curt_928

Hi everyone,

Just wanted to post a question.. having put enough into the 79' shark (5 speed) that it finally is running and I can take it out on the highway... btw the 928 is such a higher end car.. my 02 boxster is geared lower, very different highway characteristics.... anyway...

I adjusted the mixture at idle and got the car running so that it doesn't buck with deceleration and generally runs fine at lower speeds and rpm range.. at higher rpms I'm noticing some dark smoke from the tailpipe in the rear mirrors, only with the pedal to the floor. Am not getting any popping or backfiring..

What's the probability its bad valve seals versus overly rich mixture?

Many thanks!

Curt

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79 silver/blk 5sp
02 boxster blk/blk

GregBBRD

Mixture. The CIS system does not like to sit. How long since it last ran?

curt_928

Has been run weekly.. but put a new CIS distributor in it last year... have been trying to get it tweaked properly ever since. Hmmmm... I guess I'll have to lean it out some still..

Thanks for the reply! (I suspected this was the issue)

Curt

123quattro

It's just smoke from over-fueling. If you can measure A/F ratio, shoot 12.5:1 at wot. Any richer than that and you are leaving power on the table.

Jim bailey - 928 International

The warm up /control pressure regulator on the front of the engine is what determines the mixture ALL THE TIME . The vacuum line adjusts for manifold pressure , the electrical connection heats the unit to lean out the mixture as the engine gets warm (there is no choke) . The key to CIS is fuel pressure , the system pressure set by the bypass valve on the fuel distributor determines the pressure that the injectors see. The lower variable control pressure coming out of the regulator basically pushes on the end of the metering rod inside the fuel distrubutor changing the balance and thus the mixture exposing more of the slots to flow more fuel. The control pressure regulator is the "brains" of a brainless fuel injection system. To KNOW what is going on you need two fuel pressure gauges one to measure system pressure one to measure control pressure and compare those to the WSM numbers. System pressure should be in the 5.3 to 5.5 bar range with control pressure warm engine not running fuel pump bridged about 3 bar. Running engine warm with vacuum at idle 3.6 Bar ...... Cold engine ( 20 C ) running about 1.8 -2 Bar . So as you see the control pressure MUST adjust quite a bit to keep the mixture right. From as low as 1.1 Bar cold not running up to 3.8 Bar or so.