250 SL Intake Manifold Pressure at Idle Speed of 800 RPM

[glenn]

PPP  Probative Pagoda Persiflage.   Interesting page 00-0/21.  Heading says 'under load'.  So butterfly must be open some??  Also, page 00-0/7 shows distributor movement curves stopping at 400 mmHg.  A engine is an 'air pump'.  ..

[Benz Dr.]

Vacuum will always be lower under load. I use the BB book for certain things but not everything.

 Distributor movement is sometimes stated in inches of vacuum relative to degrees of movement. It takes very little vacuum to make the vacuum portion move so that can easily be taken out of context. On most units 5 - 7 inches of vacuum will give you full movement of advance/retard.

Engines are air pumps but they're also vacuum pumps of a sort. 15 inches is low but maybe not completely dead. Your engine will still run but not as good as it would with 18 - 20 inches. 250SL engines are basically the same as the 230 or the 280 so you can expect similar readings.
Just for giggles, check your cam timing. Could be out a tooth. Very small possibility the pump could be timed wrong to the engine. Supposed to be 20 ATDC but may be TDC or 20 BTDC.

[Atazman]

Thanks, Glenn and Dr. Dan.  See, I read that page 00-0/21 (the lower row) to mean at idle without any load.   

I will soon be checking my valve adjustment, and at that time I will also check out the cam timing.  I'm fairly confident that it is spot on, but the injection pump timing was done several years ago, so it remains a question.

Another thing I need to check is whether I have too much exhaust back pressure.

Oh.... per Dr. Dan's suggestion I advanced the timing to 5 Deg. BTDC (with vacuum); I have no pinging.  What would you say about me advancing another 3-5 Deg.?

I just posted on another forum.....asking if anyone knows the MB specification for engine vacuum at idle for my car.  No response from them yet.  They will probably refer me back to you guys.

Thanks to everyone......