I read with great interest your post on surging dating back 10 years. I have a '77 911S, 2.7 original engine, that has been backfiring at idle as soon as it starts and surges, as you have explained. Its been to 3 different mechanics, but the problem persists. I believe your assessment makes sense. Can you send along any pictures of your mod to the aux air valve, as I'm ready to do it myself! Thanks