Carb cleaner sprayed under CIS intake boot causes idle to lower! (Vacuum leak test)
 

My '73.5 911T CIS starts and idles fine, but stumbles and backfires whenever you try to accelerate. If you hold the throttle steady at higher RPMs it will be smooth, and decel is OK, but any acceleration causes severe bucking and backfires.

So I read here that the first thing to do was to check for vacuum leaks by spraying carburetor cleaner around the engine and seeing if the idle increases (indicating a vacuum leak.

I sprayed at all the possible vacuum leak areas, and nothing affected the idle except spraying under the CIS intake boot (the big one on top of the fuel distributor and airbox). When sprayed under there it LOWERED the idle What could cause this? The intake boot is securely fastened and I didn't notice any vacuum leaks there...any usual spots?

Thanks for any help.

perhaps the car is already running lean..when you perhaps found a leak, it caused it to momentarily run even leaner with subsequent loss of power. try a pulling a a spark plug or two and check their condition. whitish residue would indicate a lean condition. do you know how to adjust your mixture? perhaps give it a quarter turn clockwise to enrichen and see if the stumbling stops if plugs indicate lean. good luck.
ryan

Thanks for the advice. While I have that boot off, are there any other sources of vacuum leaks in that area? The previous owner installed an aluminum airbox, so I doubt there's any issues with that leaking.

I removed the boot (which has no cracks) and reinstalled it as you instructed and re-ran the tests, including intake runners, etc. (I had done that before as well) Same problem, no difference.

While the boot was off I checked the tightness of four allen bolts that hold the throttle body on the airbox, all were tight. The four allen bolts that hold down the sensor plate are spring loaded...why is this and how tight do these need to be?

So far I'm not finding anything wrong. This "Throttle Valve Control Pressure Regulator" device, located between the throttle body and the sensor plate, really killed the idle when I sprayed carb cleaner directly at it:

http://members.rennlist.com/jimwms/CIS/throttle_cpr_photo.html

Description:
"Early CIS only ('73 - '75) Operated by a cam driven off the throttle shaft. Enriches the mixture at full throttle and at idle by decreasing the control pressure.

Failure mode: Leaking diaphragm keeps fuel mixture too rich at part throttle"

Can these have vacuum leaks or is this a sign that this unit is faulty?

Thanks again for all your help Noah!