[BMWCSRegistry] idle problems

[Dutch80210]

I had trouble just two weeks ago with my carbureted Maserati having the same symptoms. 

It turns out the Ethanol in the gas today causes the boiling point of gas in the carburetor to be much lower than without Ethanol. You're having the carburetor boiling when you try to idle and have a classic case of vapor lock?!
We are at altitude here so it is even worse than at sea level but I am sure your symptoms and cause are the same.

Add some quality gas additive and your problem should be solved.
Cheap crap gas today!
When that coupe was build originally gas and lubricants were a bit different.
SV
Denver, Colorado
  ----- Original Message ----- 
  From: jmlaughead at aol.com 
  To: BMWCSRegistry at idb.ded.forest.net 
  Sent: Thursday, July 12, 2007 8:44 AM
  Subject: [BMWCSRegistry] idle problems


  Hi gang.  I've got a 71 2800CS with two Weber 38/38s.  I've been playing with the idle adjustments recently and think I have it set where I want, but now have a new problem.  Under braking (e.g., coming to a stop sign), the idle drops way down (from about 900-1000 to about 200rpm) with the A/C off.  After a few seconds at rest, foot still on the brake pedal, it comes back up.  Repeated pressing on the brake pedal while at a stop has little to no effect (it might drop the idle just a little, but not much).

  I'm assuming this is either:
   
  1. a vacuum problem (perhaps in the booster?) drawing too much vacuum from the carbs under hard braking and reducing the amount of fuel sucked into the carbs, or 

  2. Loose motor mounts causing the engine to move on deceleration and tweaking the throttle linkage or otherwise upsetting the carbs.

  As I recall, the vacuum advance on the distributor has no affect at idle, so I'm assuming it is not a problem with the lines that go to the distributor.

  Anyone have any ideas as to how to diagnose either of these issues, or any other suggestions?  Someone mentioned that there may be some sort of check valve in the brake booster that may need to be replaced.

  Thanks,
  Jim

  71 2800CS
  74 R75/6
  72 911  

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