Part throttle judders?

[HenrikH]

Over the last couple of years, I have fought with a really annoying engine-issue with my 911 2,7 (CIS) 1976.

When the engine is warm, it will judder annoyingly, when I try to keep a steady speed at part throttle.

The car starts easily, idles well and pulls beautifully, but when I try to stick to the speed limits (50, 60 and 80 kph) and maintain a steady speed, the car will judder within a range between 2000 and 2500 rpm.

The car has previously had some issues with warm start problems, and a serious vacuum leak, so it has had a fair bit of bits and bobs changed over the last couple of years:

• New Air Flow Meter
• Rubber boot between AFM and throttle housing
• New air-box
• New rubber hoses between airbox and intake runners
• Gaskets under the Airflow Meter, under the throttle housing and under the intake runners
• New injector sleeves and o-rings
• New fuel pump
• New fuel lines throughout the car
• New injectors
• Reconditioned distributor
• A new Pertronix unit
• New HT-leads

That has cured the surging and warm start troubles, but not the part-trottle juddering.

I have had a good mechanic test fuel pressures and vacuum. Pressures and vacuum are within specification, but he found that the system pressure was at the low end of the range (4,5 bar I think?). Could this be the root of the problem?

My mechanic believes the engine internals are OK (hope he is right!). The engine generally sounds healthy, and recently showed 173 bhp/230 NM when run on his rolling road.

Unfortunately we haven’t been able to cure the judders, and now I am just about running out of ideas. So I am a bit curious if anyone else on this board have experienced anything similar, and possibly have any ideas that might help solve the problem?