CIS rich-running condition under WOT (long)
 

Hi all,

Well, I have spent the last few weeks troubleshooting various CIS issues in my '75. Narrowed it down to the thermo-time switch and swapped that out with some success.

** The history** ...

What had happened (in theory, anyway) was kind of interesting. I think the bad thermotime switch caused the Cold Start Valve to intermittently spray to much fuel to the point that it backed all the way up the intake plenum (at one point, I could open up the Backfire-pressure-relief valve and saw fuel swimming around in there about 1/2" deep...scary) I think a backfire or two happened, and fuel got sprayed into the oil system via the breather tube on the right side of the airbox. (need to find the insert valve that will block that somewhwere) All of this caused the car to bog out after driving it, stall, and not restart.

This led to the oil system being WAY overfilled and consequently spraying a nasty fuel/oil mixture back into the airbox via the breather, which dripped out the drain in the back of the airbox on top of the hot exhaust manifolds. Which promptly started a fire under the engine, which we got extinguished before any damage was done (except to my nerves.)

Anyway, we changed the oil (twice) , changed out the fouled plugs (carbon deposits...too rich) for the third time, put in a new air filter, and put in the new thermotime switch. Ran the car for 50 miles this past weekend, and it ran excellently...EXCEPT...

I got on the gas ONCE, after the 50 miles. I put the gas pedal to the floor in 2nd gear and pulled it up to approximately 80 MPH in 5th, and after decelerating, it bogged out (like there was too much fuel being delivered again) and stalled. I got it started again, it blew some white smoke (excess fuel) and ran perfectly all the way back to my house without a puff.

** Back to the present **

So my issue is...what could be causing this? I initially thought the rich/lean mixture, but wouldn't this affect all driving throttle positions, and not just WOT? My second idea is the air/fuel mixture plunger device...I guess I should check its operation. Is there anything I am missing? Obviously I have too much fuel entering the cylinders at WOT...but in "normal" driving the car runs great! I am thinking about the decelleration valve as well, as it all follows after WOT. The fuel distributor was replaced a couple of years ago (I have the receipts) by the PO's wrench.

Any ideas? What am I missing?
Thanks in advance

Matt Daniels
1975 911S

Matt,

I would check the fuel pressure regulator (warm-up reg.) If I recall the regulator has a vacuum line connected to it. The change in vacuum to the regulator is how the engine adjusts the fuel mixture according to load. If the valve in the regulator is sticking, this could cause a rich mixture. I would also check the metering pin in the fuel distributor for sticking aswell.

Good Luck,
Lee Wister

Hi Lee,

Thanks for the reply.

I will check the WUR. How do I check the metering pin in the distributor...is this just by verifying free movement of the air/fuel plunger?

What do you think about the decelleration valve being involved?

Thanks!


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Matt Daniels
1975 911S
Providence, RI