[Gunter]

If there is no pop-off valve in your airbox, it is possible that a back-fire in the past created a vacuum leak that needs to be traced; search PP under Vacuum Leak on how to check.
There is also the possibility of problems with system pressure and/or control pressure checked with a special gauge designed for CIS.
I suggest that you get the Bentley SC Repair Manual and read up on the CIS.
There is a lot to understand about the system and may be too much for a novice.
Maybe a knowledgeable Pelicanite nearby can come and have a look?
Other than that, a shop with experience on these engines might be able to narrow down the problem.
Wild guessing and/or replacing parts is not the answer.

Look at the write-up and pictures here:

Air Box Pop-Off Valve:

Sometimes the CIS fuel system is prone to backfires. This can cause a small explosion within the CIS airbox. Without some type of pressure relief valve, this explosion can blow apart the entire airbox. Replacement airboxes are several hundreds of dollars. The work-around is to install a pop-off pressure relief valve in your CIS box. The process is quite easy, and it's very cheap insurance against costly backfires. Be sure to check out our Pelican Technical Article on Installing a Pop-Off Valve.


10-0843-000-M253 911 CIS Air Box Pop-Off Valve, 911 (1974-83)
Brand: APA [Photo] [Photo]

$47.50