Yes, unplugging the vacuum line one at a time does increase the warm idle speed noticeably.
So, does this make sense? I adjusted the mixture screw 1/8 turn clockwise to make it richer, and test drove it. It stalled right out of the driveway. Then I adjusted it 1/4 turn counter clockwise (1/8 turn past the original position) to lean it out and it did much better. Started up while hot right away, idle no longer bounces at throttle lift or coming to a stop. If I had to explain it, it would be "the car was running too rich so when lifting throttle which makes vacuum go away, there is not enough air sucked into the mixture and the engine bogs down or stalls. Adjusted the mixture to lean it out solved that problem. Do you all believe that?
A Euro 84 308 QV is engine type no. 105A. Mitchell, one thing not mentioned so far is idle speed in terms of troubleshooting or baselining. You mention that when warm, idle will usually return to 1,000, and with a near-stall will drop below 1,000 then return to 1,000... Idle speed is supposed to be 1,000 +/- 100 - but in my case, the tachometer indicates a bit high, and what looks like a 1,200 idle on mine is actually just 1,100 rpm. If my tach showed 1,000, then my idle speed would be only 900, which might be too low and cause issues like you describe. Setting the idle speed seems to be tricky, there are several different write-ups on how to do it that are quite different with respect to the Idle Adjusting Screw and the By-pass Screw. You might want to consider re-setting the idle speed (properly) to 1,100 on an independent tachometer to see if that in itself might solve the near-stall issue.
Maybe the plunger inside the fuel distributor is a bit slow, or the air plate lever mechanism might be corroded or something. At abrupt deceleration, the throttle body will cut off air flow, but the plunger will still allow too much fuel out to the injectors.
The car has new injectors (Brass). Anyway, it is running like a champ now with that litt;e 1/8 turn on the mixture screw to the lean side.
Afternoon Mitchell, The Euro model has a bypass valve on the butterfly body. This valve only works on deceleration and beeps the engine from stalling and popping out the exhaust. It bypasses the butterfly. I've got a brand new one, do you want it? My car is a US model, and when I removed all the check valves and piping, it would pop on deceleration. Decided to install the bypass valve to avoid the popping and finally never used it. Just let ne now. John
Last year I redid my fuel injection, wur, fuel distributor, injectors, fuel filter. When I reset the mixture and idle, my car felt a bit more responsive at a lener setting, but when I checked mixture at the exhaust on the road at near WOT it was lean and I fattened it up. I probably turned the 3mm mixture screw on the fuel distributor a 1/6 turn total richening it to get a good reading. One thing I never had was a stall or severe drop in the rpm. It may have changed a bit dependent on rich/lean, but not a hunting condition or a drop on closing throttle. I would look into getting a good reading on the mixture, lean is where real bad things can happen. But glad to hear it's running better. My US 83 also has the bypass valve, seen in this old pic coming off the TB
I have an 82 911SC and like/am always interested in CIS. I didn’t think this one would be fixed via mixture adjustment ... very interesting and good to know. John
Seems like mixture adjustment is the last stop when everything else checks out. Sent from my iPhone using FerrariChat
The vacuum hose is a known trigger of this issue (below the cooling water tank in my euro 328). However I had this issue recently after working on the engine and I noted that idle was too low so increased idle to 900 RPM and the problem was solved.