 4 when cyl 5 next to it is all the way in and flowing 6.5-7.

What else could cause that much of a discrepancy? Anything I can do to fix it?


These Zeniths are running 30mm venturis, 55 idle and 130 mains.

[1QuickS]

If it was #3 or #6 I would be suspect of a broken throttle shaft coupling.

Did your service provider flow check throttles on the bench to confirm throttle valve sealing? It is easy to slightly misalign a throttle valve and cause this issue. Another item of concern following a re-bushing effort is that the couplings need to be rotated 45 degrees from their original installation position on the throttle shafts. If not rotated to a new position then the setscrews will seek their original alignment divots which will upset throttle valve alignment.

[Optimusglen]

Originally Posted by 1QuickS

If it was #3 or #6 I would be suspect of a broken throttle shaft coupling.

Did your service provider flow check throttles on the bench to confirm throttle valve sealing? It is easy to slightly misalign a throttle valve and cause this issue. Another item of concern following a re-bushing effort is that the couplings need to be rotated 45 degrees from their original installation position on the throttle shafts. If not rotated to a new position then the setscrews will seek their original alignment divots which will upset throttle valve alignment.

Good points for sure. I haven't asked him yet but he asked me to check compression on #4 and maybe 1-2 others for comparison. If compression is good then I'll pull the carb and he'll go through it.

The topic of throttle valve sealing was discussed previously with him, he said that because of the process he follows that he is 100% confident that they were set properly.

[911 SLANT]

I would send your carbs to Paul. He is the carb guru.

[BK911]

Clogged air passage in #4?